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arxiv: 2606.30253 · v1 · pith:D7VCL3O6new · submitted 2026-06-29 · 🌌 astro-ph.GA · astro-ph.CO

No evolution in the number density of little red dots from cosmic dawn to cosmic noon

Federica Loiacono , Roberto Gilli , Marco Mignoli , Marcella Brusa , Francesco Calura , Marco Chiaberge , Andrea Comastri , Quirino D'Amato
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Roberto Decarli Ivan Delvecchio Kazushi Iwasawa Ignas Juod\v{z}balis Giorgio Lanzuisi Roberto Maiolino Stefano Marchesi Giovanni Mazzolari Colin Norman Alessandro Peca Isabella Prandoni Matteo Sapori Matilde Signorini Paolo Tozzi Eros Vanzella Cristian Vignali Fabio Vito Gianni Zamorani Anita Zanella
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Pith reviewed 2026-06-30 04:50 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.CO
keywords little red dotsactive galactic nucleiJWST observationsbolometric luminosity functionblack hole growthhigh-redshift galaxiescosmic noonsupermassive black holes
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The pith

Little red dots maintain constant number density above a luminosity threshold from redshift 2 onward.

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

The authors identify five broad-line emitters in JWST data that are red in color and undetected in X-rays within a field around a z~6.3 quasar. These sources are used to compute the bolometric luminosity function of little red dots at z=2.4 and z=4.5. Comparing these densities to measurements at higher redshifts shows no significant change for objects above L_bol of about 3 times 10^44 erg per second. This result bears on whether little red dots trace an early phase of rapid black hole growth that persists to cosmic noon or represent a high-accretion state in already formed black holes.

Core claim

Five sources selected by line width above 1000 km/s, red color, and X-ray non-detection yield a space density for little red dots with L_bol greater than or equal to 3 times 10^44 erg s^{-1} that shows no significant evolution at z greater than 2. At z=2.4 the density is only a factor of about two below that of all pre-JWST AGNs above the same luminosity, while at z=4.5 it matches estimates from wider fields. The abundance at cosmic noon reaches 3.4 plus 5.6 minus 2.4 times 10^{-5} per cubic megaparsec, roughly 350 times above recent model predictions.

What carries the argument

The sample of five broad-line emitters selected via FWHM greater than or equal to 1000 km s^{-1}, F200W minus F356W greater than 0, and X-ray non-detection, from which the bolometric luminosity function of little red dots is derived.

If this is right

  • At z=2.4 the space density of LRDs is only a factor of two below that of all pre-JWST AGNs above the luminosity threshold.
  • At z=4.5 the derived density matches independent estimates from larger sky areas.
  • The measured abundance at cosmic noon exceeds model predictions by a factor of about 350.
  • If LRDs mark the rapid early growth phase of supermassive black holes, seed formation remains efficient down to z approximately 2.
  • Alternatively the objects may represent a high-accretion episode in already mature black holes.

Where Pith is reading between the lines

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

  • Models that underpredict LRD densities at z less than 4 may require adjustments to their black-hole seeding or accretion prescriptions.
  • The constant density suggests LRDs could account for a larger fraction of the overall AGN population at cosmic noon than previously modeled.
  • Repeating the selection in independent JWST fields would test whether the small sample size in this single field biases the density estimate.

Load-bearing premise

The five selected broad-line emitters form a complete and uncontaminated sample that represents the full little red dot population for luminosity function calculations.

What would settle it

A wider-area survey applying identical selection criteria at z between 2 and 5 that measures a number density differing by more than the reported uncertainties would falsify the no-evolution result.

Figures

Figures reproduced from arXiv: 2606.30253 by Alessandro Peca, Andrea Comastri, Anita Zanella, Colin Norman, Cristian Vignali, Eros Vanzella, Fabio Vito, Federica Loiacono, Francesco Calura, Gianni Zamorani, Giorgio Lanzuisi, Giovanni Mazzolari, Ignas Juod\v{z}balis, Isabella Prandoni, Ivan Delvecchio, Kazushi Iwasawa, Marcella Brusa, Marco Chiaberge, Marco Mignoli, Matilde Signorini, Matteo Sapori, Paolo Tozzi, Quirino D'Amato, Roberto Decarli, Roberto Gilli, Roberto Maiolino, Stefano Marchesi.

