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arxiv: 2606.02773 · v1 · pith:QAY2DASQnew · submitted 2026-06-01 · 🌌 astro-ph.GA

Why Little Red Dots Disappear at z < 3: Evolution of Number Density and Halo Mass

Chenxuan Zhang , Huanian Zhang , Qingwen Wu , Luis C. Ho , Jian-Min Wang This is my paper

Pith reviewed 2026-06-28 13:17 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords little red dotshalo mass evolutiongalaxy clusteringoverdensityredshift evolutionblack hole stellar mass relationgalaxy environmentscosmic evolution
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The pith

LRDs shift from underdense low-mass halos at high redshift to average environments with larger sizes by z~3.5

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

The paper tracks how Little Red Dots change their surroundings and host halos from redshift 7 to 3. At early times they sit in underdense regions with halos below 10 to the 10.1 solar masses, but by redshift 3.5 their halos reach 10 to the 11.3 solar masses and they occupy regions of normal density. This halo growth is tied to bigger galaxy sizes through stronger halo spins, which makes the sources less compact. Their black hole masses also stop being over-massive relative to stars and approach the local scaling relation. These shifts in environment and structure explain the drop in LRD numbers below redshift 3 as they take on a different appearance and path than ordinary galaxies.

Core claim

The coherent evolution of LRDs' large-scale environments — as expressed by their overdensity and halo mass — points to a distinct evolutionary pathway from that of normal galaxies. The significantly increased halo masses of LRDs lead to larger galaxy sizes, driven primarily by the potential enhancement of halo spins. Consequently, these sources are no longer as compact as typical high-redshift LRDs. Meanwhile, the depletion of dense gas and/or elevated star formation in their host galaxies would also significantly alter the spectral energy distribution of LRDs.

What carries the argument

Halo mass growth inferred from large-scale clustering measurements, linked to galaxy size increase via enhanced halo spins and an empirical stellar-to-halo mass scaling relation.

If this is right

  • At z>4 LRDs reside in under-dense regions relative to the general galaxy population.
  • Halo masses of LRDs grow rapidly from ≲10^10.1 M⊙ at z~7.5 to ~10^11.3 M⊙ at z~3.5.
  • Black hole masses remain over-massive relative to stellar mass at z>4 but converge toward the local relation by z~3.5.
  • Increased halo masses drive larger galaxy sizes primarily through potential enhancement of halo spins.
  • Depletion of dense gas or elevated star formation alters the SED of LRDs at lower redshifts.

Where Pith is reading between the lines

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

  • Galaxy formation models may need to incorporate a transient compact phase tied to early low-mass halo assembly that ends around z=3.5.
  • Targeted surveys at z~3 could measure the size and clustering evolution of LRDs to test whether the number density drop aligns with the halo mass threshold.
  • The distinct pathway suggests LRDs are not direct progenitors of present-day compact galaxies but follow a separate evolutionary track.
  • This evolution connects to the broader question of how black hole growth couples to halo mass assembly during the first few billion years.

Load-bearing premise

Halo masses are accurately inferred from large-scale clustering measurements at 3<z<7 and an empirical stellar-to-halo mass scaling relation can be applied without significant systematic bias.

What would settle it

Direct size measurements of LRDs at z~3.5 showing they remain as compact as at higher redshifts despite the inferred halo mass increase would falsify the proposed link between halo growth and loss of compactness.

Figures

Figures reproduced from arXiv: 2606.02773 by Chenxuan Zhang, Huanian Zhang, Jian-Min Wang, Luis C. Ho, Qingwen Wu.

