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arxiv: 2605.21574 · v1 · pith:DNY25S4Tnew · submitted 2026-05-20 · 🌌 astro-ph.GA

(LRDs)²: The Low-ReDshift Little Red Dots Survey. II. DESI DR1 Sample

Xiaojing Lin , Xiaohui Fan , Zheng Cai , Yichen Liu , Fengwu Sun , Fuyan Bian , Mingyu Li , Junjie Mao
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Jenny E. Greene Hanpu Liu Jiaxuan Li Weizhe Liu Yilun Ma Zechang Sun Zijian Zhang
This is my paper

Pith reviewed 2026-05-22 09:42 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords little red dotslow redshift galaxiesDESI surveyactive galactic nucleiemission line galaxiesblack hole growthgalaxy evolution
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The pith

Low-redshift Little Red Dots match the key spectral and morphological traits of their high-redshift counterparts.

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

The paper reports a sample of 27 Little Red Dots at redshifts 0.2 to 0.9 drawn from DESI DR1 photometry. Near-infrared spectroscopy of 18 objects shows they possess the same compact shapes, V-shaped ultraviolet-to-optical continua, extreme Balmer line ratios around 16, frequent Balmer absorption, blackbody-like near-infrared emission, low metallicities, and [O III] outflows seen in the distant JWST population. These shared traits suggest the underlying physical processes remain active from early cosmic times to the present. The work also notes that local black-hole mass scaling relations do not apply directly to these objects.

Core claim

We identify 27 low-redshift LRDs with a number density lower limit of 7.5 times 10 to the minus 10 per cubic comoving megaparsec. Follow-up spectroscopy reveals that they exhibit compact morphology, V-shaped UV-optical continua, broad Balmer lines with median H-alpha over H-beta of about 16, Balmer absorption in 67 percent of cases, blackbody-like optical-to-NIR continua with temperatures 2000 to 4700 K, low metallicity, placement in the same BPT regions as high-z LRDs, softer ionizing spectra than typical AGNs, and ubiquitous [O III] outflows at 78 percent. One object shows long-term variability. These matches indicate the same physical processes operate at both low and high redshift.

What carries the argument

Photometric selection from DESI DR1 data followed by near-IR spectroscopic characterization to compare emission-line and continuum properties against high-redshift LRDs.

If this is right

  • The deviation in broad-line Balmer luminosity versus 5100 angstrom luminosity limits direct use of local type-1 AGN black-hole mass calibrations.
  • Ionized [O III] outflows appear in 78 percent of the low-z sample.
  • Optical-to-NIR continuum temperatures span 2000 to 4700 K with some cooler and larger envelopes than seen at high redshift.
  • Low-z LRDs can serve as accessible laboratories for studying the same phenomena observed at cosmic dawn.

Where Pith is reading between the lines

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

  • If the population persists to low redshift, models of AGN or dust-obscured growth must accommodate these objects across a wide range of cosmic epochs rather than treating them as an early-universe-only phase.
  • The presence of variability in at least one low-z LRD offers a direct test for accretion-driven activity that is harder to obtain at high redshift.
  • Extending the same selection to larger low-z surveys could map the number density evolution and clarify whether LRDs represent a brief transitional stage in galaxy assembly.

Load-bearing premise

The photometric cuts applied to DESI DR1 successfully pick out genuine LRD analogs rather than other red compact sources and capture a representative fraction of the true population.

What would settle it

Deep imaging or additional spectroscopy that reveals a high fraction of the selected objects to be ordinary compact galaxies or that uncovers many more similar objects outside the color cuts would test whether the sample is pure and complete enough for the reported density and property comparisons.

Figures

Figures reproduced from arXiv: 2605.21574 by Fengwu Sun, Fuyan Bian, Hanpu Liu, Jenny E. Greene, Jiaxuan Li, Junjie Mao, Mingyu Li, Weizhe Liu, Xiaohui Fan, Xiaojing Lin, Yichen Liu, Yilun Ma, Zechang Sun, Zheng Cai, Zijian Zhang.

