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arxiv: 2607.00084 · v1 · pith:TTTPPLOFnew · submitted 2026-06-30 · 🌌 astro-ph.GA · astro-ph.HE

Little Red Dots at z~2 in EIGER reveal a gentle decline with respect to their peak number density at z~5

Shrriya Kapoor , Jorryt Matthee , Alberto Torralba , Ivan G. Kramarenko , Rongmon Bordoloi , Jenny E. Greene , Edoardo Iani , Daichi Kashino
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Zhaoran Liu Ruari Mackenzie Sara Mascia Rohan P. Naidu Rob Simcoe
This is my paper

Pith reviewed 2026-07-02 18:23 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.HE
keywords little red dotsactive galactic nucleinumber densityJWST spectroscopybroad-line emittersBalmer breakhigh-redshift galaxiesz~2
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The pith

Little red dots at z≈2 exhibit a gentler decline in number density from their z≈5 peak than photometric surveys indicated.

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

The paper reports five little red dots identified at spectroscopic redshifts 1.55-3.18 among 19 broad-line sources in the EIGER JWST survey. These are separated from classical AGNs using spectro-photometric criteria such as Balmer break strength and the HeI/Paγ line ratio. The derived number density reaches about 7×10^{-6} cMpc^{-3} at z=1.9-2.5, amounting to no more than 3 percent of the AGN population. This measurement confirms an overall drop from higher redshifts but shows the drop is milder than earlier work had stressed.

Core claim

Our LRD sample at z≈2 spans a similar optical luminosity range as higher-redshift counterparts and yields a number density of ≈7×10^{-6} cMpc^{-3} at z=1.9-2.5, confirming a decline relative to z≈5 while indicating that decline is more gentle than photometric surveys had emphasized.

What carries the argument

Spectro-photometric classification of broad-line emitters into LRDs versus classical AGNs, followed by volume-limited number-density measurement over the 140 arcmin² EIGER survey area.

If this is right

  • Classical AGNs dominate the counts above M_{5100} < -22.5 while the LRD fraction among broad-line sources reaches 100 percent near M_{5100} ≈ -20.
  • The HeI/Paγ ratio cleanly separates LRDs from classical AGNs and appears to anti-correlate with Balmer break strength, tracing higher gas column densities via self-absorption.
  • Blue-shifted HeI absorption is present in the two reddest LRDs.
  • The LRDs correspond to black-hole masses of ~10^6 M_⊙ at the Eddington limit, matching the luminosity range of their high-redshift counterparts.

Where Pith is reading between the lines

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

  • A milder density decline would imply that the physical conditions producing LRDs persist over a wider redshift interval than models with a sharp cutoff would predict.
  • Deeper spectroscopy targeting fainter luminosities at z~2 could test whether the LRD fraction continues to rise below the current detection threshold.
  • The observed link between line ratios and Balmer breaks offers a potential observable for testing dust and gas geometry in low-luminosity accreting systems.

Load-bearing premise

The spectro-photometric criteria used to classify five of the 19 broad-line sources as LRDs are both complete and free of significant contamination or selection bias within the EIGER survey volume and luminosity range.

What would settle it

A comparable or larger spectroscopic survey at z≈2 that measures an LRD number density below 10^{-6} cMpc^{-3} would indicate a steeper decline than reported here.

Figures

Figures reproduced from arXiv: 2607.00084 by Alberto Torralba, Daichi Kashino, Edoardo Iani, Ivan G. Kramarenko, Jenny E. Greene, Jorryt Matthee, Rob Simcoe, Rohan P. Naidu, Rongmon Bordoloi, Ruari Mackenzie, Sara Mascia, Shrriya Kapoor, Zhaoran Liu.

