Clustering of JWST LRDs. Implications for PBHs and other early-forming seeds

Gert H\"utsi

REVIEW

Clustering of JWST LRDs. Implications for PBHs and other early-forming seeds

Gert Hütsi

Cosmology and Nongalactic Astrophysics · Submitted 16 Oct 2026

Pith's verdict: A clean analytic demonstration that PBH seeds predict b~1 for LRDs, ruled out by current b~3–8 clustering estimates; the argument is straightforward and honestly caveated.

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arXiv ID2610.23806 (version 2)

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CitationGert H\"utsi. "Clustering of JWST LRDs. Implications for PBHs and other early-forming seeds." arXiv:2610.23806 (2026). https://pith.science/paper/SZYZNJ4J

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Editor's note

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Hi,

This paper does one thing and does it cleanly: it uses the observed clustering bias of JWST LRDs (b~3–8 at z=4–8) to test whether a dynamically subdominant PBH population could be the seeds. The answer is no, because such PBHs generically predict b~1. The halo scenario naturally reproduces the observed bias with M_low ~ 10^{10.5–12} M_sun.

What's actually new: earlier work discussed PBH clustering for LRDs, but this is the first to carry through the Inman & Ali-Haïmoud isocurvature formalism to get a concrete bias prediction (b~1) and compare it directly to the data. The derivation is sound within its assumptions. The paper also extends the argument to any early-forming, dynamically subdominant seed population. Credit where due: the analysis is transparent, the limitations are stated upfront, and the conclusions are proportionate.

The soft spot is the observational constraint itself. The b~3–8 range is explicitly called "a representative range encompassing current clustering estimates" rather than a well-measured redshift evolution. If future measurements settle at b~2, the discrepancy with b~1 shrinks. The paper acknowledges this, so it's not a hidden problem — but it means the central argument rests on empirical ground that may shift. The other limitation (simple halo model, no HOD) is minor for the comparative purpose.

Overall this is a solid, useful contribution for people working on LRD origins and early BH seeds. It doesn't reshape the field, but it sharpens the case against PBH seeds in a falsifiable way.

Who it's for: the JWST high-redshift community, PBH modelers, anyone trying to understand LRD environments. Deserves a serious referee and publication in a good journal.

Recommendation: accept after minor revision — perhaps ask the authors to add a quantitative note on how much the conclusion weakens if the bias range broadens to b~2–8. But frankly it's fine as is.

Referee report

Significance

This paper provides a clean, parameter-free test of PBH seeding versus standard halo formation for high-redshift AGN-like sources by comparing clustering predictions, going beyond abundance-only arguments. The conclusion—that a randomly activated PBH population is disfavored while an environment-dependent activation cannot be ruled out by clustering—is robust under the stated observational constraints and has immediate implications for the interpretation of LRDs and for PBH models as SMBH seeds. The methodology is transparent, uses well-established theoretical tools, and the authors honestly discuss the provisional nature of the observational bias range. The results are timely given the intense interest in JWST discoveries and will guide both future observational programmes and theoretical modelling.

Minor objections (5)

[Section III, Eq. (4)] The Fry bias evolution formula is presented as an illustrative limiting case. The paper would benefit from a brief statement on its regime of validity (e.g., linear scales where bias is scale-independent and the tracer population is conserved). The current text already notes that LRDs are not obviously conserved tracers, but a more explicit mention of the linear-bias assumption would improve clarity.
[Figure 4] The legend for the dashed curve labelled 'PPBH, fPBH/MPBH=const=5x10-16' is cryptic. Consider adding a note that this curve shows the PBH auto-spectrum for a fixed number density, which is the relevant comparison across the three mass choices.
[Section IV, Eq. (9)] The shot-noise term SN is defined in the text, but explicitly writing SN = 1/\bar{n}_{\rm pbh} in the equation or immediately after would aid readability, especially since the subtraction is central to the bias definition.
[Appendix A] The Fisher analysis assumes a constant number density and no redshift evolution within the survey volume. While this is consistent with the main text, a brief note on how a more detailed treatment (e.g., including redshfit binning) would affect the signal-to-noise estimates would be helpful for future applications.
[Acknowledgments] The grant identifiers 'TARISTU24-TK10' and 'TARISTU24-TK3' are listed; the latter is repeated. Please verify the correct grant numbers.

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The paper

The paper claims

JWST little red dots cluster too strongly to come from randomly scattered primordial black holes.

The reading

The paper uses the observed abundance and spatial clustering of JWST little red dots (LRDs) at redshifts 4–8 to test two competing scenarios for their origin. In the standard picture, LRDs form in virialized dark-matter halos; this model naturally reproduces the measured clustering bias of b~3–8 and the number density of ~$10^{{-5}}$–$10^{{-4}}$ $h^{3}$ $Mpc^{{-3}}$ without fine-tuning. In contrast, if LRDs are seeded by a dynamically subdominant population of primordial black holes (PBHs) that are initially Poisson-distributed, the predicted bias is b~1, far below the observed range. The paper therefore concludes that PBH seeds are disfavored as a randomly activated LRD population, though they remain viable if their activity is tied to massive host halos (in which case clustering alone cannot distinguish them from astrophysical seeds).

What carries the argument

The analysis leans on the Sheth-Mo-Tormen (SMT) fitting formulae for the halo mass function and linear bias to compute the effective bias and number density in the halo scenario. For the PBH scenario, it uses the Inman & Ali-Haïmoud formalism for the power spectra of a discrete PBH component, including the Poisson shot-noise term and the isocurvature growth factor D_iso(z), to show that after shot-noise subtraction the PBH bias b_pbh = 1. The Fry equation describes how the bias of a conserved tracer decays toward unity after early formation, which is applied to PBHs but not to LRDs (which are not conserved).