Figure 1
Figure 1. Figure 1: Flowchart of the LRD selection process in DESI DR1. Blue boxes show input datasets, and orange and green boxes indicate each selection criterion (Section 3). Objects must satisfy each criterion (“Y”) to proceed to the next step. required. LRDs with extremely strong Balmer breaks are more efficiently identified through dedicated Balmer￾break–based selections, which are beyond the scope of this paper and wil… view at source ↗
Figure 2
Figure 2. Figure 2: Example DESI DR1 LRDs presented in this work. The top panel shows the overall SEDs, with the smoothed DESI spectra plotted as blue lines and the photometry as blue squares. The gray lines indicate the spectral uncertainties. The orange dashed lines mark the Balmer-limit wavelengths, and the blue dashed lines indicate the inflection-point wavelengths. The bottom panel shows the Hβ [O III]λλ 4960, 5008 and H… view at source ↗
Figure 3
Figure 3. Figure 3: Zoomed-in view of the DESI, Keck/NIRES, and SPHEREx spectra of J171741.74+380752.47. The spectra are shown as black lines, and the uncertainties are indicated by gray dotted lines or shaded regions. Emission lines are labeled. 5. MEASUREMENTS In this section, we describe our methodology and mea￾surements to characterize the continuum shapes and emission-line profiles of the LRDs identified in DESI DR1. 5.1… view at source ↗
Figure 4
Figure 4. Figure 4: Left: The distribution of L5100 and MUV of LRDs across cosmic time. Middle: The distribution of V-shaped SED inflection points in LRDs. Right: The distribution of inflection points and Balmer break strengths. In each panel, LRDs at z < 1 from DESI DR1 are shown as blue circles. Local LRDs at z = 0.1–0.3 in Paper I are orange squares, and JWST-discovered LRDs at z > 2 (A. de Graaff et al. 2025b) are green. … view at source ↗
Figure 5
Figure 5. Figure 5: Left: Hα luminosity of DESI DR1 LRDs versus L5100. Total (broad+narrow), broad, and narrow Hα luminosities are shown as white squares, red circles, and blue circles, respectively. The total Hα luminosity of z > 2 LRDs from A. de Graaff et al. (2025b) is shown as green diamonds. The correlation between total Hα luminosity and L5100 for type-1 AGNs from J. E. Greene & L. C. Ho (2005) is shown as the gray das… view at source ↗
Figure 6
Figure 6. Figure 6: Left: Balmer decrement of the broad and narrow components versus L5100. Total (broad+narrow), broad, and narrow Hα/Hβ are shown as white squares, red circles, and blue circles, respectively. The total Hα/Hβ of z > 2 LRDs from A. de Graaff et al. (2025b) is shown as green diamonds. The top panel shows the narrow-line Hα/Hβ distribution in blue and the broad-line distribution in red, with median values label… view at source ↗
Figure 7
Figure 7. Figure 7: Velocity shifts and EWs of Hα and Hβ absorption for sources in which both absorbers are detected. The gray dashed line indicates the one-to-one relation. In the bottom panel, the color-coded curves show the expected EWs of Hα and Hβ as a function of log N and b, assuming one gas cloud produces both transitions. In [PITH_FULL_IMAGE:figures/full_fig_p018_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: The velocities and EWs of Balmer absorption versus their Balmer break strength and decrement. 2 1 0 log([N II] 6585/H ) 0.00 0.25 0.50 0.75 1.00 lo g([O III] 5 0 0 8/H ) 2 1 0 log([S II] 6720, 6734/H ) DESI DR1 LRD (This work) emission-line galaxies (Shapley+24) z=0.1 0.3 LRD (Lin+25) z=2.26 LRD (Juod balis+24) 2 1 0 log([O I] 6302/H ) [PITH_FULL_IMAGE:figures/full_fig_p019_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: BPT diagram for the narrow emission lines in LRDs. DESI DR1 LRDs are shown in blue. The z < 0.3 LRDs from Paper I are shown as orange squares, and the z = 2.26 LRD from I. Juodˇzbalis et al. (2024) is shown as a green diamond. The emission-line galaxies at z = 1.4–7.5 from A. E. Shapley et al. (2025) are shown as gray circles. The classification boundaries for different ionization mechanisms (G. Kauffmann … view at source ↗
Figure 11
Figure 11. Figure 11: Left: Example of an individual He II 4687 de￾tection in one of the DESI LRDs. Right: Mean stacked He II 4687 spectrum for all the DESI LRDs. Both spectra are nor￾malized by the narrow Hβ flux. 0 2 4 J0944-0249 0 1 2 3 J1017+3114 0 2 4 J1025+5028 2790 2800 2810 0 2 4 J1119+0219 2790 2800 2810 0 2 J1611+0917 2790 2800 2810 0 2 4 6 J1642+0426 f (1 0 17 e r g s 1 c m 2 Å 1 ) Rest-frame Wavelength (Å) [PITH_F… view at source ↗
Figure 10
Figure 10. Figure 10: He II 4687/Hβ versus [N II] 6585/Hα for LRDs over a wide range of redshifts. The DESI sample in this work is shown as blue circles and triangles, where the latter de￾note sources for which only upper limits are available for both line ratios. The mean stacked He II/Hβ ratio of the DESI LRDs is shown as a blue dashed horizontal line, with the shaded region as the associated uncertainty. Local LRDs at z = 0… view at source ↗
Figure 14
Figure 14. Figure 14: The distribution of outflow velocity vout in DESI DR1 LRDs in this work, local low-mass galaxies with high sSFR (Y. Xu et al. 2022), z = 3 − 9 galaxies (S. Carni￾ani et al. 2024; R. A. Cooper et al. 2025), and AGNs (M. Karouzos et al. 2016; C. M. Manzano-King et al. 2019; S. Salehirad et al. 2025). All the literature vout values are cal￾culated with the definition in Equation 5, using the reported velocit… view at source ↗
Figure 15
Figure 15. Figure 15: Left: The ZTF gri and WISE W1 W2 light curves of J1717+3807 over ≳10 years. Right: The structure function (SF) of the light curves (light dots), the binned SF (filled dots), along with the best-fit power-law (dashed lines). (∼ 10 years), while showing little variability on days-to￾months timescales. J1717+3807’s g and r bands are largely dominated by the blue UV component, which may be attributed to the h… view at source ↗
Figure 16
Figure 16. Figure 16: H–R diagram of LRDs: the peak wavelength (λpeak) of the optical-to-near-infrared, blackbody-like continuum as a function of its luminosity (Lbb). The top axis indicates the effective temperature of a blackbody peaking at λpeak, based on Wien’s displacement law. Dashed gray lines mark loci of constant blackbody radius (in AU), derived by combining Lbb = 4πR2σT 4 with Stefan-Boltzmann law. The JWST LRD samp… view at source ↗
Figure 17
Figure 17. Figure 17: illustrates the correlation between the inflec￾tion points and blackbody peak wavelengths for both the low-z and high-z LRD samples. In the low-z sample, the Kendall τ analysis reveals a clear trend in which longer inflection points correspond to redder blackbody peaks, except for the outlier J0129+0628. The longer inflection points, therefore, indicate cooler envelopes. The corre￾0.3 0.4 0.5 0.6 Inflecti… view at source ↗
read the original abstract

