astro-ph.GA

The weak-field, quasi-static regime of gravity is commonly described by the Newton–Poisson equation as an effective response law. We construct this response within a cost-first discrete variational framework. The Recognition Composition Law (RCL) uniquely selects a reciprocal closure cost within the restricted quadratic symmetric composition class; together with the discrete ledger axioms AX1–AX5 (including conservation) and standard DEC refinement, the Newton–Poisson baseline is then recovered in the instantaneous-closure limit. Conditional on Assumption AS1 (scale-free latency) and Assumption AS2 (causal frequency–wavenumber ansatz), allowing finite equilibration introduces fractional memory into the response, yielding a scale-free modification of the source–potential relation characterized by a power-law kernel w ker ( k ) = 1 + C ( k 0 / k ) α in Fourier space. The kernel exponent α = 1 2 ( 1 − φ − 1 ) ≈ 0.191 , where φ = ( 1 + 5 ) / 2 , is derived from self-similarity of the discrete ledger closure; the amplitude C = φ − 2 ≈ 0.382 is identified as a hypothesis from a three-channel factorization argument. We evaluate this quasi-static kernel-motivated response against SPARC galaxy rotation curves under a strict global-only protocol (fixed M / L = 1 , no per-galaxy tuning, conservative σ tot ), using a controlled multiplicative surrogate for the full nonlocal disk operator implied by the kernel. In this deliberately over-constrained setting, the surrogate interface achieves median ( χ 2 / N ) = 3.06 over 147 galaxies (2933 points), outperforming a strict global-only NFW benchmark and remaining less efficient than MOND under identical constraints. The analysis is restricted to the non-relativistic, quasi-static sector and should be read as a falsifier-oriented galactic-regime consistency check of the scaling window, not as a relativistic completion or a claim of Solar System viability without additional UV regularization/screening.

It is of a general interest to look for signatures of stellar bodies orbiting supermassive black holes (SMBHs) in galactic nuclei other than the Galactic center. Previously stellar transits were analyzed in UV, optical, and X-ray domains as well as potential microlensing signatures due to more compact bodies orbiting SMBH accretion disks. Here we complement previous studies by considering nearby ($z=0.001$) radio-loud active galactic nuclei targeted by different facilities in the millimeter domain. At these wavelengths the radio core is sufficiently small so that it can be occulted by large evolved stars in dense nuclear star clusters. We find that in the millimeter domain evolved stars with stellar radii of $\gtrsim 500\,R_{\odot}$ can cause eclipses with the relative depth of $\sim 10\%$. Typical recurrence timescales are at least 10 years and the eclipse durations are $\sim 10$ days. Towards lower frequencies the eclipse temporal profiles become shallower and broader while towards higher frequencies they are deeper and narrower. Although expected to be rare due to selection effects and evolved stars being prone to tidal disruption, recurrent eclipses of mm radio cores can be applied to infer SMBH masses and constrain the composition of the Nuclear Star Cluster of the host nucleus.

Accurate estimates of fundamental physical properties of galaxies, such as star formation rates (SFRs) or stellar masses, are essential for testing and constraining models of galaxy formation and evolution. Spectral energy distribution (SED) modeling has become the standard method for deriving these quantities. However, the influence of the underlying stellar population synthesis (SPS) models on the inferred parameters remains poorly quantified. This work investigates how the choice of SPS models affects the estimation of SFRs and stellar masses derived from SED modeling. Four widely used SPS models are applied to a sample of 17 230 galaxies with spectroscopic redshifts, selected from recently published Hubble Space Telescope and James Webb Space Telescope photometric catalogs. SEDs are modeled using the Code for Investigating GALaxy Emission. The analysis is performed in two steps: (i) estimating galaxy properties with each SPS model, and (ii) employing synthetic catalogs to assess the relative impact of model choice on the recovered parameters. Systematic differences are found among the models, with stellar mass estimates varying by up to ~ 0.6 dex and SFRs by up to ~ 0.4 dex between certain model pairs. The choice of stellar population model introduces significant systematic uncertainties in derived galaxy properties. This dependence should be accounted for when interpreting SED-based measurements and comparing results across different studies of galaxy evolution.

The dependence of the stellar initial mass function (IMF) on star-formation environment, particularly at low metallicities and high redshifts, remains poorly constrained. Ultra-faint dwarf galaxies (UFDs) are local fossils of high-redshift galaxies hosting old, metal-poor populations, and their resolved stellar populations provide unique pathways to constrain the sub-solar IMF. We investigate the low-mass IMF in the Bo\"otes I (Boo I) UFD with JWST/NIRCam, leveraging its capability to resolve over 10,000 stars reaching $\lesssim0.15 M_{\odot}$, obtaining one of the largest, deepest resolved stellar samples for UFDs. We explore three different functional forms of the IMF with machine learning and statistical techniques, combining forward modeling of synthetic color-magnitude diagrams with simulation-based inference. We find that a single power-law IMF fails to reproduce the observed luminosity function and also deviates from the canonical Salpeter IMF. Our best-fit broken power-law and lognormal IMF parameters are consistent with the Milky Way within 68% confidence level, providing evidence that star formation at metallicities as low as [Fe/H]$\approx-2.4$ follows a similar IMF as in the Milky Way. By treating Boo I as a local relic analogous to a high-redshift galaxy with a stellar mass of $\lesssim10^5 M_{\odot}$ at $z\gtrsim6$, our results provide evidence for the universality of the IMF across both local and high-redshift environments.

Nitrogen-bearing polycyclic aromatic hydrocarbons (N-PAHs) are key precursors to complex organic molecules in both the interstellar medium and the nitrogen-rich planetary atmospheres. Despite the recent detections of nitrogen-functionalized astromolecules, their formation pathways remain an open question. The discrepancies between their predicted and observed abundances point to unknown mechanism that govern their evolution in the astrophysical environments. Employing an ion trap technique and electronic structure calculations, we unravel multiple barrier-less reactions between gas-phase pyrimidine cations (C$_4$H$_4$N$_2^+$) and acetylene (C$_2$H$_2$) which form an hitherto unreported endocyclic- N-PAHs (C$_8$H$_7$N$_2^+$). The present measurements on reactions involving a double-nitrogen subsituted aromatic heterocycle have implications to the astrochemistry of both the Titan's atmosphere and interstellar medium.

We present a multi-wavelength physical characterization of 14 luminous Ly$\alpha$ emitters (LAEs) at $z\approx6$, integrating deep ground-based Magellan/M2FS spectroscopy with heterogeneous JWST/NIRCam broad- and medium-band imaging. Identified via strong Ly$\alpha$ lines with extreme Ly$\alpha$ luminosities of ${>}10^{42.6}$ erg s$^{-1}$, the sample exhibits very large rest-frame equivalent widths (${\gtrsim}100$ \AA) and steeply blue UV continua ($\beta_{\rm median}\simeq-2.2$, $-18.2>M_{\rm 1500}>-20.2$ mag). Crucially, the integration of NIRCam medium-band photometry (F410M) breaks the degeneracy between strong rest-optical nebular emission and Balmer breaks, resolving prior mass overestimations. The tightly constrained spectral energy distribution modeling demonstrates that these luminous LAEs tend to be unequivocally low-mass, ultra-young dwarf starbursts; half the sample is characterized by stellar masses of $M_* < 10^8 M_{\odot}$, ages $\lesssim10$ Myr, and negligible dust attenuation. We also map the production efficiency of ionizing photons and Ly$\alpha$ escape fractions ($f_{\rm esc}^{\rm Ly\alpha}$). The $f_{\rm esc}^{\rm Ly\alpha}$ values are exceptionally high, with a median of ${\gtrsim}40$%, increasing for the bluer UV continua. Finally, analyzing spatial offsets between the Ly$\alpha$ centroid and the stellar counterpart, we demonstrate empirically that internal dust content, rather than neutral hydrogen gas, dominate the suppression of Ly$\alpha$ radiative transfer. Our study reveals that strong Ly$\alpha$ emission of the luminous LAEs are generally attributed to both the vigorous starburst activities and the high $f_{\rm esc}^{\rm Ly\alpha}$. Resembling Lyman continuum leakers, these extreme dwarf systems function as highly efficient ionizing engines at the conclusion of the Epoch of Reionization.

The Sagittarius (Sgr) Stream is produced by the ongoing disruption of the Sgr dwarf spheroidal (dSph) galaxy and is thought to contain multiple wraps that were stripped during different pericentric passages. In this study, we introduce a neural-network--based method trained on $N$-body simulations to infer the stripping time of Sgr Stream stars directly from their phase-space coordinates. We combine spectroscopic data from SEGUE, APOGEE DR17, and LAMOST DR7 LRS with \textit{Gaia} EDR3 astrometry and distance estimates from the latest \texttt{StarHorse} catalog to identify high-quality Sgr Stream members. Applying our method to these stars, we measure a clear metallicity gradient with stripping time, well described by a linear relation with slope $\sim 0.3~\mathrm{dex~Gyr^{-1}}$. We further predict the stripping times of globular clusters previously suggested to originate from the Sgr dSph. M 54, Terzan 7, Terzan 8, and Arp 2 exhibit stripping times consistent with being currently bound to the Sgr remnant. Pal 12, Whiting 1, and NGC 2419 are inferred to have been stripped $0.9 \pm 0.1$, $1.1 \pm 0.2$, and $2.1 \pm 0.2$ Gyr ago, respectively. For NGC 4147 and NGC 5634, whose membership in the Sgr system remains uncertain, our analysis suggests stripping times of $1.1 \pm 0.4$ and $1.1 \pm 0.1$ Gyr, respectively, if they are ultimately confirmed as genuine Sgr members. These results demonstrate that data-driven models of dynamical stripping histories offer a promising approach for reconstructing the formation and chemical evolution of the Sgr Stream.

JWST has unveiled an abundant population of compact broad-line emitters largely at $z\gtrsim4$, the Little Red Dots (LRDs), which might represent a previously unprobed supermassive black hole evolution channel predominant at high redshift. However, the LRDs have remained mostly elusive at lower redshift ($z\lesssim2$) where detailed studies are possible from ground-based observatories. We searched for low-redshift LRDs in the Dark Energy Spectroscopic Instrument (DESI) survey. Our search is primarily based on emission line properties, as opposed to earlier approaches that searched for compact sources with specific photometric spectral energy distributions. We report the discovery of eight LRDs at $z=0.2-0.45$, which show spectral features akin to the high-redshift LRDs in the rest-frame optical. The sources are characterized by broad Balmer lines, steep Balmer decrements, compact morphologies, Balmer absorption features and/or strong He I emission, but weak or absent He II, [Ne V] or other high excitation lines typical of Type I AGN. For 7 out of 8 sources, we retrieve dense-cadence light curves from time-domain surveys and for most sources we find weak to no intrinsic variability ($0.0-0.1$ mag) over $4-17$ years in the rest-frame. We also highlight the identification of a quasar with similar Balmer line profiles as LRDs, but shows differences in Balmer decrement, significant variability, and high-ionisation lines. Given the effective volume $4.9{\rm Gpc^3}$ covered by DESI DR1 at $z<0.45$, our sample corresponds to a number density of $1.6\times10^{-9}$Mpc$^{-3}$, indicating a number density $\sim$10,000 times lower than in the first billion years of cosmic time. We find a dearth of luminous and red LRDs at $z<1$ compared to higher-redshift, which could suggest lower gas feeding rates of LRD activity due to higher metallicities at later cosmic epochs.

The Galactic Center hosts the densest known stellar environment in the Milky Way, dominated by the massive black hole Sgr A* and the surrounding nuclear star cluster. Theory predicts that this region should also contain a large population of stellar compact objects (SCOs) - black holes, neutron stars, and white dwarfs - forming a "dark cluster" whose distribution and properties remain observationally unconstrained. These unseen stellar remnants are central to questions of mass segregation, cluster dynamics, and the expected rate of extreme mass ratio inspirals (EMRIs) detectable by future gravitational-wave observatories including LISA. Current evidence for SCOs in the Galactic Center is indirect, relying on dynamical mass measurements, X-ray surveys, and a small number of transient sources. Direct detections remain elusive due to crowding, extinction, and the sensitivity limits of existing instruments. We explore how upcoming facilities, in particular the Extremely Large Telescope (ELT) with its first-light imager MICADO, can fundamentally transform this field. MICADO's combination of deep photometry, high spatial resolution, and precise astrometry will enable systematic searches for SCO-star binaries via photometric variability and orbital astrometric signatures, as well as direct detection of isolated accreting black holes interacting with the gas-rich Galactic Center environment. We outline the observational pathways, technical challenges, and expected sensitivities, showing that ELT/MICADO observations can provide the first quantitative constraints on the dark cluster population. Establishing these constraints will be pivotal for understanding the dynamical evolution of the Galactic Center, the role of compact remnants in nuclear star clusters, and the astrophysical context of gravitational-wave sources in galactic nuclei.

Sgr A* is the nearest quiescent supermassive black hole, and its proximity offers a unique opportunity to study its surrounding fuel supply. We leverage extensive spatial and spectroscopic information provided by the \jwst/MIRI MRS instrument to disentangle mid-infrared ionized gas structures in the central 0.1 parsec of the Galaxy. The Galactic Minispiral's Bar and Northern Arm are revealed by their distinct morphological and kinematic signatures. Several compact ($<1$\arcsec) gas structures including X7 also appear within $\sim 0.05$ parsec of Sgr A* in the plane of the sky, moving with blue-shifted radial velocities $\gtrsim 600$ km/s. Fine structure line measurements spanning ionization energies $\sim 7 - 55$ eV are used to constrain the incident radiation field, metal abundances (neon, argon, sulfur, nickel, and iron), and dust depletion/destruction for each identified gas structure. Overall, the Minispiral gas metallicity is $\sim 1-2.5~Z_\sun$, with a Wolf-Rayet star-driven ionizing radiation field, and significant nickel and iron dust destruction. Increased flux at energies $\gtrsim 41$ eV suggests that the compact gas structures experience an additional harder ionizing radiation source, which is most likely driven by localized fast radiative shocks from stellar winds, a hypothetical Sgr A* outflow, and/or interactions with the ambient medium.

