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arxiv: 2511.07532 · v2 · pith:M4MRBBG5new · submitted 2025-11-10 · 🌌 astro-ph.GA

Changing-Look Active Galactic Nuclei in SDSS-V: Host-Galaxy Properties and Black-Hole Scaling Relations

Grisha Zeltyn , Benny Trakhtenbrot , Michael Eracleous , Scott F. Anderson , Claudio Ricci , Andrea Merloni , Jessie Runnoe , Mirko Krumpe
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James Aird Roberto J. Assef Catarina Aydar Franz E. Bauer W.N. Brandt Joel R. Brownstein Johannes Buchner Kaushik Chatterjee Laura Duffy Lorena Hern\'andez-Garc\'ia H\'ector Hern\'andez-Toledo Anton M. Koekemoer Sean Morrison Castalia Alenka Negrete Pe\~naloza Mara Salvato Donald P. Schneider Yue Shen Marzena \'Sniegowska
This is my paper

Pith reviewed 2026-05-17 23:18 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords changing-look AGNhost galaxy propertiesblack hole scaling relationsactive galactic nucleistellar populationsSDSS-Vvariability
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The pith

CL-AGNs live in typical AGN host galaxies and follow standard black-hole scaling relations.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper analyzes intermediate-resolution spectra of 23 changing-look AGNs from SDSS-V to compare their host galaxies and black-hole properties against ordinary type 2 AGNs. It shows that stellar masses, ages, young-star fractions, and star-formation rates are statistically similar, while the black holes obey the usual M_BH-sigma_* and M_BH-M_* relations with a median mass ratio of 0.38 percent. The MgII line profiles indicate that variable obscuration cannot explain most of the dramatic spectral changes. A reader would care because this evidence implies the extreme variability is an intrinsic feature of normal AGN accretion rather than a special host-galaxy condition, allowing the same objects to be studied in both AGN-dominated and host-dominated states.

Core claim

The CL-AGNs roughly follow the M_BH-sigma_* and M_BH-M_* relations of inactive galaxies, with a median black hole-to-stellar mass ratio of 0.38%. Stellar population properties including mass, age, young stellar fraction, and star-formation rate show no difference from those of type 2 AGNs in SDSS. MgII emission-line analysis rules out variable obscuration for the majority of the sample. These results indicate that CL-AGNs reside in typical AGN host galaxies and that their variability is unrelated to host environment, so they represent a phase of normal AGN activity rather than a distinct population.

What carries the argument

Direct comparison of stellar population parameters and black-hole scaling relations against a matched SDSS type 2 AGN control sample, supported by MgII line diagnostics to exclude variable obscuration.

If this is right

  • CL-AGNs can serve as useful probes of the AGN-host connection because the same systems supply both AGN-dominated and host-dominated spectra.
  • The extreme variability of CL-AGNs is likely unrelated to host-galaxy environment.
  • CL-AGNs represent a phase of normal AGN activity rather than a distinct population.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the hosts are ordinary, the variability is more likely driven by changes in accretion rate or disk structure than by external triggers.
  • CL-AGNs could be used to test whether black-hole growth tracks stellar mass assembly on the same timescales in active and changing phases.
  • Larger samples might reveal whether transition timescales correlate with black-hole mass in the same way as in steady-state AGNs.

Load-bearing premise

The control sample of type 2 AGNs from SDSS is assumed to be well-matched in selection and measurement methods to the CL-AGN targets.

What would settle it

A statistically significant difference in average stellar age or star-formation rate between a larger set of CL-AGNs and a re-matched type 2 control sample would falsify the claim that the hosts are typical.

Figures

Figures reproduced from arXiv: 2511.07532 by Andrea Merloni, Anton M. Koekemoer, Benny Trakhtenbrot, Castalia Alenka Negrete Pe\~naloza, Catarina Aydar, Claudio Ricci, Donald P. Schneider, Franz E. Bauer, Grisha Zeltyn, H\'ector Hern\'andez-Toledo, James Aird, Jessie Runnoe, Joel R. Brownstein, Johannes Buchner, Kaushik Chatterjee, Laura Duffy, Lorena Hern\'andez-Garc\'ia, Mara Salvato, Marzena \'Sniegowska, Michael Eracleous, Mirko Krumpe, Roberto J. Assef, Scott F. Anderson, Sean Morrison, W.N. Brandt, Yue Shen.