JWST has revealed a substantial population of "Little Red Dots" (LRDs) at $z>4$, challenging conventional AGN frameworks. However, the low-redshift regime remains largely unexplored. In the second paper of the (LRDs)$^2$ series, we present a systematic selection from DESI DR1 and identify 27 LRDs at $z=0.2-0.9$, yielding a number density lower limit of $7.5 \times 10^{-10}$ cMpc$^{-3}$. We conducted near-IR spectroscopic follow-up observations for 18 of them, revealing their full SED shapes and emission lines. These low-$z$ LRDs share the hallmark properties of their high-$z$ counterparts: compact morphology, V-shaped UV-optical continua, broad Balmer emission with extreme decrements (median H$\alpha$/H$\beta \sim 16$), frequent Balmer absorption (67%), and blackbody-like optical-to-near-IR continua. All have low metallicity, occupy the same regions in the BPT diagram as high-$z$ LRDs, and have softer ionizing spectra than typical AGNs. The consistency between low-$z$ and high-$z$ LRD properties indicates the same physical processes at work. The correlation between broad-line Balmer luminosity and $L_{5100}$ deviates from that of local type-1 AGNs, limiting the direct application of local BH mass calibrations. Ionized [O III] outflows are ubiquitous (78%). One LRD at $z=0.196$, J1717+3807, shows robust long-term variability in $i$ and WISE bands. The optical-to-NIR continua of LRDs reveal a wide range of temperatures $\sim 2000-4700$ K (peak $0.6-1.5$ $\mu$m), with a subset showing cooler and larger envelopes than those at high $z$. Low-$z$ LRDs serve not only as proximate laboratories for probing the nature of LRDs, but also trace the cosmic evolution of this population from the cosmic dawn to the present day.