Observations with the JWST in lensed fields have revealed that galaxies at cosmic dawn may concentrate their star formation in highly dense, compact, star clusters. The high columns and low metallicities encountered in their birth environments suggest that Lyman-alpha (Ly$\alpha$) radiation pressure may be crucial to their formation and evolution. In this study, we address this question by post-processing snapshots from radiation hydrodynamic simulations of dense star cluster-forming clouds ($\Sigma_*\gtrsim10^3{M_\odot{pc}^{-2}}$) with a range of dust abundances ($Z_d=0-0.1Z_{d,\odot}$) using the COLT Monte Carlo code. We infer that Ly$\alpha$ is likely to have mild (~10%) effects on the gas-to-star conversion efficiencies ($\epsilon_*\gtrsim60$%) for $Z_d\gtrsim0.01Z_{d,\odot}$, and even in dust-free environments, $\epsilon_*\gtrsim25$% - much higher than the <10% values typical of star-forming regions in the local Universe. This is because the densest filaments dominating stellar mass assembly ($n\gtrsim10^4{cm}^{-3}$) remain sub-Eddington ($f_{Edd}<1$). On the other hand, the bulk of the gas volume ($n\lesssim10^3{cm}^{-3}$) has $f_{Edd}>1$, with noticeable fractions having $f_{Edd}\gtrsim10$, implying that Ly$\alpha$ can launch dynamically significant winds from these systems rapidly ($\lesssim$4Myr), with possible implications for ionizing photon escape and galactic outflows. The Ly$\alpha$ force multiplier $M_F$ is highly sensitive to $Z_d$, with $M_F\lesssim3$ ($\lesssim 500$) for $0.1Z_{d,\odot}$ (dust-free) environments respectively. Nevertheless, Ly$\alpha$ dominates over UV and IR radiation pressure at all values of $Z_d\lesssim0.1Z_{d,\odot}$, by factors of ~3-500. Our results suggest that Ly$\alpha$ radiation pressure reinforces the emerging picture of locally efficient, bursty star formation accompanied by rapid outflows in galaxies at cosmic dawn.

We aim to develop a state-of-the-art tool to infer detailed star formation histories (SFHs) and age-metallicity relations from realistic observational data, while mitigating classical degeneracies and substantially reducing computational cost. In particular, we seek to exploit the complementarity of spectroscopic and photometric data to improve constraints on the spatially resolved SFH and metallicity evolution of nearby galaxies in the PHANGS collaboration. We construct and train a convolutional neural network (CNN) that combines convolutional layers, attention mechanisms, and a shared latent space to jointly predict SFHs and metallicities in 16 age bins. The network simultaneously processes integral-field spectroscopic data from PHANGS-MUSE and five-band photometric fluxes from PHANGS-HST. Training is performed on a dataset of 165\,000 synthetic spectra and photometric measurements spanning a broad range of SFH shapes, metallicity evolution, dust attenuation, and signal-to-noise ratios representative of the observations. The CNN accurately recovers SFHs and age-metallicity relations over a wide range of evolutionary scenarios. The inferred luminosity- and mass-weighted mean ages and metallicities show negligible bias, with dispersions of $\sim0.12$ dex in age and $\sim0.03$ dex in metallicity. When applied to real PHANGS-MUSE and PHANGS-HST data for NGC\,3627, the network produces smooth, spatially coherent maps of stellar age and metallicity that recover physically meaningful structures, including younger populations tracing the spiral arms and star-forming regions. The CNN is approximately $5\times10^{3}$--$2\times10^{4}$ times faster than traditional full spectral fitting codes, providing a powerful and efficient alternative for the analysis of large spectro-photometric surveys.

Recent JWST observations indicate that star formation at $z\!\sim\!4-6$ is more stochastic than previously assumed, with rms log-SFR scatter $\sim\!0.6$ dex at $M_h\!\sim\!10^{11}M_{\odot}$, growing toward smaller halos and time-correlated on $\sim\!25$ Myr. This is significantly higher than the typical $\sim\!0.3$ dex phenomenological lognormal scatter assumed in standard line-intensity mapping (LIM) forecasts. We propagate the JWST-era burstiness through to the LIM shot-noise power spectrum and show that the result is a simple multiplicative correction: the deterministic shot noise multiplied by a line-dependent boost factor $B_\lambda$ derived in closed form by convolving the SFR correlation function with the stellar-population-synthesis kernel of each line. At $z\!\sim\!6$, we find $B_{{\rm H}\alpha}\!\simeq\!7$ and $B\!\sim\!2.5$-$3.5$ for longer-window tracers ([CII], CO, UV) - factors of $\sim\!2$-$5$ above the standard prescription, and growing further toward higher redshift. The enhancement transforms the LIM landscape: it improves auto-spectrum detectability and suppresses lower-redshift interloper contamination, but degrades cosmological applications such as BAO that rely on a clean clustering measurement. Crucially, it also opens a new use of LIM as a diagnostic of high-redshift star-formation physics beyond the regime of individually resolved galaxies: redshift tomography of a single line constrains the amplitude and mass dependence of the burstiness, while cross-line shot-noise correlations probe its time coherence.

The stellar mass assembly of massive galaxies in the first few billion years of cosmic history remains a central challenge in galaxy formation. Galaxies with $M_\star \gtrsim 10^{10}M_\odot$ observed at $z \gtrsim 4$ must grow rapidly under conditions of intense gas accretion, feedback, and dust production. Observationally, their star-formation histories (SFHs) have been poorly constrained due to degeneracies inherent to broadband photometry. The advent of JWST enables direct spectroscopic access to detailed continuum shapes and rest-frame optical diagnostics at high redshift, providing a critical opportunity to reconstruct formation timescales of massive early galaxies. Here, we investigate massive galaxies using joint spectro-photometric SED fitting of JWST/NIRSpec prism spectroscopy from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS). Our sample comprises 148 galaxies selected photometrically with log $(M_\star/M_\odot) > 9.5$ at $z > 3.5$. We find that the most massive galaxies (log $(M_\star/M_\odot) > 10.5$) preferentially exhibit shallow, gray dust attenuation curves, consistent with higher dust optical depths and large grain sizes. We also find significant diversity in the time at which galaxies form 25% of their stellar mass. While formation timescales converge toward later cosmic times, galaxies with lower sSFR ($\lesssim -9$) at the observation epoch formed significantly earlier than systems with higher sSFRs. Across the full mass range, inferred assembly times are systematically earlier than model predictions, suggesting more rapid early growth than currently captured theoretically. These results underscore the importance of spectroscopic constraints and flexible SFH and dust models for reconstructing high-redshift massive galaxy formation histories.

Theoretical studies predict that dual AGNs are a critical stage of galaxy merger-driven supermassive black hole growth. Systematic searches for dual AGNs typically target late-stage mergers ($\leq10$ kpc nuclear separations) and select AGNs based on optical diagnostics. Yet, simulations predict that obscuration can occur early in the merger sequence, and that a significant fraction of dual AGNs can be found beyond $10$ kpc. Here, we report on a new sample of 157 spatially resolved mid-IR dual AGNs candidates selected based upon their mid-IR $W1-W2$ colors from the Wide-Field Infrared Survey Explorer and optically classified as galaxy merger candidates using imaging from the Dark Energy Camera Legacy Survey. Spectroscopic results are presented for approximately 2/3 of the sample. 76 candidates have been confirmed to reside in galaxy mergers; among these, 13 have been confirmed as bona fide mid-IR dual AGNs, while 63 represent strong dual AGN candidates that require further examination. 46 candidates have been rejected as non-merger contaminants (foreground-background AGNs, separations inconsistent with interacting galaxies, etc.). 35 candidates still await spectroscopic coverage. The confirmed and high confidence dual AGN candidates exhibit separations of 14.5-129 kpc; $>50$% reside at separations $>50$ kpc. Confirmed and high confidence candidates also exhibit a diversity of nuclear optical BPT classes. Seyfert-Seyferts and Seyfert-HIIs dominate the overall BPT pairs sample. 31% of confirmed mid-IR dual AGNs reside in multi-mergers involving three or more galaxies. The diversity in AGN properties and environments identified in this work highlights the importance of multiwavelength selection strategies and analyses in the quest to holistically understand dual AGNs as a population.

We present validated and forward-modelled galaxy luminosity functions and photometric predictions for the Vera C. Rubin Observatory Legacy Survey of Space and Time using the ASTRID cosmological hydrodynamical simulation. Galaxy magnitudes are computed by combining stellar population synthesis modeling with a physically motivated dust attenuation prescription in which the optical depth scales with metal surface density. The dust model is calibrated at z = 0 using SDSS luminosity functions and tested at intermediate redshifts (z = 0.5, 1.0, and 1.5) in rest-frame B, V , R, and I bands. We find that the attenuated luminosity functions reproduce observed galaxy statistics across multiple wavelengths and redshifts. Using this calibrated framework, we construct LSST-ready mock photometric catalogs over 0 <= z <= 2 in steps of Delta z = 0.1, containing ~378 million galaxies. We provide predicted apparent-magnitude luminosity functions in the LSST ugrizy bands, derive best-fit Schechter parameters as a compact analytic representation, and compute differential and cumulative galaxy number counts as a function of survey depth from Year 1 to Year 10.

Low-surface-brightness galaxies (LSBGs) are vital for understanding galaxy formation, but their diffuse nature makes them challenging to detect. Upcoming large-scale surveys are expected to uncover large numbers of LSBGs, requiring robust automated methods to identify them across heterogeneous datasets. As a precursor to the Legacy Survey of Space and Time (LSST) and Euclid, we explore domain adaptation techniques for cross-survey LSBG identification. Using models trained on the Dark Energy Survey (DES), we search for LSBGs in the Kilo-Degree Survey Data Release 5 (KiDS DR5). We used an ensemble consisting of one convolutional neural network (CNN) and two transformer models trained on DES cutouts and applied to KiDS DR5 imaging data. Structural parameters were estimated with galfitm, and photometric redshifts and stellar population properties were estimated through spectral energy distribution fitting with CIGALE. We identify 20,180 LSBGs and 434 ultra-diffuse galaxies (UDGs) in KiDS DR5. Their structural parameters are similar to known LSBGs from DES and the Hyper Suprime-Cam SSP Survey (HSC-SSP). The KiDS-LSBGs follow a continuous size-luminosity relation connecting classical dwarf galaxies and UDGs, and their colours are bimodal ($\sim73\%$ blue, $\sim27\%$ red). Cross-matching with spectroscopic and cluster catalogues provides redshifts for 4,913 systems, enabling a systematic characterisation of the star-forming main sequence of LSBGs. Strong environmental trends are evident, with cluster LSBGs and UDGs exhibiting redder colours and reduced star formation compared to non-cluster systems. We demonstrate that domain adaptation enables robust cross-survey LSBG identification with deep learning models, providing a scalable pathway for constructing homogeneous LSBG catalogues for the LSST and Euclid era.

We propose a new point-spread function (PSF) deconvolution algorithm for images of galaxies hosting an active galactic nucleus (AGN), designed to simultaneously enhance the spatial resolution of the host galaxy and remove the bright central point source. In this algorithm, an intrinsic image is reconstructed by decomposing an observed image into two components: an image $I_{\rm sm}$ of an extended component (i.e., a host galaxy) and an image $I_{\rm sp}$ of a point-source component (i.e., an AGN). During image reconstruction, three constraints are imposed: (1) a smooth constraint on the image $I_{\rm sm}$ , which spatially smooths the host-galaxy structures; (2) a sparse constraint on the image $I_{\rm sp}$ , which localizes the point source to a small number of pixels; and (3) a new constraint, the point-source balance constraint, based on the pixel-wise product $I_{\rm sm} \times I_{\rm sp}$ , which removes the point source from the host galaxy without over- or under-subtraction. As a test, we apply this algorithm to images of artificial and $z \sim 0-1$ real AGNs observed with Hyper Suprime-Cam on the Subaru Telescope. We find that the spatial resolution of the host-galaxy images is improved to a level comparable to that of images from the Hubble Space Telescope and that the bright central point sources are removed. This algorithm is expected to enable statistical morphological studies of distant AGN host galaxies when applied to wide-field survey data from the Vera C. Rubin Observatory, the Euclid Space Telescope, and the Roman Space Telescope.

We propose a new point-spread function (PSF) deconvolution algorithm for images of galaxies hosting an active galactic nucleus (AGN), designed to simultaneously enhance the spatial resolution of the host galaxy and remove the bright central point source. In this algorithm, an intrinsic image is reconstructed by decomposing an observed image into two components: an image $I_{\rm sm}$ of an extended component (i.e., a host galaxy) and an image $I_{\rm sp}$ of a point-source component (i.e., an AGN). During image reconstruction, three constraints are imposed: (1) a smooth constraint on the image $I_{\rm sm}$ , which spatially smooths the host-galaxy structures; (2) a sparse constraint on the image $I_{\rm sp}$ , which localizes the point source to a small number of pixels; and (3) a new constraint, the point-source balance constraint, based on the pixel-wise product $I_{\rm sm} \times I_{\rm sp}$ , which removes the point source from the host galaxy without over- or under-subtraction. As a test, we apply this algorithm to images of artificial and $z \sim 0-1$ real AGNs observed with Hyper Suprime-Cam on the Subaru Telescope. We find that the spatial resolution of the host-galaxy images is improved to a level comparable to that of images from the Hubble Space Telescope and that the bright central point sources are removed. This algorithm is expected to enable statistical morphological studies of distant AGN host galaxies when applied to wide-field survey data from the Vera C. Rubin Observatory, the Euclid Space Telescope, and the Roman Space Telescope.