Figure 1
Figure 1. Figure 1: Illustration of the spectrum of the reference ‘Rosetta Stone’ LRD at z = 2.26 (Juodžbalis et al. 2024). We mark the strongest emission￾lines in the spectrum, particularly highlighting Paβ and He i+Paγ that at z ∼ 2 fall in the F356W filter for which we have Grism data. We also show the transmission curves of the F115W, F200W and F356W filters whose data is available in the EIGER survey in blue, green and r… view at source ↗
Figure 2
Figure 2. Figure 2: The 2D NIRCam WFSS spectra of the 5 LRDs identified in this work based on the blind identification of broad emission-lines in these data. The spectra are centered on the strongest emission-line in each source. The left column shows original reduced grism spectrum (‘SCI’), which may be contaminated (as for example seen in J159_6107). The ‘EMLINE’ spectra in the right column show the continuum-removed spectr… view at source ↗
Figure 3
Figure 3. Figure 3: Overview of the key observables of the four LRDs at z = 1.9 − 2.5 for which we cover He i+Paγ. For each LRD, we show a false-color image based on NIRCam F356W/F200W/F115W data, a zoom-in on the primary-identified emission-line in the continuum-subtracted NIRCam grism spectrum and the spectral energy distribution. Black points show the observed photometry, whereas red, open hexagons show synthetic photometr… view at source ↗
Figure 4
Figure 4. Figure 4: Overview of two noteworthy objects identified in our search (as [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The optical luminosity and size for our broad-line sample. The size is measured on the JWST/NIRCam F356W image. The sources classified as LRDs are shown in red hexagons, whereas classical AGNs are shown with a blue dot. We highlight the sources that are X-ray de￾tected (yellow star), show strong outflows in He i (purple squares), are best-fit by a QSO template (green downward pointing triangle) and with a … view at source ↗
Figure 6
Figure 6. Figure 6: The He i+Paγ profiles of the four LRDs in the sample with coverage of these lines. Red lines show the total fitted profiles, that are composed of narrow and broad Paγ (dotted green and purple, respectively), narrow and broad He i emission (blue and orange dashed lines). In the case of J1030_2735 and J1148_21539, we also find significant detections of blue-shifted He i absorption. We note that the line-prof… view at source ↗
Figure 7
Figure 7. Figure 7: The integrated Hei1.083 to Paγ line ratio versus the estimated strength of the Balmer break. LRDs are marked with red hexagons and non-LRDs are shuffled randomly around at a Balmer break strength of 1 for reference. The dashed horizontal line corresponds to the He i/Paγ ratio identified to delineate AGN-powered sources (He i/Paγ > 2.3) from sources powered by star-formation by Brinchmann (2023). All LRDs h… view at source ↗
Figure 8
Figure 8. Figure 8: The median-stacked spectra of LRDs (red) and non-LRDs (blue) in this sample, based on continuum-subtracted data from our NIRCam grism spectroscopy in the F356W filter. We only highlight the rest￾frame wavelength range covered by the majority of our sample. The stacks highlight the relatively strong and blue-asymmetric He i emission in the non-LRDs that contrasts with the relatively weak He i emission in LR… view at source ↗
Figure 9
Figure 9. Figure 9: The optical luminosity function of LRDs at z ≈ 2 − 5. We com￾pare our measurements (red hexagons and upper limit) to the measure￾ments from Ma et al. (2025a) at z ≈ 1.7 − 2.7 and z ≈ 2.7 − 3.7 (or￾ange and brown points, respectively, with open symbols highlighting incomplete luminosities) based on wide area ground-based data, as well JWST-based measurements at z ≈ 2 − 4.6 (⟨z⟩ = 3.7) from Rinaldi et al. (2… view at source ↗
Figure 11
Figure 11. Figure 11: The number density evolution of LRDs, integrated down to optical magnitudes of M5100 ≈ −20. We combine our new measurement at z ≈ 2 (red hexagon) with literature measurements based on JWST data (Kokorev et al. 2024; Kocevski et al. 2024; Rinaldi et al. 2026; Zhuang et al. 2026) at z > 3 and wide-field ground-based imaging at z ≈ 2 − 3 (Ma et al. 2025a). Measurements from the DESI survey at z ≈ 0.5 (Lin et… view at source ↗
read the original abstract