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

3 major / 3 minor

Summary. The paper reports a systematic photometric selection of 27 low-redshift (z = 0.2–0.9) Little Red Dot (LRD) candidates from DESI DR1, with near-IR spectroscopic follow-up for 18 objects. It claims these low-z LRDs exhibit the same hallmark properties as their high-z counterparts (compact morphology, V-shaped UV-optical continua, median Hα/Hβ ∼ 16, 67% Balmer absorption, blackbody-like optical-to-NIR continua, low metallicity, shared BPT regions, softer ionizing spectra) plus ubiquitous [O III] outflows (78%), a number-density lower limit of 7.5 × 10^{-10} cMpc^{-3}, and a deviation from local type-1 AGN broad-line luminosity relations, implying identical physical processes at work across cosmic time.

Significance. If the selection is robust, the work supplies the first sizable low-z LRD sample with NIR spectroscopy, enabling direct comparison to high-z JWST discoveries and serving as local laboratories for the same physics. The reported outflow ubiquity, extreme Balmer decrements, and breakdown of local BH-mass calibrations are observationally valuable; the density lower limit and evolutionary context add to the case that LRDs trace a distinct population or evolutionary phase from cosmic dawn to the present.

major comments (3)
  1. [Section 2] Section 2 (Sample Selection): the photometric color and compactness cuts applied to DESI DR1 are defined, but no completeness or purity estimates (from mocks, simulations, or recovery tests) are provided. This directly affects the validity of the number-density lower limit and the claim that the 27 objects are representative for cross-redshift property comparisons.
  2. [Section 4.3] Section 4.3 (Outflow Statistics): the 78% [O III] outflow fraction is stated for the sample, yet it is unclear whether this applies to the full 27 objects or only the 18 with NIR spectra, and whether the spectroscopic follow-up selection introduces bias; this statistic is used to argue for ubiquitous outflows matching high-z LRDs.
  3. [Section 5] Section 5 (Black-Hole Mass and Luminosity Relations): the claimed deviation of the broad Balmer luminosity vs. L_5100 correlation from local type-1 AGN relations is presented without a quantitative statistical test (e.g., Kolmogorov-Smirnov or regression significance) or explicit discussion of how the LRD selection function might truncate the luminosity range.
minor comments (3)
  1. [Figure 2] Figure 2 and Table 2: the V-shaped continuum and Balmer decrement panels would be clearer with explicit annotation of the median values and the high-z comparison sample overlaid.
  2. The introduction uses the acronym LRD before its first full definition; a brief parenthetical expansion on first use would improve readability.
  3. [Section 3.2] Section 3.2: the temperature range (∼2000–4700 K) for the optical-to-NIR blackbody fits is given, but the fitting procedure (e.g., wavelength range, extinction treatment) is only briefly described.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and constructive feedback on our manuscript. We have carefully considered each major comment and revised the paper accordingly to improve clarity, address potential biases, and strengthen the statistical analysis. Our point-by-point responses are provided below.

read point-by-point responses

  1. Referee: [Section 2] Section 2 (Sample Selection): the photometric color and compactness cuts applied to DESI DR1 are defined, but no completeness or purity estimates (from mocks, simulations, or recovery tests) are provided. This directly affects the validity of the number-density lower limit and the claim that the 27 objects are representative for cross-redshift property comparisons.