Sulfur is one of the most abundant elements in the Universe, yet the sulfur budget inferred from the observed sulfur-bearing molecules in dense cores is significantly lower than expected. Starless and pre-stellar cores represent the earliest stages of star formation and provide a laboratory for studying the physical and chemical processes that cause sulfur depletion. We aim to constrain sulfur chemistry in dense cores by measuring abundances of sulfur-bearing molecules and how they reflect core evolution and environmental effects. We observed nine cores in the Taurus Molecular Cloud, targeting 13 sulfur-bearing molecules, including CS, CCS, C$_3$S, OCS, SO, SO$_2$, H$_2$CS, and isotopologs. Molecular abundances and six abundance ratios were compared to three evolutionary tracers: H$_2$ column density, N$_2$D$^+$/N$_2$H$^+$, and the CO depletion factor. We also compared observations with 0D chemical models with different initial sulfur abundances. We find variations in abundances across cores. L1517B exhibits low abundances and a high depletion factor, whereas L1495B shows enhanced levels in oxygen-bearing species within the L1495 filament. Ratios tracing carbon- and oxygen-bearing species (CCS/$^{34}$SO and C$^{34}$S/$^{34}$SO) decrease with increasing H$_2$ column density and N$_2$D$^+$/N$_2$H$^+$ ratio. Other species and ratios show weak or no correlation with tracers. Models reproduce OCS, H$_2$CS, and HDCS reasonably well, but not all species simultaneously, especially between carbon- and oxygen-bearing molecules. The variations and lack of consistent correlations suggest that a single evolutionary parameter cannot describe sulfur chemistry and that the local environmental conditions strongly influence the observed abundances. Reproducing the full sample of sulfur-bearing molecules would require improved chemical networks and models that account for the core's physical structure.

Wide-separation lensed quasars (WSLQs) are rare systems that arise from the chance alignment of two objects: a galaxy cluster and a background quasar. After two decades, only seven WSLQs have been found. Here, we report the discovery of COOLJ1153+0755 by the COOL-LAMPS collaboration in DECaLS imaging and its confirmation with follow-up observations with the Magellan Telescopes and the Nordic Optical Telescope. This system features two multiply-imaged quasars each lensed into four images by the same $z=0.4301$ cluster: a classic broad-line Type I quasar at $z=1.524$ (COOLJ1153A) and a dust-obscured Type II quasar at $z=1.939$ (COOLJ1153B), with maximum image separations of $25.''6$ and $26.''0$, respectively. We construct a lens model to estimate a projected cluster mass of $M(<500\,{\rm kpc})\sim3.3\times10^{14}{\rm M}_{\odot}$ and relative time delays between the three brightest images of each quasar of $\Delta t_{\rm \,A3,A1}\sim800$, $\Delta t_{\rm \,A2,A1}\sim1200$, $\Delta t_{\rm \,B1,B3}\sim800$, and $\Delta t_{\rm \,B2,B3}\sim1000$ days. COOLJ1153A resides in a dense environment with three nearby galaxies, two of which are also strongly lensed. We identify COOLJ1153+0755 without making a morphological cut in the DECaLS catalog; none of its multiple images are classified as point sources in those data, implying that morphology-based selection would miss such systems. COOLJ1153+0755 expands the WSLQ sample from 7 to 8 systems (9 individual quasars), adding two powerful laboratories for probing black hole-galaxy co-evolution at Cosmic Noon and for time-delay cosmography constraints on the Hubble constant, $H_0$.

UV absorption line indices trace both chemical enrichment and star formation histories in high-redshift galaxies, yet their reliability as tracers of stellar metallicity (\(Z_\star\)) remains uncertain. In this study, we combine synthetic spectral modelling and cosmological simulations to establish a theoretical framework for interpreting these features in the early Universe. Using the forward modelling package \texttt{Synthesizer}, we compute equivalent widths for a suite of UV indices based on BPASS stellar population synthesis models and investigate their sensitivity to metallicity, star formation history (SFH), and model assumptions. Certain indices, particularly the \(1719\,\text{\AA}\) feature, exhibit strong and consistent correlations with stellar metallicity, while others display increased sensitivity to SFH. To assess the impact of realistic galaxy assembly histories, we apply these models to galaxies drawn from the First Light and Reionization Epoch Simulations (\flares). The simulations provide diverse stellar populations with realistic metallicity distributions and SFHs, enabling an investigation of UV index behaviour within complex enrichment environments. We examine the relationship between galaxy properties and metallicity in \flares\ and reproduce a synthetic mass--metallicity relation (MZR). Across most indices, equivalent widths increase monotonically with metallicity, consistent with predictions from simple stellar population models. The \(1719\,\text{\AA}\) index emerges as one of the most reliable tracers of stellar metallicity, while the \(1460\,\text{\AA}\) feature shows enhanced sensitivity to nebular emission and bursty star formation. These results provide a theoretical benchmark for interpreting rest-frame UV spectra of high-redshift galaxies observed with \textit{JWST}.

Protostellar cores located near supernova remnants are considered potential analogues of the birth environment of the solar system. However, the extent to which supernovae influence their chemical evolution remains unclear. We report the first detection of hot molecular cores in a supernova remnant using the Atacama Large Millimeter/submillimeter Array. The detected hot cores (HC1 and HC2) are located inside the X-ray shell of the young supernova remnant RX J1713.7-3946, and both sources are associated with Class I intermediate-mass protostars. This paper focuses on a detailed chemical analysis of HC1, in which a variety of carbon-, oxygen-, nitrogen-, sulfur-, and silicon-bearing species are detected. Excitation analyses indicate that HC1 harbors dense (~10^7 cm-3), compact (<500 au), and high-temperature (>100K) molecular gas. Despite being located within a supernova-feedback region, the column density ratios of complex organic molecules (HCOOCH3/CH3OH, CH3OCH3/CH3OH, and CH3CHO/CH3OH), a deuterated molecule (CH2DOH/CH3OH), and sulfur- and nitrogen-bearing species (OCS/CH3OH and C2H5CN/CH3CN) in HC1 are indistinguishable from those observed in hot cores/corinos in more typical star-forming environments. HC1 is located near the outer edge of the supernova shell, and the surrounding region has likely begun to be exposed to such a harsh environment only recently. The elapsed time since the onset of exposure to high-energy particles and photons may be too short for the chemical composition of the hot core to be significantly altered, and/or the hot-core region may be shielded by magnetic fields amplified by supernova feedback, which could suppress the penetration of enhanced cosmic rays.

Improving the accuracy of photometric redshifts (photo-$z$) is essential for reliable statistical studies of cosmology and galaxy evolution. However, missing photometric bands are a common observational challenge that can significantly degrade photo-$z$ estimation accuracy. In this work, we present a systematic evaluation of data imputation methods aimed at improving photo-$z$ performance. We benchmark a range of representative machine learning (ML) and deep learning (DL) architectures, identifying k-nearest neighbors (KNN) and the attention-based SAITS model as the leading performers. These models are then applied to China Space Station Survey Telescope (CSST) mock data to assess their performance under realistic observational conditions. Our results show that KNN yields the highest accuracy under idealized missing completely at random (MCAR) conditions with complete training sets, whereas robustness tests reveal that SAITS significantly outperforms KNN when training data is incomplete or when applied to realistic mixed-mechanism scenarios. We find that domain consistency between training and testing missingness patterns is a prerequisite for optimal performance, highlighting the risks of domain shift in supervised regression tasks. Furthermore, our analysis demonstrates that while general imputation models are highly effective for MCAR and missing at random (MAR) data, they are detrimental when applied to missing not at random (MNAR) data arising from flux limits, as statistical models fail to capture the physical information inherent in these non-detections. Consequently, we advocate for more sophisticated architectures capable of disentangling stochastic missingness from physical non-detections to address these distinct mechanisms individually.

Active galactic nucleus (AGN) feedback is a key ingredient in galaxy evolution, yet its impact on the cold atomic gas reservoir -- the neutral hydrogen (HI) phase -- remains poorly constrained. We present the most extensive spatially resolved HI 21-cm survey of Seyfert AGN hosts to date, based on observations with the Giant Metrewave Radio Telescope (GMRT). Our high-resolution HI maps of eight Seyfert galaxies reveal detailed kinematics and surface density distributions of their atomic gas disks. We find that AGN-host galaxies exhibit a slightly shallower HI mass-size relation than the canonical relation or the SIMBA simulation predictions; however, the measured slope remains consistent with the canonical value within $2\sigma$ uncertainties. This result suggests that AGN feedback does not significantly disrupt the global extent or large-scale structure of atomic gas reservoirs. To investigate the internal HI kinematics in greater detail, we perform a 3D kinematic forward modeling of the HI disk in UGC 4503. Our analysis reveals an elevated intrinsic velocity dispersion of $\sigma = 14.9^{+6.1}_{-3.8}$ km/s and a reduced level of rotational support, with $V/\sigma = 14.28_{-4.17}^{+4.97}$, compared to large-sample star-forming spirals. These kinematic signatures, together with localized residuals in the velocity field, indicate that AGN-driven outflows or jets may inject or indirectly affect the turbulence in the atomic gas disk, potentially regulating the cold gas reservoir. Future GMRT observations, combined with optical integral-field spectroscopy from MaNGA, will enable quantitative constraints on the role of AGN feedback in regulating star formation efficiency across a larger and more representative galaxy sample.

Ultra-faint dwarf galaxies (UFDs, $M_* < 10^5 M_\odot$) offer unique insights into early chemical evolution in low-mass systems. However, interpreting their metallicity distribution functions (MDFs) has been challenging due to limited spectroscopic samples, especially beyond the red giant branch. We present metallicities from the Ca II K absorption feature, measured from low-resolution ($R \sim 1000$) Keck/LRIS spectroscopy of 40 stars in the UFD Segue 1 ($M_* \approx 500 M_\odot$), including both red giant branch and main-sequence turnoff stars, resulting in a metallicity sample more than six times larger than previously published data for Segue 1. The resulting MDF has an average [Fe/H] $= -2.52 \pm 0.10$ dex and a dispersion of $\sigma = 0.59 \pm 0.06$ dex, with no evidence for distinct subpopulations. This is consistent with a continuous, short-duration ($\lesssim 1$ Gyr) episode of star formation and chemical enrichment prior to reionization. The nonzero metallicity spread reaffirms its classification as a galaxy. Segue 1 highlights the rich chemical enrichment histories present even in the least massive galaxies, and underscores the importance of deep spectroscopic follow-up to fully characterize these ancient stellar systems.

We use Hubble Space Telescope optical imaging from the Panchromatic Hubble Andromeda Southern Treasury (PHAST) to measure the spatially resolved recent star formation history (SFH) across the southern disk of M31. We fit color-magnitude diagrams (CMDs) of over 6500 individual 0.01 kpc$^2$ regions to measure SFHs over the last $\sim$500 Myr. The resulting maps show coherent structure that traces the ringed morphology of the disk. We find a clear global decline in the recent SFR, with a pronounced drop in the last $\sim$40 Myr that is most evident in the region closest to M32. Combining PHAST and PHAT measurements, we now cover two thirds of M31's star-forming disk with homogeneous SFHs, yielding the highest-resolution spatially resolved SFHs of M31. Inside the joint footprint, we measure mean SFRs of $0.445 \pm0.006$ M$_\odot$ yr$^{-1}$ over the last 100 Myr and $0.285 \pm 0.014$ M$_\odot$ yr$^{-1}$ over the last 20 Myr, implying total disk SFRs of $\sim$0.67 and $\sim$0.43 M$_\odot$ yr$^{-1}$, respectively. The observed decline is interpreted as the late stage of a multi-Gyr wind-down from a previously more active state. Because recent star formation in M31 is concentrated primarily in the rings, the global decline is driven mainly by decreasing activity within those features. We also compare the CMD-based SFR surface densities to those inferred from FUV+24 $\mu$m prescriptions and find that the FUV-based calibration underestimates the CMD-based 100 Myr average by a factor of $\sim$2.1. However, the PHAST SFHs produce a synthetic GALEX FUV image that agrees well with observations, indicating that the CMD-derived SFHs provide an accurate description of recent star formation. The mismatch with the FUV+24 $\mu$m estimates underscores that tracers implicitly averaged over $\sim$100 Myr are not reliable when the recent SFR is evolving.

We present first results from SIMPLIFI (Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments), a SOFIA/HAWC+ $214~\mu\rm{}m$ polarimetric survey of Galactic molecular cloud filaments. We trace magnetic field morphology from the DR21 Main Ridge into surrounding sub-filaments at $\sim{}0.1~\rm{}pc$ resolution, extending polarimetric detections for the first time beyond high-column-density regions probed by prior submillimeter observations. We compare the plane-of-sky orientations of the magnetic field $\hat{B}_{\rm{}pos}$, the projected gravitational acceleration $\vec{g}_{\rm{}pos}$, and the intensity gradient rotated by $90^{\circ}$. The relative orientation of $\hat{B}_{\rm{}pos}$ and the rotated gradient transitions from preferentially parallel in sub-filaments to perpendicular in the Main Ridge at $N({\rm{}H_2})\sim{}2\times{}10^{22}~\rm{}cm^{-2}$, consistent with thresholds seen with Planck. This is expected in clouds formed from strongly magnetized, sub-Alfvenic, magnetically sub-critical gas. We find region-to-region and pixel-to-pixel variations at fixed column density, indicating that column density alone is not sufficient to encode changes in magnetic field structure. Our central finding is that $\vec{g}_{\rm{}pos}$ and $\hat{B}_{\rm{}pos}$ remain aligned throughout the cloud regardless of column density or environment, unlike the environment-dependent behavior of either quantity vs. the intensity gradient. This persistent alignment is consistent with magnetically-guided accretion: sub-filaments channel material along field lines at several $10^{-3}\,M_{\odot}\,\rm{}yr^{-1}$, sufficient to assemble the Ridge within $\sim{}1~\rm{}Myr$ and sustain high-mass star formation. The framework also explains why observed radial velocities $\sim{}2~\rm{}km\,s^{-1}$ fall well below free-fall expectations $\sim{}8~\rm{}km\,s^{-1}$ due to projection effects.