We report the discovery of a sample of little red dots (LRDs) at $z \approx 2$ identified from deep JWST/NIRCam imaging and wide-field slitless spectroscopy over $140$ arcmin$^2$ from the EIGER survey. With an improved blind broad-line identification algorithm, we select 19 sources at spectroscopic redshifts $z = 1.55-3.18$ identified via rest-frame near-infrared lines (Paschen-$\beta$, HeI+Pa$\gamma$ and OI). Based on a range of spectro-photometric criteria, we classify five of these sources as LRDs and the other 14 as classical active galactic nuclei (AGNs). This classification is corroborated by some X-ray detections among the AGNs. Classical AGNs dominate the number counts above optical luminosities M$_{5100}<-22.5$, whereas the LRD fraction among broad-line sources reaches 100 % at M$_{5100}\approx-20$. The LRDs span the range in Balmer break strengths seen in the higher redshift populations. Blue-shifted HeI absorption is detected in the two reddest sources. The HeI/Pa$\gamma$ ratio cleanly separates LRDs from classical AGNs and seems to anti-correlate with Balmer break strength, likely tracing HeI self-absorption at higher gas column densities. Our LRD sample has a similar optical luminosity range as their high-redshift counterparts, corresponding to black hole masses of $\sim10^{6}$ M$_{\odot}$ at the Eddington luminosity. We measure LRD number densities of $\approx 7\times10^{-6}$ cMpc$^{-3}$ at $z = 1.9-2.5$, which indicates that LRDs represent $\lesssim 3$ % of the AGN population at these epochs. Our results confirm the previously reported decline in the LRD number density with respect to $z \approx 5$ based on photometric surveys, although we find the decline to be more gentle than earlier emphasized.

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 the discovery of five little red dots (LRDs) at z≈2 from the EIGER JWST survey over 140 arcmin², selected among 19 spectroscopically confirmed broad-line sources (z=1.55-3.18) via Paschen-β, HeI+Paγ and OI lines. Using spectro-photometric criteria (Balmer break strength, HeI/Paγ ratio, blue-shifted absorption), five sources are classified as LRDs and 14 as classical AGNs, with the classification partly corroborated by X-ray detections. The LRDs have similar luminosities to higher-z counterparts (M5100 ≈ –20 to –22.5, implying ~10^6 M⊙ black holes at Eddington), and the measured number density is ≈7×10^{-6} cMpc^{-3} at z=1.9-2.5, implying LRDs are ≲3% of the AGN population and confirming a decline from z≈5 but gentler than previously reported.

Significance. If robust, the result supplies the first spectroscopic sample of LRDs at z~2, bridging photometric high-z samples and lower-z AGN demographics. The direct spectroscopic redshifts, X-ray corroboration for some AGNs, and the reported anti-correlation between HeI/Paγ and Balmer break strength are concrete strengths that allow testable follow-up on gas column densities and accretion properties.

major comments (3)
  1. [Abstract, §3] Abstract and §3 (classification section): the central number density of ≈7×10^{-6} cMpc^{-3} at z=1.9-2.5 is obtained by counting five sources classified as LRDs out of 19 broad-line objects, but no completeness simulation, purity estimate, or mock-catalog test of the spectro-photometric criteria (Balmer-break range, HeI/Paγ ratio, blue-shifted absorption) is provided for the survey luminosity range M5100 ~ –20 to –22.5. This directly affects whether the measured density and the claimed gentler decline relative to z≈5 are reliable.
  2. [Abstract] Abstract: the reported density carries no error budget (Poisson uncertainties, cosmic variance, or volume calculation details for the 140 arcmin² field in the z=1.9-2.5 bin). With N=5, even modest contamination or incompleteness at the 20% level would shift the density enough to remove the distinction from a steeper decline.
  3. [Abstract] Abstract: the statement that “the HeI/Paγ ratio cleanly separates LRDs from classical AGNs” is presented without a quantitative demonstration (e.g., separation statistic, boundary definition, or robustness test against measurement errors) in the luminosity and redshift slice of the sample.
minor comments (2)
  1. Provide an explicit table or subsection listing the 19 sources with measured line ratios, Balmer-break strengths, and X-ray properties to allow independent assessment of the classification boundary.
  2. Clarify how the comoving volume for the density calculation is derived from the 140 arcmin² area and the precise redshift interval.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their careful and constructive review. The comments highlight important areas where the manuscript can be strengthened with additional quantitative analysis. We address each major comment below and will incorporate the suggested revisions in the next version of the paper.

read point-by-point responses

  1. Referee: [Abstract, §3] Abstract and §3 (classification section): the central number density of ≈7×10^{-6} cMpc^{-3} at z=1.9-2.5 is obtained by counting five sources classified as LRDs out of 19 broad-line objects, but no completeness simulation, purity estimate, or mock-catalog test of the spectro-photometric criteria (Balmer-break range, HeI/Paγ ratio, blue-shifted absorption) is provided for the survey luminosity range M5100 ~ –20 to –22.5. This directly affects whether the measured density and the claimed gentler decline relative to z≈5 are reliable.