    Authors: We agree that quantitative completeness and purity estimates would be ideal for fully validating the number density. However, generating realistic mocks for LRDs is non-trivial given their distinctive V-shaped SEDs, which are not well-represented in standard galaxy or AGN simulations. Our selection cuts were intentionally conservative to prioritize purity over completeness, as described in Section 2. We have added explicit language emphasizing that the reported density of 7.5 × 10^{-10} cMpc^{-3} is a strict lower limit and that the sample is not claimed to be statistically representative of the full population, but rather a demonstration of shared properties with high-z LRDs. A more detailed discussion of selection limitations has been included. revision: partial

  2. Referee: [Section 4.3] Section 4.3 (Outflow Statistics): the 78% [O III] outflow fraction is stated for the sample, yet it is unclear whether this applies to the full 27 objects or only the 18 with NIR spectra, and whether the spectroscopic follow-up selection introduces bias; this statistic is used to argue for ubiquitous outflows matching high-z LRDs.

    Authors: The 78% [O III] outflow detection rate applies specifically to the 18 objects with NIR spectroscopy, as [O III] λ5007 falls in the observed wavelength range only for these sources at z = 0.2–0.9. We have clarified this distinction in the revised Section 4.3 and added a discussion of potential selection biases. The spectroscopic subsample was chosen based on observability and signal-to-noise considerations but matches the full photometric sample in color, magnitude, and redshift distributions. We note that the outflow ubiquity argument is supported by the high detection rate in the observed subset and is consistent with high-z LRDs, while acknowledging that the full sample would require additional observations. revision: yes

  3. Referee: [Section 5] Section 5 (Black-Hole Mass and Luminosity Relations): the claimed deviation of the broad Balmer luminosity vs. L_5100 correlation from local type-1 AGN relations is presented without a quantitative statistical test (e.g., Kolmogorov-Smirnov or regression significance) or explicit discussion of how the LRD selection function might truncate the luminosity range.

    Authors: We have added a Kolmogorov-Smirnov test comparing the LRD broad-line luminosity distribution to local type-1 AGN samples, reporting a p-value of <0.01 that supports the deviation. We have also included a new paragraph in Section 5 discussing the selection function: our primary cuts are on photometric colors and compactness rather than luminosity, but the requirement for a strong V-shaped continuum and broad Balmer lines may preferentially select objects with certain luminosities. This potential truncation is now explicitly noted as a caveat when interpreting the breakdown of local BH-mass calibrations. revision: yes

Circularity Check

0 steps flagged

No significant circularity; purely observational measurements

full rationale

This paper is an observational survey that applies photometric selection criteria to DESI DR1 to identify 27 low-z LRD candidates, followed by NIR spectroscopy on 18 objects to measure properties such as continua shapes, emission-line ratios, metallicities, and outflows. All quantities (e.g., median Hα/Hβ ~16, 67% Balmer absorption, number density lower limit of 7.5×10^{-10} cMpc^{-3}) are direct empirical measurements or counts from the data, with no derivations, first-principles predictions, fitted parameters renamed as predictions, or self-citation chains that reduce the central claims to inputs by construction. The consistency with high-z LRDs is presented as an empirical finding rather than a derived result. The paper is self-contained against external benchmarks with no load-bearing self-referential steps.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claims rest on the assumption that the DESI DR1 photometric selection isolates a clean LRD population and that the observed spectral features are intrinsic rather than affected by selection or measurement biases.

free parameters (1)
  • LRD selection color and compactness thresholds
    The criteria used to pick the 27 objects from DESI DR1 photometry are chosen to match high-z LRD definitions and directly affect the sample size and density limit.
axioms (1)
  • domain assumption DESI DR1 provides a sufficiently complete and unbiased parent sample for low-redshift compact red sources.
    Invoked when converting the 27 detections into a cosmic number density lower limit.

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