We explore the three-dimensional structure of a sample of $\sim$ 16000 young giant stars in the Galactic disc out to $\sim$8 kpc in heliocentric distance. This population traces a thin disc with a local vertical scale height of $h_{Z \odot} = 77 \pm 4$ pc, that progressively thickens toward the outer Galaxy with a prominent Galactic flare, rising exponentially with a radial scale length of $h_{fl} = 3.5 \pm 0.3 \, \rm{kpc}$. Our analysis incorporates both the survey selection function and the vertical displacements caused by the Galactic warp and corrugations, which, if neglected, would lead to significant biases in the derived disc scale height. In the Galactic plane, the young giants trace coherent spiral arm segments, extending previous maps based on upper main sequence (UMS) and OB stars by 2-4 kpc depending on the considered direction. The obtained map supports a pitch angle of roughly 20 degrees for the Perseus Arm, and shows that the Local/Orion arm stretches at least 10 kpc in length. Unlike earlier and more local maps based on UMS and OB stars, where the relatively small sampled portion of the Perseus Arm appeared as a short, nearly straight feature, our map reveals it as an extended structure with a gentle curvature, as expected for spiral arms on large scales. In the inner Galaxy, we also identify a new segment likely associated with the Scutum Arm, clearly detached from the Sagittarius-Carina Arm in the fourth Galactic quadrant.

Galaxy mergers are fundamental drivers of galaxy evolution and black hole (BH) growth across cosmic time. We use the Horizon-AGN simulation to investigate the fraction of galaxy pairs, the merger fraction, and the galaxy merger rate over a wide range of stellar masses and redshifts. To identify physically connected pairs, we adapt the Matthews Correlation coefficient (MCC) framework, optimizing thresholds in projected distance and redshift difference, and compare our selection to commonly used criteria in the literature. We then connect the derived galaxy merger rates to supermassive BH mergers, tracking the evolution from galaxy interactions to BH coalescences, thereby reconstructing the full merger history. We find that the galaxy pair fraction, merger fraction, characteristic timescale, and merger rate all evolve strongly with both stellar mass and redshift, with higher-mass galaxies and earlier galaxies showing elevated merger activity. BHs exhibit a similar evolutionary trend, with the volume-averaged BH merger rate peaking around cosmic noon ($z\sim2\mbox{--}3$). Our results demonstrate a close correspondence between galaxy and BH cosmic histories. This work provides a comprehensive, simulation-based framework for linking galaxy and BH merger populations, and offers refined selection criteria for future observational studies, for forecasts of gravitational wave detections with LISA, and interpretation of Pulsar Timing Array results.

The standardized magnitude of a type Ia supernova (SN Ia) correlates with host-galaxy properties, and a host mass-step correction is now routinely included in SN Ia luminosity standardization. Given that host mass cannot directly influence SN Ia luminosity, the root cause of the step must be another latent parameter associated with host mass. Identifying this driver is essential because different host properties evolve differently with redshift, so corrections based on them can lead to divergent cosmological inferences. In recent years, direct and extensive age measurements have revealed a significant relation between host age and Hubble residual (HR). Here, using a new dataset, we confirm that this relation arises from the age dependence of the SN Ia luminosity standardization process and the resulting overcorrection. Specifically, we show that while the mass-step correction reduces the age bias by about half, the host age-bias correction fully eliminates the mass step, supporting a progenitor-age origin of the host-age--HR relation. We further demonstrate that the SN Ia magnitude steps with host mass (and specific star formation rate; sSFR) emerge from a nonlinear, step-like relation between mass (and sSFR) and progenitor age, combined with a linear progenitor-age--HR relation: the SN Ia magnitude steps are therefore projected manifestations of an underlying dependence on progenitor age. Taken together, our results show that progenitor age is the primary driver of both the strong host-age--HR relation and the apparent host-mass and host-sSFR steps.

Gravitational collapse is fundamental to star formation, yet direct kinematic evidence of infall at the core scale in high-mass star-forming regions remains poorly constrained. We present the first large-scale statistical study of infall signatures in 304 dense cores within 24 massive 70 $\mu$m-dark clumps from the GLASHES (Global and Local Infall in the ASHES Sample) survey. Using ALMA Band 6 observations of the optically thick tracers HCO$^+$ and HNC (J=3-2), we systematically characterize blue asymmetry line profiles indicative of infalling motions. We employ two complementary metrics, the velocity difference parameter ($\delta_v$) and the asymmetry parameter ($A$), to quantify infall signatures, finding consistent results across both tracers. Blue asymmetry profiles are detected in $\sim$50-60% of cores ($\delta_v<$0 or A>0). Spectral classification reveals that $\sim$60% of cores exhibit double-peaked profiles, and 34% and 39% show blue asymmetry profiles in HCO$^+$ and HNC, respectively, with the percentage increasing with core mass and surface density. Accounting for geometric effects that can obscure infall signatures, our results suggest that gravitational collapse is prevalent in and around the cores. Importantly, infall signatures are detected from the prestellar stage and become more dominant as cores' evolution proceeds. Even cores with virial parameters $\alpha_{vir} > 2$ show infall signatures, suggesting that external compression may trigger collapse in addition to self-gravity or that linewidth may include inward motion in addition to turbulence. Furthermore, a moderate correlation between clump-scale and core-scale asymmetry supports a hierarchical collapse scenario, implying a dynamic and multi-scale process of high-mass star formation.

The study of our Solar System -- its formation, evolution, and long-term stability -- has been ongoing for centuries and is now a standard part of scientific education. While the formation of other Solar-like exoplanetary systems is generally explained using the same mechanisms that describe our own, the discovery of exoplanets around pulsars in 1990s has raised new questions about their origin. Several scenarios were proposed, including formation by capture during a close encounter of a compact stellar-mass remnant and a pre-existing planetary system. It was, however, also conjectured that captured planets should exhibit high eccentricities and -- if more planets are captured -- their evolution would lead to chaos We revisit classical mechanics as applied to planetary systems. As an example and follow-up to previous works, we use an open-source high-precision $N$-body code to investigate dynamical interactions between planetary systems and stellar remnants, the orbital properties of captured planets, and their long-term stability over gigayears. We corroborate that the captured planets often exhibit high eccentricities (unlike some observed pulsar planetary systems), but we also present a student's simulation where a Jupiter-like planet undergoes a series of planet-planet encounters and planetary ejections, eventually stabilising at a low eccentricity of ~0.146. This shows that a chaotic post-capture evolution may eventually lead to long-term stability, making the dynamical formation channel viable for producing low-eccentricity systems. These results warrant more detailed investigation in future work. Beyond their astrophysical significance, they also illustrate general principles of non-linear dynamics and computation, where aspects of the analysis can even be carried out at the high-school or undergraduate level, making this type of research accessible to students at an early stage.

We present an automatic method based on machine-learning convolutional neural network (CNN) architecture to detect Lyman alpha emitters (LAE) hidden in the Data Release 1 spectroscopic dataset of the Dark Energy Spectroscopic Instrument (DESI). Those LAEs mostly have incorrect redshift estimations because the current DESI pipeline is not designed to detect and measure the redshifts of galaxies at $z>2$. To uncover those sources, we first visually inspect thousands of DESI spectra and construct a sample, consisting of both LAEs and non-LAEs, for training and testing the CNN-based model to (1) detect LAEs in DESI spectra and (2) determine their Ly$\alpha$ redshifts. The final model yields $95.2\%$ purity and $95.9\%$ completeness for detecting LAEs. We apply this model to approximately $2\times10^{6}$ spectra of sources targeted as emission-line galaxies and detect 19,685 LAEs from $z\sim2$ to $3.5$ within 12 minutes with a single GPU, illustrating the high efficiency of this model for identifying LAEs. The detected LAEs are mostly at the bright end of the luminosity function with Ly$\alpha$ luminosity $L_{\rm Ly\alpha} \gtrsim 10^{43}$ erg/s. The high signal-to-noise composite spectrum of the detected LAEs further shows various spectral features, including P-Cygni profiles of metal lines and MgII emission lines, possible indicators of Lyman continuum escape fraction, revealing the rich astrophysical information in this LAE sample. Finally, this sample can be used to train and validate the pipelines for redshift determination of LAEs for the preparation of the DESI-II survey.

We studied the properties of star formation and the characteristics of young stars in a quiet region located beyond the outskirts of the prominent star-forming cluster NGC 346 in the Small Magellanic Cloud (SMC). Utilising observations from the Hubble Space Telescope across the broad V and I bands, as well as the narrow Halpha band, we identified populations with ages of roughly 10, 60, 400 Myr and of 5 Gyr through isochrone comparison. We successfully identified 137 bona fide pre-main sequence (PMS) candidates exhibiting Halpha excess with a significance level of 5 sigma, accompanied by an Halpha line emission equivalent width exceeding 20 \AA. Physical parameters for these PMS stars were determined, including mass, age, accretion luminosity, and mass accretion rate. Most PMS stars have an age around 16 Myr and an average mass of 0.80 \pm 0.16 M_sun. The median mass accretion rate for all 137 PMS stars is estimated to be about 8.0 x 10^(-9) M_sun/yr. While this rate is lower than that observed in the NGC 346 cluster itself, it is comparable with those measured for PMS stars in low-density star-forming regions in the SMC, despite the absence of apparent clustering and nebulosity. Furthermore, our analysis reveals that the ratios of accreting and non-accreting PMS stars to non-PMS stars and their mass accretion rate correlate with their distance from a group of hot massive stars in the vicinity. This suggests that the ultraviolet radiation emitted by these massive stars might erode the circumstellar discs of nearby PMS stars. Lastly, the overlap between our studied region and observations from the James Webb Space Telescope reveals that some of the identified PMS stars display near-infrared excess.

We present an update to the frequency analysis method for measuring the properties of a galactic bar. The method involves computing the fundamental frequencies of orbits in rotating, N-body-derived potential models, classifying the stars as members of bar supporting orbits, and finding the extent of the apo-centre distribution. In this work, we apply an updated classification criterion designed to isolate the so-called "Warm" inner Lindblad resonance (ILR) orbits. These orbits have been shown to contain the looped x1 orbits, which dominate the "shoulder regions" of the bar and largely contribute to the radial extent. We apply this method to existing Gaia, APOGEE, and OGLE data of more than 200,000 stars to constrain the properties of the Milky Way bar. We find that multiple bar lengths and pattern speeds are consistent with the data to within 5 percent.

Infrared dark clouds (IRDCs) contains cold dense gas at the earliest stage of massive star and cluster formation. In studying the IRDCs, a universal and fundamental task is to resolve their internal hierarchical structures. Various packages and algorithms were developed for this purpose, but with most of them mainly focused on certain individual steps in data processing. In this work, we build a more automatic procedure for multi-band structure measurement HONKAI (Hierarchical cOre ideNtification and Kinematic property AssIgnment), which can resolve the elemental components including cores and clumps, disentangle the velocity components in spectral data, measure their physical properties, and generate a catalogue for all the measured properties. We use {\sc honkai} for a joint study towards three IRDCs observed in 850 $\mu$m dust continuum with James Clerk Maxwell Telescope (JCMT) and the $^{13}CO$ $(1-0)$ data cube with the Purple Mount Observatory 14-m telescope. 193 dense cores in 16 clumps are identified. As major dynamical properties, a large amount of the cores (136 out of 193) are measured to have large virial ratio of $R_{\rm vir}>1$, but their mass-size relation is bellow the threshold for massive star formation. Meanwhile, core mass function (CMF) also exhibits a steeper slope towards high-mass end compared to more evolved core samples. These three properties in accordance suggest that although many IRDC cores are self-gravitating, only a small fraction are seemingly possible to form high-mass stars. In subsequent core evolution, some further mass assembly trend may be involved to facilitate the high-mass star formation.

Active galactic nuclei (AGN) profoundly affect the interstellar medium of their host galaxies through intense radiation fields and powerful winds. Characterising this feedback is essential for understanding galaxy formation and evolution. Here we revisit the origin of proximate damped Lyman-$\alpha$ absorbers (PDLAs), which trace cold gas within 3000 km/s of the quasar redshift, and interpret their kinematics and physical properties within a unified framework. We searched the SDSS DR16 database for low-ionisation metal absorption-line systems at the quasar redshift (referred to as ProxSys). This approach enables us to identify and classify different types of proximate absorbers, including so-called Ghostly systems, coronagraphic DLAs (DLA-Cor), standard DLAs, and sub-DLAs, based on the presence of strong Lyman-alpha absorption, partial covering signatures, or excited atomic transitions such as SiII*. We find that about 13% of ProxSys belong to the Ghostly or DLA-Cor classes and exhibit strong absorption from excited species. The different classes of ProxSys form a continuous sequence characterised by decreasing SiII*, CIV, and NV absorption strengths and dust content. Their velocity distributions reveal multiple kinematic components. Standard DLAs cluster within 1000km/s of the quasar systemic redshift, consistent with gas in the host galaxy, whereas Ghostly and SiII* bearing systems display broader distributions, including outflows reaching -2000 km/s and a smaller population of inflowing clouds up to +1200 km/s. Median stacked spectra confirm that Ghostly and coronagraphic systems arise in dense, compact gas partially covering the quasar emission regions. These results support a scenario in which cold, dense clouds participate in a dynamic cycle of inflow and outflow in the vicinity of quasars, consistent with chaotic cold accretion.

We investigate whether atmosphere models traditionally used for massive stars with strong winds can produce synthetic spectra morphologically similar to those of Little Red Dots (LRDs). We compute atmosphere models and synthetic spectra with the code CMFGEN. The models assume a thermalized radiation field at the inner boundary, parameterized by a temperature varying between 5000 and 12000~K. We adopt a typical luminosity of 1e10 Lsun. The models are spherical, assume an expanding atmosphere, and are computed under non-LTE conditions and for several metallicities. The spectral energy distribution (SED) is different from a blackbody, with a blue optical spectrum. Broad hydrogen emission lines are produced, their wings being formed by electron scattering. The SED near the Balmer and Paschen limit is rather continuous. A Balmer break is predicted for the coolest temperature models provided the wind density is reduced. The SED and Balmer decrement of most LRDs is reproduced by the models, provided they are dust-attenuated with Av~1.9-2.7. Assuming the absorbed luminosity is re-radiated in the infrared, the energy output at these wavelengths is consistent with observational constraints. The models predict FeII, oxygen and calcium lines. OI lines at 8446 A and 1.129 um are produced mostly by Lybeta fluorescence. The strength of emission lines from metals depends on input temperature, metallicity, and details of the radiative transfer models. CMFGEN atmosphere models predict a large number of spectral properties observed in many LRDs. They struggle to simultaneously produce a genuine Balmer break and strong emission lines. Whether they are more relevant or not to explain LRDs' spectra compared to alternative models is unclear, leaving open the question of the physical conditions in LRDs.