Figure 1
Figure 1. Figure 1: Left: The distribution of the Z24 sample of CL-AGNs on the luminosity-redshift plane, along with the subsample observed with VLT/X-shooter and Gemini-N/GMOS studied here (red circles). Middle:: The MBH-normalized density distri￾butions of SDSS DR16 z < 0.6 quasars (Wu & Shen 2022, gray region) and of the CL-AGNs analyzed in this work (red). The shape of the logarithmic BH mass function (BHMF) of broad-line… view at source ↗
Figure 2
Figure 2. Figure 2: Example of the pPXF fitting and σ∗ measurement for J0206−0414. The X-shooter data are shown in black, the best-fitting model in red, and the residuals are displayed near the bottom, where green points were considered for the fit while blue ones were masked (masked spectral regions are indicated in gray and listed in [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Broad Mg ii to broad Hα line flux ratios for our CL-AGNs, compared with SDSS/DR16 quasars. The F(Mg ii)/F(Hα) distribution of DR16 quasars, taken from Wu & Shen (2022), is shown in light blue and the 90th per￾centile is marked with a vertical line. The median and range of the observed “dim-state” flux ratios of our 13 sources with detectable Mg ii are shown in yellow, with arbitrary verti￾cal scaling. Also… view at source ↗
Figure 4
Figure 4. Figure 4: The MBH–σ∗ relation of our CL-AGNs (red sym￾bols). Comparison samples include elliptical galaxies and classical bulges (Kormendy & Ho 2013, blue), BASS type 1.9 AGNs (Koss et al. 2022; Mejía-Restrepo et al. 2022, gray), low-MBH AGN from Xiao et al. (2011), AGNs from Bennert et al. (2021) and Winkel et al. (2025, orange), and ‘turn￾on’ CL-AGNs from Yang et al. (2025, green). The relation from KH13 (dashed l… view at source ↗
Figure 5
Figure 5. Figure 5: The virial scaling factor f as a function of σ∗ for our CL-AGN sample, assuming CL-AGNs follow the KH13 MBH–σ∗ relation for inactive galaxies. Top: f values derived using the broad-line FWHM as the BLR velocity proxy. Bot￾tom: f values using the line dispersion (σ) instead. Horizon￾tal dashed lines mark the values from Woo et al. (2015); solid lines and shaded regions show our inferred f and the accom￾pany… view at source ↗
Figure 7
Figure 7. Figure 7: CL-AGNs (red symbols) along with SF galaxies (left panel) and type 2 AGN (right panel; gray regions) in the M∗–z plane. M∗ values for all samples were derived using FIREFLY: for CL-AGNs from their SDSS-V spectra, and for the comparison samples from the DR14 SDSS FIREFLY catalog (Comparat et al. 2017). SF galaxy and type 2 AGN classifications were obtained from the SDSS DR7 (Abazajian et al. 2009) MPA/JHU c… view at source ↗
Figure 8
Figure 8. Figure 8: CL-AGNs (red symbols) along with SF galaxies (left panel) and type 2 AGN (right panel; gray regions) in the M∗–t∗,M plane. M∗ and mass-weighted stellar ages for all samples were derived using FIREFLY: for CL-AGNs from their SDSS-V spectra, and for the comparison samples from the DR14 SDSS FIREFLY catalog (Comparat et al. 2017). SF galaxy and type 2 AGN classifications were obtained from the SDSS DR7 (Abaza… view at source ↗
Figure 9
Figure 9. Figure 9: CL-AGNs (red circles; inverted triangles indicate upper limits) compared with SF galaxies (left panel) and type 2 AGNs (right panel; gray regions) in the SFR-M∗ plane. For both panels, the solid blue line marks the moving median of the SF population, and the dashed blue line 1 dex below, roughly marks the boundary of the quenched-galaxy regime. SFRs for all samples were derived using the [O ii]-based estim… view at source ↗
Figure 10
Figure 10. Figure 10: CL-AGNs (red symbols) along with SF galaxies (left panel) and type 2 AGN (right panel; gray regions) in the f1Gyr–age plane. f1Gyr and mass-weighted stellar ages for all samples were derived using FIREFLY: for CL-AGNs from their SDSS-V spectra, and for the comparison samples from the DR14 SDSS FIREFLY catalog (Comparat et al. 2017). SF galaxy and type 2 AGN classifications were obtained from the SDSS DR7 … view at source ↗
Figure 11
Figure 11. Figure 11: Median offsets between the synthetic and cmodel r-band magnitudes (∆r) for our CL-AGN sample, shown as a function of redshift. The points mark the median offsets for each source, and the error bars indicate the 16th –84th percentile range of the comparison sample. The upper axis shows the physical scale of a 2′′ SDSS fiber corresponding to each redshift. aperture corrections using the same method. The MPA… view at source ↗
Figure 12
Figure 12. Figure 12: Spectra of our CL-AGN sample (continued on next page). Each panel displays the legacy-SDSS bright-state spectrum (blue), SDSS-V dim-state spectrum (red), and X-shooter/GMOS spectrum (black). All spectra were smoothed over 20 Å [PITH_FULL_IMAGE:figures/full_fig_p029_12.png] view at source ↗
Figure 12
Figure 12. Figure 12: Spectra of our CL-AGN sample (continued). Each panel displays the legacy-SDSS bright-state spectrum (blue), SDSS-V dim-state spectrum (red), and X-shooter/GMOS spectrum (black). All spectra were smoothed over 20 Å [PITH_FULL_IMAGE:figures/full_fig_p030_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: pPXF fitting and σ∗ measurement for our CL-AGN sample (continued on next page). The X-shooter/GMOS data are shown in black, the best-fitting pPXF model in red, and the residuals are shown near the bottom, where green points were considered for the fit while blue ones were masked out (masked spectral regions are indicated in gray and listed in [PITH_FULL_IMAGE:figures/full_fig_p031_13.png] view at source ↗
Figure 13
Figure 13. Figure 13: pPXF fitting and σ∗ measurement for our CL-AGN sample (continued). The X-shooter/GMOS data are shown in black, the best-fitting pPXF model in red, and the residuals are shown near the bottom, where green points were considered for the fit while blue ones were masked out (masked spectral regions are indicated in gray and listed in [PITH_FULL_IMAGE:figures/full_fig_p032_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: FIREFLY host-galaxy spectral fits for the CL-AGN sample (continued on next page). Each panel shows one object: the SDSS-V dim-state spectrum (red; smoothed over 5 pixels), the corresponding FIREFLY best-fitting host-galaxy model (blue), and the residuals (gray; also smoothed over 5 pixels). The prominent features visible in the residuals correspond to AGN emission lines that were masked during the FIREFLY… view at source ↗
Figure 14
Figure 14. Figure 14: FIREFLY host-galaxy spectral fits for the CL-AGN sample (continued). Each panel shows one object: the SDSS-V dim-state spectrum (red; smoothed over 5 pixels), the corresponding FIREFLY best-fitting host-galaxy model (blue), and the residuals (gray; also smoothed over 5 pixels). The prominent features visible in the residuals correspond to AGN emission lines that were masked during the FIREFLY fitting [PI… view at source ↗
read the original abstract