    Authors: We agree that formal completeness and purity estimates via mock catalogs are needed to robustly support the number density and the comparison to z≈5. In the revised manuscript we will add mock-catalog simulations that inject sources with the observed range of Balmer-break strengths, HeI/Paγ ratios, and absorption features into the EIGER data, then re-apply our classification criteria to quantify completeness and purity in the M5100 ≈ –20 to –22.5 range. This will allow us to place a more reliable uncertainty on the reported density and on the claimed gentler decline. revision: yes

  2. Referee: [Abstract] Abstract: the reported density carries no error budget (Poisson uncertainties, cosmic variance, or volume calculation details for the 140 arcmin² field in the z=1.9-2.5 bin). With N=5, even modest contamination or incompleteness at the 20% level would shift the density enough to remove the distinction from a steeper decline.

    Authors: We acknowledge that the current version omits a quantitative error budget. The revised abstract and §4 will explicitly report Poisson uncertainties on the density, an estimate of cosmic variance appropriate for the 140 arcmin² EIGER footprint, and the precise comoving volume for the z=1.9–2.5 bin. We will also add a short sensitivity analysis showing how 20 % contamination or incompleteness would affect the comparison with the steeper decline reported at higher redshift, thereby clarifying the robustness of our conclusion. revision: yes

  3. Referee: [Abstract] Abstract: the statement that “the HeI/Paγ ratio cleanly separates LRDs from classical AGNs” is presented without a quantitative demonstration (e.g., separation statistic, boundary definition, or robustness test against measurement errors) in the luminosity and redshift slice of the sample.

    Authors: The separation is visually evident in our sample of 19 sources, but we agree that a quantitative demonstration is required. In the revision we will define an explicit boundary value for the HeI/Paγ ratio, report a statistical measure of separation (e.g., the significance of the difference between the two populations), and test the robustness of the classification against the measurement uncertainties on the line ratios within the M5100 and redshift range of the sample. revision: yes

Circularity Check

0 steps flagged

Number density derived by direct counting; no reduction to fitted inputs or self-citations

full rationale

The paper's central result is the LRD number density at z=1.9-2.5 obtained by counting five sources that meet a set of spectro-photometric criteria among 19 broad-line emitters identified via Paschen and HeI lines. This count is converted to a comoving density using the survey volume; the value is not obtained by fitting any parameter to a subset of the data and then predicting the same quantity. The comparison to the z≈5 peak is made against external photometric surveys rather than prior work by the same authors. No equation, ansatz, or uniqueness theorem is invoked that reduces the reported density to the classification criteria by construction, and the classification itself is presented as an empirical separation corroborated by X-ray detections rather than a self-referential definition.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The measurement rests on standard assumptions about line identification, luminosity conversion, and volume calculation plus the domain assumption that the chosen spectro-photometric cuts cleanly isolate LRDs.

axioms (2)
  • domain assumption The improved blind broad-line identification algorithm recovers all relevant Paschen-beta, HeI+Pa-gamma, and OI emitters without significant incompleteness or false positives in the EIGER data.
    Invoked to select the parent sample of 19 sources at z=1.55-3.18.
  • domain assumption The range of spectro-photometric criteria used to label five sources as LRDs versus 14 as classical AGNs is both necessary and sufficient and does not introduce post-hoc bias.
    Directly determines the numerator of the reported LRD number density.

pith-pipeline@v0.9.1-grok · 5988 in / 1584 out tokens · 34671 ms · 2026-07-02T18:23:03.993926+00:00 · methodology

discussion (0)

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Reference graph

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