The first generation of stars, Population III (Pop III), is believed to be massive, with some potentially having masses in the range 140 M$_\odot$ to 270 M$_\odot$ and capable of exploding as a pair-instability supernova (PISN). Such events release large amounts of energy and produce substantial quantities of metals, suggesting that they should leave characteristic signatures in the abundance patterns of extremely metal-poor (EMP) stars observed in the local Universe. No clear imprint of PISNe is seen in the local EMP star population, implying either that these events were rare or that stars forming from PISN-enriched gas had metallicities too high to find them in the EMP population. Previous work explored the latter possibility by investigating the enrichment by PISNe with masses and explosion energies at the upper end of the theoretical range (270 M$_\odot$, $10^{53}$ erg). Here, we complement that work at the opposite extreme: Pop III stars at the lower mass (140 M$_\odot$) and explosion energy ($5\cdot 10^{51}$ erg) limit. Using a cosmological hydrodynamic simulation, we self-consistently track the formation of Pop III stars, their radiative and mechanical feedback, and the subsequent formation of second-generation stars in metal-enriched gas. We find that all second-generation stars are exclusively internally enriched by their progenitor within the same halo, thereby imprinting the abundance pattern of a single first-generation star. The median [Fe/H] abundance of second-generation stars is ~ -5.5 which is 2.9 dex smaller than in the high-energy PISN case. Our results demonstrate that if Pop III PISNe were common, we would expect to find stars with the characteristic odd-even abundance pattern produced by PISNe within the observed EMP population. Their absence in observations therefore strongly disfavours PISNe as the dominant channel of early metal enrichment.

Context. Giant Low Surface Brightness galaxies, such as Malin 1, host extended discs exceeding 100 kpc. Their formation and evolution remain debated, with interactions with satellite galaxies and accretion streams proposed as key contributors. Malin 1 hosts satellites and exhibits two giant stellar streams, likely the result of past interactions. Aims. We investigate the orbital dynamics of Malin 1's satellites and their possible connections with observed stellar streams, testing their nature with different formation scenarios. Methods. We constructed gravitational potentials using optical and HI data, including stellar, gaseous, and dark matter components, and explored a wide parameter space while testing NFW and ISO halo profiles. Results. Some scenarios produced bound solutions. The ISO halo model ($M_{\text{Virial}} \approx 2.6 \times 10^{12}~M_{\odot}$) favours bound satellite orbits more than the NFW model ($M_{\text{Virial}} \approx 1.4 \times 10^{12}~M_{\odot}$). Giant stellar streams could be substructures of some satellite galaxies along their leading and trailing trajectories. The most distant Malin 1 satellite could have reached pericenter $\sim 1.6$ Gyr ago, while closer companions interacted as early as $\sim 100$ Myr ago. At the same time, one close companion displays both leading and trailing arms in radial and polar orbits. Furthermore, we also identify some unbound solutions linking satellites with streams. Conclusions. Satellites and stream alignment indicate that past interactions shaped Malin 1's morphology. Our modelling constrains progenitors and orbital histories, providing insights into the dynamical evolution of gLSBGs. Findings are consistent with recent studies using Malin 1 kinematic data.

Context. Giant Low Surface Brightness galaxies, such as Malin 1, host extended discs exceeding 100 kpc. Their formation and evolution remain debated, with interactions with satellite galaxies and accretion streams proposed as key contributors. Malin 1 hosts satellites and exhibits two giant stellar streams, likely the result of past interactions. Aims. We investigate the orbital dynamics of Malin 1's satellites and their possible connections with observed stellar streams, testing their nature with different formation scenarios. Methods. We constructed gravitational potentials using optical and HI data, including stellar, gaseous, and dark matter components, and explored a wide parameter space while testing NFW and ISO halo profiles. Results. Some scenarios produced bound solutions. The ISO halo model ($M_{\text{Virial}} \approx 2.6 \times 10^{12}~M_{\odot}$) favours bound satellite orbits more than the NFW model ($M_{\text{Virial}} \approx 1.4 \times 10^{12}~M_{\odot}$). Giant stellar streams could be substructures of some satellite galaxies along their leading and trailing trajectories. The most distant Malin 1 satellite could have reached pericenter $\sim 1.6$ Gyr ago, while closer companions interacted as early as $\sim 100$ Myr ago. At the same time, one close companion displays both leading and trailing arms in radial and polar orbits. Furthermore, we also identify some unbound solutions linking satellites with streams. Conclusions. Satellites and stream alignment indicate that past interactions shaped Malin 1's morphology. Our modelling constrains progenitors and orbital histories, providing insights into the dynamical evolution of gLSBGs. Findings are consistent with recent studies using Malin 1 kinematic data.

The internal dust attenuation of the Hii region reduces the observed emission-line fluxes. Turbulent density fields within each Hii region change the degree of the line-of-the-sight obscuration of the emission-line fluxes. In this paper, we implement the dust Monte-Carlo radiative transfer in the latest M3D code, creating the emission-line maps attenuated by the internal turbulent dust obscuration with the varying Mach numbers. The internal density and temperature fluctuations of Hii regions make the radiative transfer of hydrogen lines neither Case A nor Case B conditions, resulting in the global H{\alpha} to H\b{eta} ratio of approximately 3.02-3.03, differing from the widely-used value of 2.86. This deviation from Case B is because the temperature of these Hii regions is cooler than 10,000 K. We further derive the internal nebular attenuation curve from the attenuated Hydrogen lines, finding that the clumpy structures within Hii regions do not change the slope of the internal attenuation curve. This is because the heavy dust obscuration of dense clumps is canceled out by the high in-situ production of emission-line intensities.

M71 is a nearby, metal-rich globular cluster at low Galactic latitude, where field contamination and spatially variable extinction complicate colour-magnitude diagrams (CMDs) and the identification of cluster member variable stars. Our aims are (i) to construct a homogeneous census of variable stars in M71 by refining their periods and classifications and identifying new candidates, and (ii) to derive a decontaminated, differentially dereddened CMD to constrain its physical properties. We obtained Johnson-Kron-Cousins $VI$ time-series CCD photometry and reduced it using difference image analysis. Cluster membership was established from \textit{Gaia}~DR3 proper motions, and a differential-reddening correction was applied across the field of view. The resulting CMD, cleaned of field stars, was compared with tailored isochrones to estimate age ($12.9^{+0.9}_{-0.8}$ Gyr), metallicity ([Fe/H] =$-0.88^{+0.13}_{-0.15}$), mean reddening ($E(B-V)$ = $0.21 \pm 0.02$), and distance modulus ($(m-M)_{0}$ = $13.01 \pm 0.06$). Variable stars were identified using two complementary approaches: a periodogram-free string-length scan refined with phase dispersion minimisation, and a robust inter-site screening based on median statistics combined with a generalised Lomb-Scargle significance criterion. We identified 21 variable stars not previously reported in the Catalog of Variable Stars in Globular Clusters and provided their periods, amplitudes, classifications, membership status, and light curves. This combined strategy yields a consistent picture of M71, expanding its known variable-star population and confirming parameters typical of metal-rich Galactic disk globular clusters.

The JAGB method has been proposed in recent years as a possible distance indicator for galaxies in the Local Group and beyond. However, the nature of the stars populating the J region, and the conditions required for the direct application of this method, still need to be clarified. We investigate the robustness of the JAGB method through a detailed theoretical analysis of the stars populating the J region of the (J-Ks, J) diagram. The main goal is to identify the properties of the corresponding J luminosity function (JLF) that are minimally affected by the previous evolutionary history of the host galaxy, particularly its metal enrichment history. We use a population synthesis approach based on AGB stellar evolution models coupled consistently with dust formation in the stellar wind. Synthetic stellar distributions in the (J-Ks, J) diagram and the related JLFs are calculated for different assumptions on the metallicity evolution of the interstellar medium, in order to study how the JLF depends on the efficiency of metal enrichment. We find that the JAGB population is dominated by stars formed between about 1 and 6 Gyr ago, while stars formed outside this interval contribute only marginally to the JAGB region. The shape of the JLF strongly depends on the metal enrichment history, and the position of the J-band peak varies by more than 0.3 mag among the different cases explored. Conversely, the mean J-band magnitude, MavJ, is much less sensitive to the previous history of the galaxy and therefore represents a more reliable distance indicator. For all the cases investigated we find MavJ = -6.2 +/- 0.05 mag. We also discuss the uncertainties related to the still poorly constrained mass-loss process experienced by low-mass stars during the red giant branch phase.

An update of the evidence that radio galaxies and clusters of galaxies are more common than average near the plane of the de Vaucouleurs Local Supercluster shows that in the distance range 100 to 200Mpc objects whose positions are correlated with the plane of the Local Supercluster include galaxies that are exceptionally luminous at two microns, radio galaxies, and clusters of galaxies. There can be little doubt about this property of cosmic structure. I also argue for detection of this correlation for the galaxies at 400Mpc distance that are exceptionally luminous at two microns. It will be interesting to learn whether these results are expected in the standard cosmology.

We present a radio continuum study of photometrically selected cosmic noon (0.5<z<3) post-starburst galaxies (PSBs) in the UKIDSS Deep Survey (UDS) field to assess if radio-mode Active Galactic Nuclei (AGN) are linked to the quenching of star formation at cosmic noon. Our cross-matching using the deep Very Large Array (VLA) imaging at 1.4 GHz results in a mean radio detection fraction ($f_{det}$) of only 0.8$\%$ for PSBs above a radio luminosity threshold of $L_{\rm 1.4 GHz} \geq 10^{24}$ W Hz$^{-1}$, increasing to 5$\pm2\%$ for massive PSBs with stellar masses M$_*>10^{11}$M$_\odot$. Massive PSBs have a comparable detection fraction to that of massive quiescent galaxies ($f_{det}=8\pm1\%$), and both classes have lower fractions than that of massive star-forming galaxies ($f_{det}=13\pm1\%$) in the same field. The radio luminosities of detected PSBs, ${\rm L}_{1.4}\sim 10^{22.8}-10^{24.9}$W/Hz, exceed those from star formation by a median factor of 37 indicative of a possible AGN origin. Their compact morphologies ($\lesssim15$ kpc at $z_{med}=1.5$) suggest low-luminosity AGN with less powerful jets. Stacking the undetected PSBs reveals a weak radio detection ($3.9\sigma$) in the highest mass bin (M$_*>10^{11}$M$_\odot$). In contrast, 1.4 GHz detected quiescent galaxies have radio luminosities reaching radio-loud levels, and a higher prevalence of extended morphologies indicative of large-scale jetted AGN. The AGN contribution is also detected in stacked measurements of quiescent galaxies. Overall, our results support a short radio AGN duty cycle for PSBs, characterized by weak radio jets, suggesting radio-driven maintenance mode feedback may become important at older ages.

As part of the ongoing work of the $R$-Process Alliance (RPA), detailed abundance measurements of 29 heavy elements in three metal-poor stars, 2MASS J14592981$-$3852558, 2MASS J19445483$-$4039459, and 2MASS J15211026$-$0607566, are presented based on an analysis of high-resolution ($R\sim 80,000$), high signal-to-noise ``portrait'' spectra from the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory. The selected targets were identified as $r$-process-enhanced metal-poor stars in previous RPA snapshot analyses. They have also been linked to possible chemodynamically tagged groups, indicating that the stars may have formed in dwarf galaxies that were later accreted into the Milky Way halo. These stars have also been tentatively linked to the Thamnos structure. The detailed chemical abundances in this work confirm that 2MASS J14592981$-$3852558 and J15211026$-$0607566 are $r$-II stars, while 2MASS J19445483$-$4039459 is found to lie just below the threshold for $r$-I status. The $r$-II stars show signs of slight enhancement in fission fragments compared to 2MASS J19445483$-$4039459. Based on radioactive age dating with Th, the $r$-process material in the two $r$-II stars is found to be old (with ages $>10$ Gyr); neither star shows signs of an actinide boost. The varying elemental compositions suggest that these stars likely did not originate in the same environment, though each could be consistent with originating in the Thamnos progenitor.

The standard theory of galaxy formation predicts that all galaxies should contain dark matter, yet a handful of recently discovered galaxies appear to lack it, challenging our understanding of galaxy formation. We investigate whether such dark-matter deficient objects can be identified from their baryonic properties alone, analogously to the radial-acceleration relation, which tightly links baryon and dark matter distributions in spiral galaxies. Using a sample of ultra-diffuse and dwarf spheroidal galaxies -- systems whose baryonic properties resemble those of the confirmed dark-matter-deficient galaxies -- we systematically search for a formula to predict baryonic fractions from stellar mass, effective radius, distance to the host, and the host's baryonic mass. We find that baryonic fraction correlates most strongly with the gravitational acceleration expected from baryons alone, $a_\mathrm{bar}$, or equivalently, with mean surface brightness, following an approximately $a_\mathrm{bar}^{-1}$ dependence. This scaling resembles the radial-acceleration relation but differs in functional form and applies to a different galaxy population. Strikingly, the dark-matter-deficient galaxies occupy the extreme end of the correlation. This suggests that they result from standard formation processes operating at unusual intensities rather than from exotic mechanisms. Importantly, the correlation predicts that all ultra-diffuse galaxies brighter than approximately 25 mag arcsec$^{-2}$ in the $g$-band should have very low dark matter content, offering a straightforward observational criterion for identifying these rare objects.