Figure 1
Figure 1. Figure 1: Exposure maps of the F115W, F200W, F356W imaging mosaics (size ∼ 6.8 ′ ×3.6 ′ ) of the J1030 field. The central tiles reach the maximum exposure time, which is 17500 s (F115W), 25800 s (F200W), and 7300 s (F356W). raw data ("uncal" files) through the Stage1 and Stage2 of the pipeline. At the end of Stage2, we subtracted the correlated read￾out noise (1/ f noise) from the calibrated exposures ("cal" files) … view at source ↗
Figure 2
Figure 2. Figure 2: The JWST/NIRCam F115W + F200W + F356W mosaic of the J1030 field. It covers a sky area of ∼ 6.8 ′ × 3.6 ′ . We also show the MUSE, HST/ACS, and HST/WFC3 footprints (yellow and cyan, respectively). The red circles show the positions of the 18 point-like sources with color F200W − F356W > 0, excluding the X-ray AGNs (see Sect. 3.2 and [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Color-color (left) and color-magnitude (right) diagrams of point-like sources in the J1030 field. The line emitters are shown with filled green circles. There are four X-ray "classical" AGNs, which are marked with a cross. Red sources are located in the F200W − F356W > 0 plane. On the other hand, stars are located at F200W − F356W < 0 ("stars locus") of the color-color plot. We show the redshift tracks for… view at source ↗
Figure 4
Figure 4. Figure 4: Line fit of the X-ray silent BLAGNs at z ∼ 2.4 (ID 1017 and ID 3646) and at z ∼ 4.5 (ID 8511 and ID 9008). We fit the He i and Paγ emission in ID 1017 and ID 3646 using both a narrow (solid lines) and a broad (dotted lines) Gaussian component. For ID 3646, we also show a zoomed panel to highlight the Paγ broad component. We fit the Hα emission of ID 8511 with both a narrow (solid line) and a broad (dotted … view at source ↗
Figure 5
Figure 5. Figure 5: X-ray bolometric correction Lbol/L2−10 keV,rest vs bolometric lu￾minosity Lbol. Lower limits to Lbol/L2−10 keV,rest for the BLAGNs in our sample are shown with empty triangles. We also show the lower lim￾its corresponding to the stacked samples at z ∼ 2.4 and z ∼ 4.5. The lower limits for the X-ray silent, BLAGNs at z > 4 of Maiolino et al. (2024) are shown with indian red triangles. For comparison, we sho… view at source ↗
Figure 6
Figure 6. Figure 6: UV slope βUV vs optical slope βopt for the X-ray silent BLAGNs in the J1030 field (circles). The red half-plane with βopt > 0 encloses the little red dots (LRDs), while little blue dots (LBDs) have βopt < 0 (blue half-plane). BiRD, ID 8511, and ID 9008 are clearly LRDs. On the other hand, ID 3646 has slopes consistent with LBDs. ID 1017 shows intermediate features between the two populations. We also show … view at source ↗
Figure 7
Figure 7. Figure 7: Bolometric LF of LRDs at z ∼ 2.4 and z ∼ 4.5. Left panel: we show with orchid pentagons the abundance of LRDs estimated from the J1030 field. The estimate from Loiacono et al. (2025) is shown with a purple pentagon. The LF from Ma et al. (2025) is shown with filled circles and includes their complete bins after converting the de-extinguished M5500 magnitude to bolometric luminosity (see the text for the de… view at source ↗
Figure 8
Figure 8. Figure 8: Active black hole mass function (BHMF) of LRDs at z ∼ 2.4 and z ∼ 4.5. Left panel: we show with orchid pentagons the abundance of LRDs estimated from the J1030 field. The estimate from Loiacono et al. (2025) is shown with a purple pentagon. We also show the predicted active BHMF from broad line quasars at similar redshift (dark cyan; Shen & Kelly 2012) The shaded areas correspond to the 16th and 84th perce… view at source ↗
Figure 9
Figure 9. Figure 9: Abundance of LRDs with Lbol ≳ 3 × 1044 erg s−1 as a function of redshift. We compare our estimate at z ∼ 2.4 (orchid pentagon) with the number density of LRDs from Matthee et al. (2024) and Kokorev et al. (2024) over the same integration limits. We show the abundance of classical pre-JWST AGNs (Shen et al. 2020) and X-ray selected AGNs (Peca et al. 2023) integrated over the same bolometric luminosity range… view at source ↗
read the original abstract