Changing-look active galactic nuclei (CL-AGNs) exhibit dramatic spectral variability on unexpectedly short timescales, challenging standard accretion flow models. Despite growing samples, the physical drivers of this extreme variability, and the potential link to host-galaxy properties, remain unknown. Regardless of the underlying mechanism, the transition between AGN-dominated and host-dominated spectra offers a unique opportunity to study relations between AGNs and their hosts within the same objects. We present intermediate-resolution spectroscopy of 23 CL-AGNs identified by the Sloan Digital Sky Survey V (SDSS-V), obtained with the Very Large Telescope/X-shooter and Gemini-N/GMOS. An analysis of the Mgii emission line observed in the spectra demonstrates that the majority of these sources cannot be driven by variable obscuration. Our CL-AGNs roughly follow the M_BH-sigma_* and M_BH-M_* relations of inactive galaxies, with a median black hole-to-stellar mass ratio of 0.38%. We find no evidence that the stellar population properties of our CL-AGNs, including stellar mass, age, young stellar fraction, and star-formation rate, differ from those of type 2 AGNs in SDSS. These results suggest that CL-AGNs reside in typical AGN host galaxies and that their extreme variability is likely unrelated to host-galaxy environment, supporting the idea that CL-AGNs are not a distinct population, but rather represent a phase of normal AGN activity. This result, in turn, implies that CL-AGNs can serve as useful probes of the AGN-host connection, providing access to both AGN-dominated and host-dominated spectra of the same systems.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript presents intermediate-resolution spectroscopy of 23 CL-AGNs identified in SDSS-V, obtained with VLT/X-shooter and Gemini-N/GMOS. Mg II line analysis is used to argue that variable obscuration cannot explain the variability for the majority of the sample. The CL-AGNs are shown to follow the M_BH-σ_* and M_BH-M_* relations of inactive galaxies (median black-hole-to-stellar-mass ratio 0.38%). Stellar population properties (stellar mass, age, young stellar fraction, SFR) show no significant differences from a control sample of SDSS type 2 AGNs, leading to the conclusion that CL-AGNs reside in typical AGN host galaxies, that their variability is unrelated to host environment, and that they represent a normal phase of AGN activity rather than a distinct population.