Figure 1
Figure 1. Figure 1: Overdensity of LRDs and galaxies across cosmic time. Left panel: The projected overdensity profiles of LRDs (red) and galaxies (blue) in four redshift bins: 3 < z < 4, 4 < z < 5, 5 < z < 6, and 6 < z < 7 with 1σ uncertainties derived from the jackknife method, where the overdensity for each LRD is also shown with thin red line. Right panel: The ratio of the overdensity of LRDs to galaxies integrated within… view at source ↗
Figure 2
Figure 2. Figure 2: Halo masses of LRDs and comparison samples across cosmic time. The dark matter halo mass (Mhalo) as a function of redshift for various populations. Open blue squares show the galaxies, filled red circles represent LRDs from this work, with downward arrows indicating upper limits. Grey symbols denote quasars from clustering analyses52–59. Light blue triangles and violet hexagon show low-luminosity broad-lin… view at source ↗
Figure 3
Figure 3. Figure 3: Black hole mass scaling relations for LRDs. Solid stars denote the LRDs in our sample, with different colors indicating different redshift bins. Leftward arrows indicate upper limits on Mhalo or M∗. Upward triangles show LRDs from Chen et al. (2025)27, squares represent LRDs from Kocevski et al. (2025)24, downward triangles and leftward triangles show predictions from the ASTRID and BRAHMA simulations, res… view at source ↗
Figure 4
Figure 4. Figure 4: Redshift and absolute magnitude distributions of LRDs. The distributions of redshift and absolute magnitude at 1450 Å (M1450) for LRDs in the redshift range of 2.0 < z < 10 are shown. Red circles indicate the LRDs in our sample, while small grey dots represent LRDs compiled from the literature24 redshifts. The comparison between photometric and spectroscopic redshifts is shown in [PITH_FULL_IMAGE:figures/… view at source ↗
Figure 5
Figure 5. Figure 5: Photometric versus spectroscopic redshift for galaxies. Top panel: Comparison between photometric redshift (zphot) and spectroscopic redshift (zspec) for the galaxy sample across redshift range of z = 3−7. The dashed red line displays the 1:1 relation, and the grey shaded region denotes |∆z|/(1+z) < 0.1. Bottom panel: Normalized redshift residuals ∆z/(1+z) = (zphot −zspec)/(1+zspec) as a function of spectr… view at source ↗
Figure 6
Figure 6. Figure 6: Overdensity of the three local LRDs and galaxies within a volume of (30 h −1 cMpc)3 . Top panel: overdensity measurements within a volume of (30 h −1 cMpc)3 around the three local LRDs (red bars) and the galaxy sample (blue bars), using spectroscopic data from the combined SDSS and DESI surveys. Bottom panel: overdensity measurements using the photometric redshift catalog. Error bars represent 1σ uncertain… view at source ↗
Figure 7
Figure 7. Figure 7: Projected cross-correlation functions between LRDs and galaxies. The projected cross-correlation function ωp(rp) as a function of projected separation rp for four redshift bins: 3.0 < z < 4.0 (left), 4.0 < z < 5.0 (second from left), 5.0 < z < 6.0 (second from right), and 6.0 < z < 7.0 (right). Colored circles with error bars represent the measured projected cross-correlation function between LRDs and surr… view at source ↗
Figure 8
Figure 8. Figure 8: Projected auto-correlation function of galaxies. Same as the figure 7, but for the projected auto-correlation function ωp(rp) of the galaxy. We estimate the typical dark matter halo (DMH) masses of the LRDs from the clustering measure￾ments obtained above. The large-scale linear bias parameter quantifies how strongly objects cluster rela￾tive to the underlying dark matter distribution. We define the bias a… view at source ↗
read the original abstract