We present our search for little red dots (LRDs) in the "J1030 field", a region of the sky around the $z\sim 6.3$ quasar SDSS J1030+0524, observed by the JWST EIGER program. Over 154 point-like sources selected in a JWST-based photometric catalog, we find five broad line emitters (with $FWHM \gtrsim 1000\ \rm km s^{-1}$) that are red ($F200W - F356W > 0$) and are undetected in the X-rays. We use these sources to derive the bolometric luminosity function (LF) of LRDs at $z = 2.4$ and $z = 4.5$. At $z = 2.4$, the space density of LRDs is only a factor of $\sim 2$ lower than that of all pre-JWST active galactic nuclei (AGNs) with bolometric luminosity $L_{\rm bol} \gtrsim 3 \times 10^{44}\ \rm erg\ s^{-1}$. At $z = 4.5$, our estimate is consistent with those derived for LRDs based on larger areas of the sky. A similar behaviour is observed in the black hole mass function. More importantly, we study the number density of LRDs from cosmic dawn to cosmic noon. We find that there is no significant evolution in the abundance of LRDs with $L_{\rm bol} \gtrsim 3 \times 10^{44}\ \rm erg\ s^{-1}$ at $z > 2$. We speculate that the drop at $z < 4$ seen by other studies is due to their sampling of only the bright-end of the LRDs LF. At cosmic noon, the abundance of LRDs is $n = 3.4^{+5.6}_{-2.4} \times 10^{-5}\ \rm Mpc^{-3}$, which is a factor of $\sim 350$ larger than recent model predictions and is comparable with that of X-ray selected AGNs with similar bolometric luminosity. Our result may imply that, if LRDs are the early, rapid stages of supermassive black hole growth, as suggested by some models, then the formation of black hole seeds can be efficient down to epochs as recent as cosmic noon. Alternatively, LRDs may simply be a high-accretion phase in already mature black holes.

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 reports a search for little red dots (LRDs) in the J1030 field using JWST EIGER data, identifying five broad-line emitters (FWHM ≳1000 km s^{-1}) that are red (F200W-F356W>0) and X-ray undetected among 154 point-like sources. These are used to derive the bolometric luminosity function at z=2.4 and z=4.5, yielding a space density of n=3.4^{+5.6}_{-2.4}×10^{-5} Mpc^{-3} at z=2.4 for L_bol≳3×10^{44} erg s^{-1}. The central claim is that there is no significant evolution in LRD abundance with this luminosity threshold from z>2 to cosmic noon, with the z=2.4 density comparable to X-ray AGNs but ~350× higher than model predictions; implications for black hole seed formation or accretion phases are discussed.

Significance. If the no-evolution result holds after addressing sample limitations, it would indicate that LRDs remain abundant at z~2.4, challenging models and suggesting either efficient black hole seed formation persisting to cosmic noon or a common high-accretion phase in mature systems. The direct comparison to pre-JWST AGN densities and higher-z LRD estimates from wider fields provides useful context, and the JWST-based identification of broad-line LRDs at lower redshift is a concrete observational contribution.

major comments (3)
  1. [Abstract and LF section] Abstract and luminosity function derivation: The no-evolution conclusion at z>2 rests on the z=2.4 density from only five sources (n=3.4^{+5.6}_{-2.4}×10^{-5} Mpc^{-3}); the large Poisson uncertainties mean the claimed statistical consistency with z=4.5 and other works may reflect low statistical power rather than true constancy. The manuscript must detail the exact LF computation, volume corrections, and any completeness factors applied to this small sample.
  2. [Sample selection] Sample selection criteria: The five sources are defined by FWHM≳1000 km s^{-1}, red color, and X-ray non-detection; this selection risks incompleteness if LRDs lacking broad lines or with alternate X-ray properties exist, directly affecting whether the derived density represents the full population for the no-evolution claim.
  3. [Observational setup and results] Field and variance: All five sources come from the single J1030 field (centered on a z~6.3 quasar); cosmic variance is not quantified, which is load-bearing for the z=2.4 density and the cross-redshift comparison.
minor comments (2)
  1. [Abstract] The abstract states 'a similar behaviour is observed in the black hole mass function' without numbers or a figure reference; add a brief quantitative statement or pointer to the relevant panel.
  2. [Throughout] Notation for bolometric luminosity (L_bol) and redshift bins should be checked for consistency between text, tables, and any equations.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments, which have helped clarify several aspects of the analysis. We address each major comment below and have revised the manuscript accordingly to improve clarity on the luminosity function derivation and limitations.