Significance. If the central results hold after addressing sample matching, the work adds new spectroscopic data on a growing class of variable AGNs and supports the interpretation that CL-AGNs are useful probes of the AGN-host connection because the same objects can be observed in both AGN-dominated and host-dominated states. The quantitative comparison to standard scaling relations and the null result versus type 2 hosts are potentially valuable for accretion-disk models.

major comments (2)
  1. [§4] §4 (host-galaxy properties comparison): The claim of no differences in stellar mass, age, young fraction, and SFR relative to the SDSS type 2 AGN control sample is load-bearing for the conclusion that CL-AGNs are not a distinct population. However, the manuscript provides no explicit matching criteria or distributions for redshift, AGN luminosity (e.g., [O III] or bolometric), or host mass range. Without these, selection or measurement systematics could produce an artifactual null result, as raised by the control-sample concern.
  2. [§3.1] §3.1 (Mg II analysis): The statement that Mg II emission demonstrates variable obscuration is ruled out for the majority lacks reported details on line-fitting procedures, velocity-width measurements, error bars, and quantitative thresholds used to reach this conclusion. This step is central to excluding an extrinsic driver and therefore to the intrinsic-variability interpretation.
minor comments (2)
  1. [Figures 5-7] Ensure that all scaling-relation figures include the full sample size, individual error bars, and the exact functional form of the reference relations from the literature.
  2. [§3.2] Clarify the exact definition and measurement method for the young stellar fraction and SFR in the stellar-population fitting section.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed report. We address each major comment below and have revised the manuscript to incorporate additional details and clarifications as suggested.

read point-by-point responses

  1. Referee: [§4] §4 (host-galaxy properties comparison): The claim of no differences in stellar mass, age, young fraction, and SFR relative to the SDSS type 2 AGN control sample is load-bearing for the conclusion that CL-AGNs are not a distinct population. However, the manuscript provides no explicit matching criteria or distributions for redshift, AGN luminosity (e.g., [O III] or bolometric), or host mass range. Without these, selection or measurement systematics could produce an artifactual null result, as raised by the control-sample concern.

    Authors: We agree that explicit documentation of the control-sample matching is necessary to support the null result robustly. In the revised manuscript we will add a dedicated paragraph in §4 describing the matching criteria applied to redshift, [O III] luminosity (as an AGN-power proxy), and stellar-mass range. We will also include a new figure (or table) showing the distributions of these quantities for the CL-AGN sample and the control sample, allowing readers to assess the quality of the match and any residual systematics. revision: yes

  2. Referee: [§3.1] §3.1 (Mg II analysis): The statement that Mg II emission demonstrates variable obscuration is ruled out for the majority lacks reported details on line-fitting procedures, velocity-width measurements, error bars, and quantitative thresholds used to reach this conclusion. This step is central to excluding an extrinsic driver and therefore to the intrinsic-variability interpretation.

    Authors: We acknowledge the need for greater methodological transparency. In the revised §3.1 we will expand the description of the Mg II fitting procedure to include the functional form and number of Gaussian components employed, the precise definition and measurement of velocity width (FWHM), the method used to derive uncertainties (e.g., Monte-Carlo resampling or covariance-matrix errors), and the quantitative thresholds (detection significance, minimum line width, or flux-ratio criteria) applied to conclude that variable obscuration is ruled out for the majority of the sample. These additions will make the intrinsic-variability argument fully reproducible. revision: yes

Circularity Check

0 steps flagged

No significant circularity; claims rest on new observations vs external catalogs

full rationale

The paper reports new intermediate-resolution spectroscopy for 23 SDSS-V CL-AGNs, uses Mg II line analysis to argue against variable obscuration as the driver, and directly compares measured stellar mass, age, young fraction, SFR, and black-hole scaling relations against SDSS type 2 AGN catalogs and standard inactive-galaxy relations. These comparisons use external benchmarks and standard relations rather than quantities defined or fitted by the authors themselves. No load-bearing step reduces by construction to a self-citation, self-defined parameter, or fitted input renamed as a prediction. The central claim that CL-AGNs represent a normal AGN phase follows from the observed lack of differences, which is independent of the paper's own prior fits.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

Based on abstract only; the work relies on standard domain assumptions for black hole mass estimation from emission lines and stellar population synthesis models, with no free parameters or invented entities explicitly introduced in the summary.

axioms (2)
  • domain assumption Black hole mass can be estimated from the width and luminosity of the MgII emission line using established virial calibrations
    Invoked to derive M_BH values for the scaling relations.
  • domain assumption Stellar velocity dispersion and stellar mass measurements from spectra are comparable across CL-AGN and type 2 AGN samples
    Required for the claim that CL-AGNs follow the same relations as inactive galaxies.

pith-pipeline@v0.9.0 · 5747 in / 1546 out tokens · 45707 ms · 2026-05-17T23:18:40.497934+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Identifying Changing-Look AGN Transitions in Light Curve Data with the Zwicky Transient Facility

    astro-ph.GA 2026-04 unverdicted novelty 6.0

    A criterion of |Δg| > 0.4 mag and |Δ(g-r)| > 0.2 mag detects photometric CL-AGN transitions in 9.6% of known hosts with 1.6% false positive rate from simulations.

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Works this paper leans on

3 extracted references · 3 canonical work pages · cited by 1 Pith paper

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    work page doi:10.5281/zenodo.831784 2019