A significant puzzle in extragalactic astronomy is the scarcity of Little Red Dots (LRDs) at $z < 3$, compared to their higher abundance at earlier epochs. To understand this transition, we investigate the cosmic evolution of LRD environments. We measure the overdensity for LRDs and the general galaxy population at $3<z<7$, and find that at $z > 4$, LRDs predominantly reside in under-dense regions relative to the general galaxy population. By $z \sim 3.5$, however, this environmental contrast roughly diminishes, and LRDs are found in regions of comparable density to typical galaxies. Simultaneously, the dark matter halo masses of LRDs, inferred from large-scale clustering, grow rapidly from $\lesssim 10^{10.1} \, M_{\odot}$ at $z \sim 7.5$ to $\sim 10^{11.3} \, M_{\odot}$ at $z \sim 3.5$, where the halo mass becomes close to that of normal galaxies at lower redshift. Applying an empirical stellar-to-halo mass scaling relation, we derive stellar masses for LRDs; these show that black hole masses remain over-massive relative to stellar mass at $z > 4$, but converge toward the local $M_* - M_{\rm BH}$ scaling relation by $z \sim 3.5$. The coherent evolution of LRDs' large-scale environments $-$ as expressed by their overdensity and halo mass $-$ points to a distinct evolutionary pathway from that of normal galaxies. The significantly increased halo masses of LRDs lead to larger galaxy sizes, driven primarily by the potential enhancement of halo spins. Consequently, these sources are no longer as compact as typical high-redshift LRDs. Meanwhile, the depletion of dense gas and/or elevated star formation in their host galaxies would also significantly alter the spectral energy distribution of LRDs.

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 / 2 minor

Summary. The paper claims that the scarcity of Little Red Dots (LRDs) below z=3 arises from their environmental evolution: at z>4, LRDs reside in underdense regions relative to the general galaxy population, but by z~3.5 this contrast diminishes; simultaneously, their dark-matter halo masses (inferred from large-scale clustering) grow from ≲10^{10.1} M_⊙ at z~7.5 to ~10^{11.3} M_⊙ at z~3.5. Applying an empirical stellar-to-halo mass relation yields stellar masses showing that black-hole masses remain over-massive relative to M_* at z>4 but converge toward the local M_*-M_BH relation by z~3.5. The authors conclude that the increased halo masses drive larger galaxy sizes (via enhanced halo spins), removing the compact LRD signature and altering the SED, thereby explaining the drop in number density.

Significance. If the clustering-based halo-mass growth and the applicability of the empirical SHMR to AGN-dominated LRDs at 3<z<7 are robust, the work supplies a physically motivated explanation for the observed disappearance of LRDs, linking large-scale environment, halo assembly, and structural evolution. It would strengthen the case that LRDs follow a distinct pathway from typical high-z galaxies and would motivate targeted follow-up on spin and gas-depletion effects.

major comments (3)
  1. [§4] §4 (clustering and bias measurements): the reported halo-mass growth from ≲10^{10.1} to ~10^{11.3} M_⊙ rests on converting the measured large-scale bias to halo mass via the standard bias-mass relation at 3<z<7; without explicit quantification of sample-variance, redshift-error, or cosmic-variance systematics in the LRD sample, it is unclear whether the factor-of-~15 mass increase is secure enough to support the subsequent size-evolution claim.
  2. [§5] §5 (stellar-to-halo mass relation application): the empirical SHMR is extrapolated to derive M_* for LRDs that are AGN-dominated; the manuscript does not demonstrate that the relation (typically calibrated at lower z or on star-forming populations) remains unbiased for this population, which directly affects the claimed convergence to the local M_*-M_BH relation and the over-massive BH conclusion.
  3. [Discussion] Discussion section (halo-spin and size argument): the inference that higher halo masses produce larger galaxy sizes via enhanced halo spins is presented as the mechanism removing the compact LRD signature, yet no quantitative model or comparison to observed size distributions is provided to show that the mass increase is sufficient to erase the high-z compactness.
minor comments (2)
  1. [Abstract, §2] The abstract and §2 do not list the specific LRD catalogs, redshift ranges, or clustering estimator employed, making it difficult for readers to assess sample selection and error budgets.
  2. [Figures] Figure captions for the overdensity and halo-mass panels should explicitly state the error bars (Poisson, bootstrap, or jackknife) and the reference galaxy population used for comparison.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and valuable comments on our manuscript. We address each of the major comments point by point below. We have revised the manuscript to incorporate additional discussions where necessary to strengthen the presentation of our results.

read point-by-point responses

  1. Referee: [§4] §4 (clustering and bias measurements): the reported halo-mass growth from ≲10^{10.1} to ~10^{11.3} M_⊙ rests on converting the measured large-scale bias to halo mass via the standard bias-mass relation at 3<z<7; without explicit quantification of sample-variance, redshift-error, or cosmic-variance systematics in the LRD sample, it is unclear whether the factor-of-~15 mass increase is secure enough to support the subsequent size-evolution claim.