The unexpectedly high abundance of bright, blue, super-early galaxies ($z\gtrsim10$) has challenged most pre-JWST models of early galaxy formation and motivated a wide range of proposed explanations. We systematically investigate whether galaxy clustering can discriminate among representative scenarios that reproduce the observed UV luminosity function. Using the Shin-Uchuu dark-matter-only simulation, we populate $z \approx 11$ halos with galaxies according to solutions based on i) attenuation-free, ii) feedback-free bursts, iii) bursty star formation, and iv) primordial black hole models. For each model, we compute the two-point correlation function and predict the galaxy bias for flux-limited samples at different thresholds in the $-20 < {\rm M_{UV}} < -16$ magnitude range. We find that all models predict similar bias values ($b \approx 7$) for faint galaxies (${\rm M_{UV}}\approx-16$), but diverge at ${\rm M_{UV}}\lesssim-18$, as the underlying halo-mass to ${\rm M_{UV}}$ relations differ significantly. In particular, the primordial black hole scenario predicts an almost luminosity-independent bias, whereas the other models generally predict increasing bias with luminosity, reaching $b \approx 14$ for ${\rm M_{UV}} \approx -19$. Current observational estimates of the bias cannot yet rule out any of the models at a significant statistical confidence. More precise measurements from future JWST programs, together with improved theoretical predictions, will be required to break the present degeneracies. Ideally, constraints from a complete sample of galaxies with ${\rm M_{UV}} < -18$ would probe the knee of the $b({\rm M_{UV}})$ function, taking advantage of the difference in model predictions and strengthening our analysis. Although requiring further refinement, galaxy clustering is confirmed to be a promising probe of the physical origin of the JWST high-redshift luminosity function.

Modified gravity theories such as Modified Newtonian Dynamics (MOND) and Scalar-Tensor-Vector Gravity (STVG) have been proposed as alternatives to dark matter, but decisive tests have been hindered by degeneracies between baryonic structure and gravitational laws. Here we break this degeneracy using independent, high-precision constraints: the Milky Way radial rotation curve, vertical phase-space spirals from Gaia, and a broken-exponential stellar disk. A joint reconstruction of the radial and vertical gravitational fields reveals a structural inconsistency in modified gravity -- no model can simultaneously reproduce both observations. Our results strongly disfavor MOND at $>13\sigma$ and STVG at $>4\sigma$. In contrast, dark matter halo models naturally explain the observations, providing a self-consistent test of gravity on galactic scales.

Wide-field imaging surveys now provide photometry for billions of sources, while spectroscopic observations remain limited, motivating methods that can extract spectroscopic information from photometric data. We present a generative framework for the joint probabilistic inference of galaxy redshifts and rest-frame spectra from broadband photometric fluxes. The model provides a sampling-based estimate of the photometric-redshift probability density function (PDF) for each galaxy, from which accurate point estimates are derived, and reconstructs rest-frame spectra that preserve key spectral properties. We pre-train a spectral autoencoder, SPENDER, on 5 million DESI DR1 spectra to learn a low-dimensional latent space that represents rest-frame spectra. Conditioned on galaxy broadband photometric fluxes, a diffusion model jointly infers the corresponding spectral latent representation and photometric redshift. The inferred latent representation is decoded into a high-resolution rest-frame spectrum, which can be transformed to the observed frame by redshifting and resampling. Sampling from the conditional diffusion model yields a full photometric-redshift PDF for each galaxy, with the resulting point estimates showing a precision comparable to that of a gradient-boosted decision tree model. In most cases, the reconstructed rest-frame spectra reproduce the overall continuum shape and capture the presence of prominent spectral features. For galaxies with sufficiently high signal-to-noise ratios in their observed spectra, the Dn4000 index shows good agreement between the reconstructed spectra and the observed spectra. On average, the spectral reconstruction residuals are close to the noise level of the observed spectra. Latent-diffusion generative modeling enables joint inference of galaxy photometric-redshift PDFs and rest-frame spectra from photometric fluxes.

The neutral hydrogen (Hi) 21-cm line serves as a powerful tracer of the neutral interstellar medium (ISM). Thermal stability analysis suggests that the neutral ISM is bistable in nature, consisting of the cold neutral medium (CNM) embedded within the warm neutral medium (WNM), both in approximate thermal pressure equilibrium. When turbulence is incorporated into the numerical simulations, a third thermally unstable medium (UNM) emerges between the CNM and the WNM. Although observational studies support the existence of this intermediate phase, a clear empirical correlation between the fraction of the UNM gas and the strength of the turbulence remains elusive. In this study, we investigate the various phases of neutral ISM using Hi 21-cm emission-absorption spectra from the publicly available GWA and LAB surveys and compare it with TIGRESS-NCR and TIGRESS-CLASSIC numerical simulations. From our observational modeling, we find that 19.8% of the gas reside in the CNM phase, 32.5% in the UNM phase, and 47.8% in the WNM phase, assuming phase boundaries defined by spin temperature: T_s < 250 K for the CNM, 250 K < T_s < 5000 K for the UNM, and T_s > 5000 K for the WNM. These results are entirely in agreement with the TIGRESS-NCR numerical simulation. We further expect that deep, sensitive absorption studies with the Square Kilometre Array (SKA) or the Next Generation Very Large Array (ngVLA) or existing Upgraded Giant Metrewave Radio Telescope (uGMRT) capable of robustly detecting WNM clouds in absorption will place more tighter observational constraints on the fraction of the gas in three different phases of the neutral ISM.

Observational data have revealed a clear dichotomy in the [{\alpha}/Fe] vs. [Fe/H] diagram of the Milky Way thick and thin disc stars. Many recent studies have shown evidences of a co-evolution phase between the high- and low-{\alpha} disc sequences as well as the presence of very old low-{\alpha} stars. We aim to revise the parallel chemical evolution model that assumes two parallel histories of star formation for the two discs, by considering a pre-enriched delayed second infall episode in our revised scenario. By means of our chemical evolution models, we aim to explore the effects of a phase of co-evolution and the presence of old low-{\alpha} stars, as recently observed. We consider a new version of the parallel scenario for the Milky Way thick and thin disc formation, which consists into two distinct infall episodes of slightly pre-enriched gas. The gas is considered to be extragalactic but possibly contaminated by chemically enriched gas of a massive dwarf galaxy as Gaia-Enceladus, which merged with the Milky Way at least 10 Gyrs ago. Moreover, we test in our model observationally derived star formation histories of kinematically selected thick and thin discs, suggesting that the star formation is triggered by the passages of the Sagittarius galaxy. Our models can well explain the [{\alpha}/Fe] vs. [Fe/H] diagram from APOGEE DR17. Our revised chemical evolution model with a pre-enriched and delayed (roughly 1 Gyr) second infall episode, explains not only the abundance patterns of high- and low-{\alpha} stars but also stellar age distributions for the selected observational sample. We predict a short co-evolution period in between the two phases and we can explain the observed old low-{\alpha} stars, but still further data for precise stellar ages would be needed to put more stringent constraints on their physical nature.

Combined HI emission-absorption studies constrain the spin temperature and phase structure of the neutral atomic hydrogen interstellar medium (ISM), but have largely been limited to the Milky Way and the Local Group. We extend this technique to galaxies at distances of 7-22 Mpc using deep data from the MeerKAT HI Observations of Nearby Galactic Objects - Observing Southern Emitters (MHONGOOSE) survey, and quantify the detection fraction and Cold Neutral Medium (CNM) properties at these distances. We search for HI absorption toward 56 background continuum sources in 21 out of the 30 MHONGOOSE galaxies (with nine galaxies lacking suitable background sources), and detect absorption associated with the galaxies' HI disks in three cases: one sight line in NGC 289 and two in NGC 7424. This corresponds to detection rates of 3/56 (5 percent) for the full sample and 3/31 (10 percent) for a clean sub-sample of sight lines, considering only unresolved background sources behind 14 low-inclination galaxies. Detections occur only where both the continuum flux and the foreground \HI column density are high, with optical-depth sensitivity as the primary limiting factor. For the detected sight lines, we model the absorption and emission spectra to derive spin temperatures and CNM fractions using the standard combined emission-absorption method. The CNM spin temperatures and line widths are comparable to Local Group measurements, but the inferred CNM fractions are systematically lower. We argue that this difference is primarily a resolution effect: at the distances of our galaxies, the emission spectra average over several hundred parsecs, diluting structured CNM relative to the smoother Warm Neutral Medium (WNM). This demonstrates that emission-absorption analyses can be extended beyond the Local Group, provided that care is taken in constructing representative emission spectra.

This work spotlights the Statistical Adiabatic Channel Model as an efficient and accurate method for deriving low temperature (de)-excitation rate coefficients for collisions induced by heavy projectiles. For such systems, fully quantum treatments become intractable, while quasi-classical methods fail at low temperature. Here, we demonstrate that the Statistical Adiabatic Channel Model overcomes these limitations by combining statistical sampling with an adiabatic channel representation. Its application to the HCN and HNC isomers colliding with CO yields rate coefficients in quantitative agreement with full quantum results benchmarked for the lowest total angular momentum. These systems are relevant for modeling cometary comae, where reliable molecular data remain scarce. Remarkably, this approach also reproduces near-resonant energy transfer and isomeric effects, demonstrating that essential quantum and structural features can be captured within a statistical framework.

The formation timescale of the Milky Way thick disk is one of the central debates in Galactic archaeology. The age-metallicity relation (AMR), formation timescale, and chemical evolution gradients are frequently used to infer a rapid assembly, short-timescale enrichment, and bursty formation history of the thick disk. However, stellar ages are not directly observable, introducing the potential risk that inferred ages may harbor a systematic compression tied to observational quality. In this paper, we use the same stellar sample and identical physical covariate matching conditions, but two independent age scales--spectroscopic inferred ages (astroNN) and asteroseismic ages (APOKASC-3)--to compare the observable signatures of the thick-disk formation history. We find that several key observables previously supporting a rapid thick-disk formation are systematically weakened under seismic anchoring: the AMR slope flattens from -3.29 to -1.86 Gyr dex-1 (Delta a = +1.43), the formation timescale widens from 3.04 to 3.55 Gyr, and the peak formation age shifts from 9.1 to 6.0 Gyr. Through transport inversion experiments, we further show that additive noise can only broaden the age distribution and cannot reproduce the above pattern, whereas a compressive transport map (lambda < 1) simultaneously reproduces a narrower age distribution, a steeper AMR, and rapid-formation-like observables. This result indicates that the compression transformation itself is sufficient to generate rapid-formation-friendly observables without requiring an intrinsically bursty formation history. Our findings reveal that statistical interpretations of the Milky Way formation history may depend sensitively on the stellar age definition itself.

Identifying remnant radio-loud active galactic nuclei (AGNs) is challenging due to their diverse morphological and spectral characteristics. Using three-dimensional hydrodynamic simulations of 15 radio galaxies, we investigate how the spectral evolution of remnants depends on progenitor power, active lifetime, environment, and underlying dynamics. The simulations span low-density group and high-density cluster environments re-gridded from smooth-particle-hydrodynamic cosmological simulations. The resulting remnants exhibit a wide range of morphologies, from amorphous structures to double-lobed forms. We find that jet power correlates with the spectral slope. As the remnant lobes evolve, we find surface brightness depends strongly on environment: group remnants are systematically dimmer and more amorphous than cluster remnants, highlighting a potential observational bias against these low-surface-brightness sources. In our models, we estimate that the peak surface brightness of a low-redshift, 50 Myr-old remnant from a low-power progenitor in a 10^{13} M_sun group environment should be routinely detectable at the 3{\sigma} level with LOFAR, although 20-30% of the emission would remain undetectable within a reasonable integration time. We find young remnants exhibit low-frequency (150-1400 MHz) spectral indices that overlap with active sources, and follow a consistent and established spectral-evolution sequence: significant curvature ({\alpha}_{1400}^{6000} - {\alpha}_{150}^{1400} > 0.5) develops before an ultra-steep low-frequency index ({\alpha}_{150}^{1400} > 1.2). The results presented in this work are intended as a reference point for current and upcoming low-frequency studies of radio remnants.

Aims. We constrain the dynamical perturbations of the spiral arms in the Milky Way disk, based on the non-axisymmetric streaming motions of RGB stars revealed by APOGEE and \textit{Gaia}. Methods. We develop a revised steady-state radial-velocity response model that incorporates both the \(V_{R,\sin}\) and the dynamically important \(V_{R,\cos}\) components for a two-armed logarithmic spiral potential. The model is validated using orbit integrations with \texttt{AGAMA} and Bayesian parameter recovery with \texttt{dynesty}, and is applied to the smoothed two-dimensional radial-velocity field of RGB stars while accounting for Lindblad and corotation resonances. Results. The revised model reproduces the phase and amplitude of the mock radial-velocity field to the \(\sim2\%\) level, substantially improving upon earlier \(V_{R,\sin}\)-only formulations. Applied to the observational data, it yields a robust pitch angle of \(p \simeq 10^\circ\) and a local surface density contrast of \(\xi \simeq 5\)--\(18\%\) at the solar radius. The radial scale length is less well-constrained (\(h_{R,1} \simeq 40\)--\(50\,\mathrm{kpc}\)) due to intrinsic parameter covariance. Resonance effects strongly shape the velocity field, thus affecting the fitting: the radial velocity becomes extremely large near the Lindblad resonances, whereas it vanishes close to the corotation resonance. Conclusions. Our results demonstrate that including both the \(V_{R,\sin}\) and \(V_{R,\cos}\) terms is essential for a physically consistent interpretation of stellar streaming motions induced by a spiral potential. The observed kinematics constrain the spiral pattern speed to \(\Omega_{p} \approx 10\)--\(20\,\mathrm{km\,s}^{-1}\mathrm{kpc}^{-1}\).