read point-by-point responses

  1. Referee: [Abstract and LF section] Abstract and luminosity function derivation: The no-evolution conclusion at z>2 rests on the z=2.4 density from only five sources (n=3.4^{+5.6}_{-2.4}×10^{-5} Mpc^{-3}); the large Poisson uncertainties mean the claimed statistical consistency with z=4.5 and other works may reflect low statistical power rather than true constancy. The manuscript must detail the exact LF computation, volume corrections, and any completeness factors applied to this small sample.

    Authors: We agree that the small sample yields large Poisson uncertainties and that the no-evolution statement is formally consistent within errors rather than a high-significance detection of constancy. In the revised manuscript we have expanded Section 3 to provide the explicit LF formula, the comoving volume calculation for the J1030 field at each redshift slice, the 1/V_max weighting applied to each source, and the photometric completeness correction derived from the point-source selection efficiency. We also added a sentence noting that the result is limited by small-number statistics. revision: yes

  2. Referee: [Sample selection] Sample selection criteria: The five sources are defined by FWHM≳1000 km s^{-1}, red color, and X-ray non-detection; this selection risks incompleteness if LRDs lacking broad lines or with alternate X-ray properties exist, directly affecting whether the derived density represents the full population for the no-evolution claim.

    Authors: We acknowledge that the broad-line plus red-color plus X-ray-undetected criteria may miss LRDs without detectable broad lines or with different X-ray properties. The revised text now includes an explicit limitations paragraph stating that the reported density applies specifically to this spectroscopically confirmed broad-line subset and may therefore be a lower limit to the total LRD population. This selection was chosen to enable direct comparison with higher-redshift LRD samples that use the same criteria. revision: partial

  3. Referee: [Observational setup and results] Field and variance: All five sources come from the single J1030 field (centered on a z~6.3 quasar); cosmic variance is not quantified, which is load-bearing for the z=2.4 density and the cross-redshift comparison.

    Authors: We agree that cosmic variance is not quantified in the original submission. In the revision we have added a paragraph estimating the field-to-field variance using the analytic prescription of Somerville et al. (2004) for the probed volume and number density; the estimated fractional variance is ~30%, which remains smaller than the Poisson uncertainty. We also note the consistency of our z=4.5 point with wider-area measurements as supporting evidence that the single-field result is not strongly biased. revision: yes

Circularity Check

0 steps flagged

No circularity: direct observational count of LRDs yields number densities without reduction to fitted inputs or self-citations

full rationale

The paper's central result is an empirical space density n = 3.4^{+5.6}_{-2.4} × 10^{-5} Mpc^{-3} at z=2.4 derived from counting five qualifying sources among 154 point-like objects in a single JWST field, then comparing this count-based value to independent literature estimates at z>4.5. No equations define a quantity in terms of itself, no fitted parameters are relabeled as predictions, and no uniqueness theorems or ansatzes are imported via self-citation; the 'no significant evolution' statement follows from direct comparison of these independent measurements with stated Poisson uncertainties. The derivation chain is therefore self-contained as a standard observational luminosity-function estimate.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim depends on the domain assumption that the five selected sources accurately represent the LRD population without major selection biases or incompleteness.

axioms (1)
  • domain assumption The five sources with broad lines, red colors, and no X-ray detection are LRDs suitable for deriving the bolometric luminosity function.
    Invoked to convert the detected objects into a space density measurement.

pith-pipeline@v0.9.1-grok · 6137 in / 1115 out tokens · 47307 ms · 2026-06-30T04:50:49.526757+00:00 · methodology

discussion (0)

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