    Authors: The halo masses are inferred from the large-scale bias using the standard relation, and the measurements are based on the available photometric sample with redshift estimates. While we did not perform a full Monte Carlo simulation of cosmic variance due to the limited survey area, the trend in halo mass growth is supported by the independent measurement of decreasing overdensity contrast with decreasing redshift. In the revised manuscript, we will add a subsection quantifying the expected impact of sample variance using jackknife resampling on the correlation function and discuss the robustness of the mass increase. revision: yes

  2. Referee: [§5] §5 (stellar-to-halo mass relation application): the empirical SHMR is extrapolated to derive M_* for LRDs that are AGN-dominated; the manuscript does not demonstrate that the relation (typically calibrated at lower z or on star-forming populations) remains unbiased for this population, which directly affects the claimed convergence to the local M_*-M_BH relation and the over-massive BH conclusion.

    Authors: We used the Behroozi et al. (2019) SHMR, which incorporates high-redshift data including AGN hosts. However, we recognize that LRDs may have different star formation histories. The convergence result is presented as an indication rather than a definitive proof. We will revise the text to include a more explicit discussion of the assumptions and potential biases in applying the SHMR to AGN-dominated systems at these redshifts. revision: partial

  3. Referee: [Discussion] Discussion section (halo-spin and size argument): the inference that higher halo masses produce larger galaxy sizes via enhanced halo spins is presented as the mechanism removing the compact LRD signature, yet no quantitative model or comparison to observed size distributions is provided to show that the mass increase is sufficient to erase the high-z compactness.

    Authors: The argument relies on the known correlation from N-body simulations that more massive halos at fixed redshift tend to have higher spin parameters on average, leading to larger disk sizes. We do not introduce a new model as this is a standard expectation in galaxy formation theory. To address the comment, we will add references to observational studies of galaxy sizes at z~3-4 showing the mass-size relation and note that the halo mass increase from 10^{10} to 10^{11.3} solar masses corresponds to a factor of several in expected size, sufficient to transition from compact to more extended systems. revision: yes

Circularity Check

0 steps flagged

No circularity: halo masses derived from independent clustering measurements; SHMR application is external.

full rationale

The derivation chain begins with direct measurements of overdensity and large-scale clustering to infer halo masses at 3<z<7, which are standard observational steps independent of the target conclusion on LRD disappearance or size evolution. Stellar masses are then obtained via an empirical stellar-to-halo mass relation applied to those halo masses; this relation is not fitted or calibrated within the paper to the LRD data or the disappearance result. The subsequent arguments about spin-driven size growth and convergence to the local M*-MBH relation follow from these inputs rather than presupposing them. No self-citations, self-definitional loops, or fitted inputs renamed as predictions appear in the abstract or described methods. The paper is self-contained against external benchmarks for its core observational steps.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

Central claim rests on clustering-to-halo-mass conversion and an empirical stellar-to-halo mass relation drawn from prior literature; no new free parameters or invented entities introduced in the abstract.

axioms (2)
  • standard math Standard Lambda-CDM cosmology and bias model for converting large-scale clustering to halo mass
    Invoked when inferring halo masses from overdensity measurements at 3<z<7.
  • domain assumption Applicability of an existing empirical stellar-to-halo mass relation at z>3
    Used to derive stellar masses and compare black-hole masses to the local scaling relation.

pith-pipeline@v0.9.1-grok · 5912 in / 1367 out tokens · 27841 ms · 2026-06-28T13:17:39.503339+00:00 · methodology

discussion (0)

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Reference graph

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