Intensity ratios of aromatic emission features are widely used to diagnose the size and ionization state of polycyclic aromatic hydrocarbons (PAHs) in astronomical environments. However, PAHs are known to typically carry aliphatic side chains, a structural feature that may compromise the reliability of traditional diagnostic methods. This study systematically investigates the effects of aliphatic components on the aromatic emission properties of PAHs. Based on theoretical data from the NASA Ames PAH IR Spectroscopic Database, we compare the emission behavior of purely aromatic PAHs with those containing aliphatic substituents, revealing that aliphatic functionalization may modify the intensity ratio of the 11.2 $\mu$m band relative to the 7.7 $\mu$m and 3.3 $\mu$m bands. This potentially leads to misidentification of their ionization state if molecular structural effects are neglected. Further analysis indicates that the impact of aliphatic components on diagnostic band ratios strongly depends on PAH size: small PAHs exhibit significant emission ratio shifts, deviating from traditional size/ionization trends, while larger PAHs are minimally affected. Despite these shifts, the classic $(I_{11.2/7.7})$ versus $(I_{11.2/3.3})$ diagnostic grid remains largely applicable to mixed aromatic-aliphatic PAHs, although some systematic calibration may be needed. Our findings emphasize the necessity for caution when interpreting PAH band ratios in aliphatic-rich environments, as variations in PAH molecular composition may distort inferences about physical conditions.

We simulate star formation and star cluster assembly inside a molecular cloud with parameters we derive directly from observations of the Aquila Rift. We model the evolution of stars and gas together while resolving close encounters between stars, the formation of new stars, and stellar feedback to follow cluster formation up to the expulsion of the surrounding gas. We find that star formation takes place in clumps spaced unevenly along Serpens South and that these clumps accrete surrounding gas to grow and form new stars. Gas flows along the filament promote the merger of these clumps into a star cluster inside the Serpens South filament. The imprints of these mergers are seen in the dynamics of the Serpens South cluster in the form of velocity space anisotropies, cluster rotation, and cluster expansion. Before gas is removed from the simulation, the Serpens South cluster merges with the nearby cluster W40 non-monolithically resulting in a fractal cluster at the end of the simulation. The dynamics inherited from the mergers throughout the simulation are still seen in the final bound stellar system after the gas has been removed. We compare these results with recent observations of Milky Way clusters to comment on their formation histories. We also study how our results change when lowering the mass resolution of our simulation and removing observations of dense gas tracers from our initial condition setup. Each of the three simulations result in different final cluster configurations pointing towards the importance of gas in cluster assembly.

Recent JWST observations suggest that galaxies at z > 9 may be dominated by low-opacity SNe-produced dust before efficient ISM grain growth is established. This transition in dust origin and opacity could explain both the prevalence of galaxies with extremely low dust attenuation and the excess of UV-bright galaxies relative to most pre-JWST predictions. We investigate whether this transition, combined with evolving star-formation efficiency, can reproduce these observed properties. We develop a physically motivated attenuation framework combining (i) extinction laws for reverse-shock-processed SNe dust, (ii) metallicity- and dust-to-metal-dependent opacity scalings, and (iii) porous radiative-transfer geometries allowing partial UV-photon leakage. Unlike outflow-driven scenarios requiring large-scale gas evacuation, our approach preserves gas reservoirs while reducing effective UV opacity through dust composition and geometry. We introduce extinction-based, gas-based, and hybrid attenuation prescriptions linking SNe-dominated and ISM grain-growth dust regimes. We find that the observed A_FUV-M_star relation at z > 9 is best reproduced for an intrinsic FUV dust opacity kappa_UV(dust)=1000 cm2/g, characteristic of low-opacity SNe dust, naturally producing very low attenuation even in gas-rich galaxies. This regime reproduces galaxies with extremely low dust attenuation (GELDAs), which dominate observed samples at z > 9. Applied to intrinsic UV luminosity function models, our SNe-dominated and hybrid prescriptions mainly suppress the brightest galaxies, bringing predictions into agreement with JWST measurements without requiring extreme star-formation efficiencies or dust-free interstellar media. Our results suggest that the UV-bright galaxy excess at z > 9 reflects a transition in dust origin and opacity during the earliest phases of galaxy evolution.

We investigate the morphology and structural properties of 246 massive compact quiescent galaxies (MCGs; $\log M_{\star} \sim 10$-$11$, $\sigma_{\mathrm{e}} \sim 150$-$350\,$km\,s$^{-1}$, $R_{\mathrm{e}} \sim 0.7$-$2.5\,$kpc) at $z \sim 0$, selected as outliers in the stellar mass-velocity dispersion and velocity dispersion-size relations, using $g$-, $r$-, and $i$-band Hyper Suprime-Cam images. We compare them to a control sample of average-sized quiescent galaxies (CSGs) matched in stellar mass, star formation rate, redshift, and $g-i$ color. Both samples are dominated by S0 galaxies, comprising $93\%$ of MCGs and $71\%$ of CSGs, while ellipticals account for $4\%$ and $11\%$, respectively. The fraction of interacting or morphologically disturbed systems is low in both samples ($13\%$ for MCGs and $16\%$ for CSGs). Multi-component decompositions of the $g$- and $r$-band images show that $75\%$ of MCGs require a three-component model (bulge, disk, and envelope), while $21\%$ are best fit by two components and $4\%$ by a single S\'ersic profile. Two-component MCGs are preferentially low-inclination systems, suggesting that the three-component fraction represents a lower limit. In contrast, only $7\%$ of CSGs exhibit a comparable three-component structure. Bars are present in $29\%$ of CSGs but are absent in MCGs. For three-component systems, MCGs and CSGs have similar bulge ($R_\mathrm{e}=0.39$ vs.\ $0.45$\,kpc) and envelope ($R_\mathrm{e}=6.4$ vs.\ $5.8$\,kpc) sizes, while MCG disks are significantly more compact ($R_\mathrm{e}=1.9$ vs.\ $3.3$\,kpc). The envelope component shows a broad ellipticity distribution ($\epsilon_\mathrm{Envelope} \sim 0.0$-$0.6$), which we interpret as corresponding to either a stellar halo or a thick disk.

The identification of physically associated kiloparsec-scale quasar pairs is important for understanding galaxy evolution, the growth of supermassive black holes, and their co-evolution with host galaxies. However, their rarity and the high contamination from stellar superpositions and projected alignments require efficient pre-selection methods. We develop a machine-learning framework to produce photometric-redshift point estimates and redshift probability density functions for quasars, with the main goal of identifying high-probability quasar pair candidates in the MGQPC catalogue. We construct two large spectroscopically confirmed quasar samples with multi-wavelength photometry, based on SDSS and DESI Legacy Imaging Surveys data. CatBoost is used for point-estimate photometric-redshift regression, and FlexZBoost is used for full redshift-PDF estimation. The workflow achieves robust performance, with a normalised median absolute deviation of 0.036 and an outlier fraction of 5.6% on the test sample. Applying the trained model to the MGQPC catalogue, we identify 185 high-probability quasar pair candidates based on photometric-redshift consistency. Among them, 20 systems have been subsequently confirmed as genuine physical pairs by independent spectroscopic observations. The resulting MGQPC photometric-redshift catalogue provides a useful resource for future spectroscopic follow-up of quasar pairs and dual supermassive black holes.

Hypercompact HII regions (HC) are regions of ionized gas associated with the early stages of high-mass star formation. With the aim of better understanding their characteristics, we studied five candidate HC HII regions. Here, we present observations with the Jansky Very Large Array (VLA) at 2 and 6 cm, with angular resolutions in the range of $\sim$1 -- 3\arcsec and report the images of the detected sources and the measured parameters. In addition, we explore several possible scenarios, considering the regions as both uniform and non-uniform spheres, and as winds, both spherical and collimated. In most cases, the sources were unresolved, but by applying the models, we estimate that their sizes vary in a range of 0.3 to 3.7 mpc while their electron densities are in the range of $1.3 \times 10^{5}$ to $2.4 \times 10^{6}$ cm$^{-3}$, indicating that most sources are consistent with small, weak UC HII regions, although a few remain viable candidates for HC HII regions, with G40.28$-$0.22 as the strongest case. We do not rule out the possibility that some sources are jets or stellar winds.

We quantify systematic biases in rotation curves inferred from steady, axisymmetric Jeans modelling when the underlying stellar velocity field is non-axisymmetric. Using a high-resolution $N$-body/hydrodynamic simulation of an isolated Milky Way-like disk galaxy, we construct mock stellar-kinematic measurements for two observer azimuths relative to the bar. One observer is placed at a Solar-like viewing angle of $25^\circ$ from the bar major axis, and the other at $115^\circ$. For each configuration, we analyse multiple snapshots and compare the Jeans-inferred circular-speed curve, $V_{\rm c,Jeans}(R)$, with a reference axisymmetric circular-speed curve, $V_{\rm c,axi}(R)$, defined from the azimuthally averaged ($m=0$) component of the gravitational field. The Jeans analysis is performed in a wedge-shaped mock observational volume that mimics limited sky coverage. For the $25^\circ$ configuration, the mean azimuthal streaming is typically higher than the azimuthally averaged expectation by $\approx 10$--$15~\mathrm{km\,s^{-1}}$, which leads to an average overestimate of the axisymmetrically defined circular speed by $\approx 4\%$ ($\approx 10~\mathrm{km\,s^{-1}}$) in the inner disk. Across snapshots, the mean offset corresponds to a $\sim 1.5$--$2\sigma$ systematic deviation of $V_{\rm c,Jeans}$ from $V_{\rm c,axi}$. For the $115^\circ$ configuration, the bias reverses sign and $V_{\rm c,Jeans}$ tends to underestimate $V_{\rm c,axi}$. Under the usual spherical approximation, a $\approx 4\%$ bias in $V_{\rm c}$ corresponds to an $\approx 8\%$ bias in the enclosed dynamical mass at fixed radius. These results imply that steady, axisymmetric Jeans modelling of Milky Way stellar kinematics can overestimate the axisymmetrically defined circular-speed curve at the percent level unless non-axisymmetric streaming is modelled explicitly or included in the error budget.

Obtaining robust galaxy number counts is crucial for understanding galaxy evolution, and submillimetre counts in particular have proven valuable for revising subgrid physics models in cosmological simulations. In confusion-limited surveys, which are common at these wavelengths, statistical methods such as $P(D)$ fluctuation analysis are required to recover counts of faint, unresolved galaxies. However, the standard $P(D)$ framework assumes that galaxies are Poisson-distributed, whereas in reality galaxies are clustered. Using simulations, we demonstrate that this clustering systematically biases $P(D)$-derived number counts, and present an empirical method that simultaneously measures and corrects for this bias by combining the 1- and 2-point statistics in the map, thereby maximising the information extracted from the data. Applying this method to deep Herschel-SPIRE observations of the GOODS-N field, we provide revised galaxy number counts at 250, 350 and 500$\mu$m. Our results indicate that at 500$\mu$m clustering inflates the apparent counts by a factor of 1.6 around 10mJy and slightly suppresses the faintest sub-mJy counts, with milder effects at 350$\mu$m and 250$\mu$m owing to the smaller beam sizes. This methodology is broadly applicable to other confusion-limited data sets with well-characterised beam and noise properties, including SCUBA-2 and CCAT, enabling unbiased exploitation of the full statistical information in current and future far-infrared and submillimetre surveys.

We present new JWST/MIRI-MRS data of the z=4.601 extremely luminous obscured quasar WISEA J224607.56-052634.9 (W2246-0526). Our fits of its spectral energy distribution (SED) with the SED fitting code SMART (Spectral energy distributions Markov chain Analysis with Radiative Transfer models) predict an active galactic nucleus (AGN) fraction in the range 72-81 per cent, an intrinsic AGN luminosity of 4.2-7.2 x 10^14 Lo, a polar dust luminosity of 1.6-1.7 x 10^14 Lo, a black hole mass of 1.3-2.3 x 10^10 Mo (assuming the quasar is accreting at the Eddington limit), a star formation rate (SFR) of 360-2900 Mo/yr and a stellar mass of 4.8-5 x 10^11 Mo. The stellar and black hole masses of W2246-0526 are typical of a giant elliptical galaxy at z=0. We find statistically significant evidence for the presence of a hot dust component, which we interpret as polar dust in the context of a torus geometry, based on recent results obtained for nearby AGN. We explore two smooth and two two-phase models for the AGN torus, to put constraints on the AGN fraction of the galaxy, the black hole mass and its SFR. We show that the presence of polar dust affects the estimate of the AGN luminosity and we recommend to take into account this component in SED fits of other high-redshift obscured AGN/quasars. Despite the large difference in luminosity, we discuss possible links between the presence of this hot dust component in W2246-0526 and in some local AGN, suggesting that they may have a different origin.

The search for supermassive black hole binaries (SMBHBs) has, in recent years, seen the dawn of exploration with several hundred candidates claimed from photometric and spectroscopic surveys monitoring AGNs. While only a handful persist to date, the advent of upcoming high-precision wide-field photometric missions motivates continuing the pursuit of confirming SMBHBs in the optical. We explore the possibility of using the ESA Plato space mission to detect photometric signatures of SMBHBs. Motivated by the Kepler observation of Spikey, the best known self-lensing flare (SLF) candidate to date, this work aims to benchmark the scientific outcome if Plato were to observe Spikey-like objects via its Guest Observer programme. Starting from the Gaia database, we assemble a catalogue of 12,226 bright ($G < 19$) high-probability Quasars for the two pointing fields of Plato's nominal mission. This Plato Quasar catalogue will be pivotal for future follow-up observations of larger photometric searches such as the Vera Rubin LSST survey. We use the Plato camera simulator, PlatoSim, to realistically explore the noise budget in Plato's faint limit, while generating mock light curves to benchmark Plato's ability to recover signatures of SMBHBs. We show that, although not at all designed for the purpose, Plato is capable of detecting Spikey-like SMBHB candidates through their relativistic photometric signatures using Bayesian inference and evidence. Plato will in particular be able to confirm or rule out Spikey and Spikey-like objects with a limiting magnitude of $G\leq18$. With a minimum 2-yr baseline per pointing field, we show that Plato not only could play an essential role in future SMBHB research, but may be an integrated part of the observational fleet of continuous high-precision facilities monitoring SMBHB candidates in the near future.

Active galactic nuclei (AGNs) exhibit complex variability across multiple wavelengths, reflecting diverse physical processes near their central engines. This work investigates the temporal variability of four AGNs Mrk~509, NGC~5548, NGC~4151, and NGC~4593 using multifractal detrended moving average (MFDMA) analysis and Fisher-Shannon information plane applied to their X-ray, ultraviolet, and optical light curves. These methods quantify the scaling behavior and complexity of the variability, revealing persistent correlations and distinct variability patterns across energy bands. The Fisher-Shannon analysis further characterizes the degree of stochasticity and structural complexity in the emission processes. Our findings support the interpretation that multifractal and information-theoretic measures provide effective diagnostics of the physical mechanisms driving AGN variability. This study demonstrates the utility of advanced time series techniques as effective diagnostics of AGN variability mechanisms.

Accurate estimates of the absorption of X-rays by interstellar gas and dust are of crucial importance for the analysis and interpretation of almost all astronomical soft X-ray observations. However, the present X-ray absorption data extensively used by the community were derived from a reduced interstellar abundance (~70% of solar) and ignoring dust scattering. Therefore, these X-ray absorption data, although highly popular, could have been substantially underestimated. Here we update the interstellar X-ray absorption and scattering by making use of updated atomic cross sections, updated interstellar abundances, and realistic X-ray dust physics, and appropriately distributing metal elements in gas and dust. The resulting X-ray absorption and scattering data are publicly available on GitHub.

We present James Clerk Maxwell Telescope (JCMT) observations of the $^{12}$CO(J = 2-1) emission of 42 low-mass, star-forming disk galaxies of morphological type Scd or later from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. The sample, which probes the M33-like stellar-mass regime, is complemented with metallicities, star formation rates, and \hi\ masses used to investigate the star formation process and to test scaling relations involving molecular gas mass in low-mass systems. We detect CO emission in 55% of the sample and derive H$_2$ masses using both a constant Galactic and a metallicity-dependent CO-to-H$_2$ conversion factor. The 12 $\mu$m luminosity, which includes polycyclic aromatic hydrocarbon features, exhibits a tight linear correlation with the CO line emission, making it a robust tracer of global molecular gas content. The molecular gas mass - star formation rate relation, i.e. the Kennicutt-Schmidt law, is the most fundamental one and it is found to remain linear down to low stellar masses. We also find that the mean molecular gas depletion time is slightly shorter in low-mass late-type galaxies than in more massive systems, consistent with their higher specific star formation rates. Finally, while the specific molecular gas mass ($M_{\rm H_2}/M_*$) shows no significant dependence on stellar mass and a large intrinsic scatter, the HI-to-stellar mass ratio ($M_{\rm HI}/M_*$) decreases with increasing stellar mass and molecular fraction ($M_{\rm H_2}/M_{\rm gas}$), highlighting the progressive transition from atomic- to molecular-dominated interstellar medium along the galaxy population.

Filamentary structures are ubiquitous in the interstellar medium, yet the extent to which magnetic fields influence the morphology of cold atomic gas remains an open question. The nearby Riegel-Crutcher cloud, composed of long and narrow H I filaments observed in self-absorption, provides a critical test case. We present the most extensive optical polarimetric survey of this region to date, comprising more than 90,000 high signal-to-noise stellar polarization measurements combined with Gaia DR3 data. Using stellar polarization, extinction estimates, and archival Na I absorption data, we locate the cloud at a distance of $150 \pm 15$ pc, consistent with that of the Pipe Nebula. The plane-of-sky magnetic field traced by optical starlight polarization closely matches that inferred independently from Planck 353 GHz dust-emission polarization, revealing a coherent large-scale magnetic field across the region. A Rolling Hough Transform analysis shows that the H I filaments are tightly aligned with this field orientation. Together, these results provide strong observational evidence that the structure of the cold neutral medium in the Riegel-Crutcher cloud is closely linked to a highly ordered magnetic field. This level of coherence supports a scenario in which magnetic fields play a dynamically important role in shaping the cloud structure, and suggests that the Riegel-Crutcher cloud is part of a larger magnetized complex influencing gas flows in the solar neighborhood.

Linear C4H and cyclic c-C3H2, as small unsaturated hydrocarbons, are the key precursors to complex organic molecules and are critical components of the interstellar medium. We present on-the-fly mapping observations of C4H 9-8 lines, c-C3H2 2-1, H13CO+ 1-0, and H42 toward a sample of 22 massive star-forming regions using the IRAM 30m telescope. Our aim is to further explore the evolution of these carbon-chain molecules by combining observational results obtained in cold cores. We employed H13CO+ 1-0 and H42 as tracers to probe the positions of molecular cloud cores and ionised hydrogen regions (HII regions), respectively. One chemical model in particular, which includes gas, dust grain surface, and icy mantle phases for C4H and c-C3H2 molecules, was used to make comparisons with observed abundances. From mapping observations targeting 31 regions across 22 sources, C4H 9-8 (J = 19/2-17/2) and C4H 9-8 (J = 17/2-15/2) were detected in only 17 regions, while H13CO+ 1-0 and c-C3H2 2-1 were successfully detected in all 31 regions. We find that the emission of C4H 9-8 and c-C3H2 2-1 is concentrated at the edges of H42 emission regions. The C4H/H13CO+ and c-C3H2/H13CO+ relative abundance ratios range from 0.17 to 1.77 and 1.42 to 6.69, respectively, with a median C4H/c-C3H2 ratio of 0.13. By combining the observational results of cold cores, we find that C4H/H13CO+ and c-C3H2/H13CO+ ratios show a strong decreasing trend as molecular cores evolve. The decreasing trends in C4H/H13CO+ and c-C3H2/H13CO+ ratios imply that small unsaturated hydrocarbons can be consumed and converted into other organic molecules during the evolution of molecular cores. The spatial concentration of C4H and c-C3H2 emission at the edges of H42 regions further supports their role as precursors in the chemical pathways that lead to complex organic molecules in the interstellar medium.

We present an analysis of specific star formation rates (sSFR) across the galaxy merger sequence using visual classifications from the Zooniverse citizen science project "Cosmic Disco: Characterizing Galaxy Collisions". Our sample comprises 4884 galaxy systems pre-selected as merger candidates from SDSS DR17 ($0.01 < z < 0.05$, $M_* > 10^{8.5}M_\odot$) using Zoobot, of which 3690 were classified as mergers spanning pre-interaction through post-coalescence stages by citizen scientist volunteers. We find a weak but statistically significant positive correlation between $\log(\mathrm{sSFR})$ and visual merger stage ($r = 0.161$, $p = 7.23 \times 10^{-23}$), with a best-fit relation $\log\left(sSFR\right)=(0.148\pm0.015)\, S_{\rm Merg}-(1.865\pm0.038)$. The large RMS scatter (0.661 dex) reflects visual merger stages capturing wide merger timescales, and our results corroborate previous findings of increasing SFR enhancement with merger progression. This work shows that citizen science is a viable complement to automated and pair-based approaches to evaluate timescales for galaxies across the merger sequence.

The identification and tracking of stellar feedback-driven galaxy bubbles is an important topic in star formation and galactic structure research. However, current observational analysis of bubbles is limited in scope; information on bubble lifetime is inaccessible. Simulation data thus provides a unique opportunity to glean some of these characteristics at high resolution. We present an investigation into the characteristics and evolution of hot, ionized bubbles in the interstellar medium of a dwarf spiral (NGC300-like) galaxy. We calculate the average radius, lifetime, temperature, density, and spatial distribution of the simulated feedback-driven bubbles using Lagrangian gas parcels, and we examine the relationship between these characteristics and the local galactic environment. We find exponential distributions of bubble lifetime and size, and we find a positive correlation between bubble lifetime and galactocentric radius. Finally, we predict how the data would appear in H$\alpha$ tracers and compare the simulated values to observations. We find an additional positive correlation between the size of the bubbles and the galactocentric radius using their H$\alpha$ tracers.

Our knowledge of the space velocity of Messier 87, which is the dominant galaxy in the Virgo cluster, has been limited to the radial velocity component. Using a cadence of precision position measurements with the global geodetic very long baseline interferometry (VLBI) system over 28 years, we determined the proper motion vector of the radio-emitting core by a robust statistical method involving 1-norm optimization and bootstrapping. The proper motion vector is directed at a position angle $189.2\degr \pm 3.5\degr$ in the equatorial International Celestial Reference Frame, and its magnitude is $10.19$ $\mu$as yr$^{-1}$ with an uncertainty of $0.64$ $\mu$as yr$^{-1}$. The projected velocity of the AGN in the tangential sky plane is ($787\pm50$)~km~s$^{-1}$. The peculiar velocity of Messier 87 with respect to the preferred rest frame of the cosmic microwave background field is approximately 1037 km s$^{-1}$ (assuming a distance of 16.1 Mpc) with an angle of 65$^\circ$ to the current line of sight, which implies a tangential relative motion of M87 and the Galaxy. The peculiar velocity of M87 is directionally concordant with the reconstructed and $\Lambda$CDM-simulated motion of the Virgo filament towards the Great Attractor, but the Milky Way moves slower by 470 ~km~s$^{-1}$ in that direction.

We identify compact groups of galaxies (CGs) in the IllustrisTNG-300 simulation using a Friends-of-Friends (FoF) algorithm. Our approach is designed to be comparable to systematic CG searches based on spectroscopic surveys, while avoiding the conventional Hickson selection criteria, which can bias samples toward relatively low-density environments. We construct two CG catalogs: one based on a three-dimensional distance linking length of 73 kpc (i.e., $50~h^{-1}$ kpc), and another based on projected and radial linking lengths of 73 kpc and $1000~\rm km~s^{-1}$. We refer to these as the position-position-position (PPP) and position-position-velocity (PPV) CG catalogs, respectively. The PPV catalog provides a direct analog to observed CG samples. At $z = 0$ in TNG300, we identify 383 PPP CGs and 1666 PPV CGs. A large fraction ($\sim 80\%$) of PPV CGs are not physically compact systems but are contaminated by line-of-sight interlopers. We demonstrate that the scaling relation between total group stellar mass and velocity dispersion is an effective diagnostic for identifying false positives with line-of-sight interlopers. We further examine the large-scale environments of CGs and show that they reside in a wide range of densities, including the central regions of galaxy clusters. These CG catalogs provide a robust foundation for studying the formation and evolution of CGs in cosmological simulations.

Using Extreme Value Statistics applied to the observed galaxy stellar mass and the UV luminosity functions, we empirically estimate masses and luminosities of the most extreme galaxies in cosmological surveys, including the full sky. We incorporate uncertainties in stellar mass measurements (Eddington bias) and the scatter in the stellar-halo mass relation to derive empirical limits for galaxies residing in the most massive halos. The maximum observed $M_\ast$ strongly depends on survey area and redshift, ranging from $M_\ast \sim 7 \times 10^{12} M_\odot$ for full-sky surveys at $z\sim0$ to $M_\ast \sim 10^{10}M_\odot$ at $z\sim16$. Massive galaxies, particularly at high redshift, approach the theoretical maximum baryonic mass available in halos $M_\ast \sim 0.16 \times M_{\mathrm{vir}}$, consistent with previous claims. Accounting for measurement uncertainties significantly reduces the inferred maximum $M_\ast$ by up to $\sim1$ dex at $z\gtrsim10$, yielding stellar masses consistent with $M_\ast < 0.16$ at all redshifts. Assuming a perfect rank-order correspondence between the most massive halos and galaxies would guarantee this inequality at all redshifts. At 2 $\lesssim$ z $\lesssim$ 6, the most massive galaxies have stellar masses comparable to the total cold gas reservoir from cold and cooling flows, suggesting near-maximal star formation efficiencies, SFEs. At higher redshifts, halos are predicted to host galaxies undergoing starburst phases. When accounting for dust attenuation and adopting empirically inferred SFEs, we find good agreement between the model and the brightest observed UV galaxies at high redshifts. At lower redshifts, however, observed UV galaxies are too bright. Overall, our results indicate that current observations remain broadly consistent with $\Lambda$CDM once statistical and observational effects are properly accounted for.

Using Extreme Value Statistics applied to the observed galaxy stellar mass and the UV luminosity functions, we empirically estimate masses and luminosities of the most extreme galaxies in cosmological surveys, including the full sky. We incorporate uncertainties in stellar mass measurements (Eddington bias) and the scatter in the stellar-halo mass relation to derive empirical limits for galaxies residing in the most massive halos. The maximum observed $M_\ast$ strongly depends on survey area and redshift, ranging from $M_\ast \sim 7 \times 10^{12} M_\odot$ for full-sky surveys at $z\sim0$ to $M_\ast \sim 10^{10}M_\odot$ at $z\sim16$. Massive galaxies, particularly at high redshift, approach the theoretical maximum baryonic mass available in halos $M_\ast \sim 0.16 \times M_{\mathrm{vir}}$, consistent with previous claims. Accounting for measurement uncertainties significantly reduces the inferred maximum $M_\ast$ by up to $\sim1$ dex at $z\gtrsim10$, yielding stellar masses consistent with $M_\ast < 0.16$ at all redshifts. Assuming a perfect rank-order correspondence between the most massive halos and galaxies would guarantee this inequality at all redshifts. At 2 $\lesssim$ z $\lesssim$ 6, the most massive galaxies have stellar masses comparable to the total cold gas reservoir from cold and cooling flows, suggesting near-maximal star formation efficiencies, SFEs. At higher redshifts, halos are predicted to host galaxies undergoing starburst phases. When accounting for dust attenuation and adopting empirically inferred SFEs, we find good agreement between the model and the brightest observed UV galaxies at high redshifts. At lower redshifts, however, observed UV galaxies are too bright. Overall, our results indicate that current observations remain broadly consistent with $\Lambda$CDM once statistical and observational effects are properly accounted for.