Extended reading notes

Core claim

The central claim is that the spatial clustering of JWST LRDs strongly favors the standard halo formation picture over a PBH-based scenario. In the halo model, the observed bias range b=3–8 is reproduced for a lower mass cutoff M_low ~$10^{{10.5}}$–$10^{{12}}$ $h^{{-1}}$ M_sun, while the number density sets the duty cycle f_on <=1. In the PBH scenario, matching the LRD number density requires an extremely small PBH mass fraction f_pbh ~$10^{{-13}}$–$10^{{-10}}$, making PBHs dynamically negligible. The power-spectrum analysis then shows that such a Poisson-like distribution yields b~1 at z~5, regardless of the PBH mass in the range $10^{3}$–$10^{5}$ $h^{{-1}}$ M_sun, because the seeds are dynamically subdominant and simply trace the adiabatic matter field. This rules out PBHs as a randomly activated fraction of the LRD population; however, if PBHs become active only inside massive halos, they inherit the halo bias and become indistinguishable from astrophysical seeds.

Load-bearing premise

The adopted LRD clustering bias range b=3-8 is a representative estimate from current data, not a precisely measured redshift evolution; if the true bias is closer to 2-3, the PBH prediction of b~1 would be less strongly disfavored.

Editorial extensions

If this is right

If the PBH-seed scenario is ruled out for LRDs, other early-galaxy populations that show strong clustering will similarly disfavor PBH seeds unless the seeds are confined to massive halos.
Future wide-area JWST surveys (e.g., tens of square degrees) can measure LRD clustering with higher significance, potentially discriminating between the halo model and a PBH model even if the latter is rescued by halo occupancy.
Gravitational-wave observatories like LISA and mid-band atom interferometers will provide complementary probes of the seed origin by sensitive to merger rates of massive black holes at z>10.
The analysis implies that the simplest PBH-seed model for high-z supermassive black holes is incompatible with LRD clustering data, shifting the burden to more elaborate models (non-Gaussian initial clustering or halo-dependent activation).

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The argument applies more broadly than PBHs: any dark-matter component that is dynamically negligible and initially Poisson-distributed (e.g., compact minihalos or early quasi-stars) would predict b~1 and be ruled out by the same reasoning.
If current LRD clustering measurements are later refined and the bias turns out to be lower (b~2–3), the exclusion of PBHs would become weaker, and the halo model would require a higher minimal mass or earlier formation to match.
The fact that only PBHs in massive halos can survive the clustering test means that the LRD clustering data alone cannot prove an astrophysical origin for the seeds; they only require the seeds to be hosted by halos of a certain mass, regardless of whether the seeds themselves are primordial or stellar.

Original abstract

We use the observed abundance and spatial clustering of JWST little red dots (LRDs) over z = 4--8 to confront two formation scenarios: the standard picture in which LRDs form in virialized dark-matter halos, and one in which they are seeded by a dynamically subdominant population of primordial black holes (PBHs). The halo formation scenario accounts for both observables naturally and without fine-tuning. In the PBH scenario, matching the observed LRD abundance requires a negligibly small PBH mass fraction, and a power-spectrum analysis shows that such a Poisson isocurvature component predicts a clustering signal far below what is observed. The same conclusion holds for any population of dynamically subdominant early-forming seeds, unless their initial clustering is finely tuned or their activity is tied to massive host halos. We conclude that the spatial clustering of JWST LRDs strongly favours the standard halo formation picture and disfavours PBHs, or similar early-forming seeds, as a randomly activated LRD population; PBHs remain viable as seeds only if their activity is governed by massive host halos, in which case clustering alone cannot distinguish them from astrophysical seeds.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-20: 1 request, 4,551 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.87 · human participants 0.01 · controlled comparison 0.30 · randomized assignment 0.02 · states quantitative result 0.16 · compares with prior results 0.50 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; show every role.

We use the observed abundance and spatial clustering of JWST little red dots (LRDs) over z = 4--8 to confront two formation scenarios: the standard picture in which LRDs form in virialized dark-matter halos, and one in which they are seeded by a dynamically subdominant population of primordial black holes (PBHs). The halo formation scenario accounts for both observables naturally and without fine-tuning. In the PBH scenario, matching the observed LRD abundance requires a negligibly small PBH mass fraction, and a power-spectrum analysis shows that such a Poisson isocurvature component predicts a clustering signal far below what is observed. The same conclusion holds for any population of dynamically subdominant early-forming seeds, unless their initial clustering is finely tuned or their activity is tied to massive host halos. We conclude that the spatial clustering of JWST LRDs strongly favours the standard halo formation picture and disfavours PBHs, or similar early-forming seeds, as a randomly activated LRD population; PBHs remain viable as seeds only if their activity is governed by massive host halos, in which case clustering alone cannot distinguish them from astrophysical seeds.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	We use the observed abundance and spatial clustering of JWST little red dots (LRDs) over z = 4--8 to confront two formation scenarios: the standard picture in which LRDs form in virialized dark-matter halos, and one in which they are seeded by a dynamically subdominant population of primordial black holes (PBHs).	contribution 0.90	0.83	0.03	asserted 0.77
s1	The halo formation scenario accounts for both observables naturally and without fine-tuning.	result 0.85	0.56	0.10	asserted 1.00
s2	In the PBH scenario, matching the observed LRD abundance requires a negligibly small PBH mass fraction, and a power-spectrum analysis shows that such a Poisson isocurvature component predicts a clustering signal far below what is observed.	result 0.99	0.86	0.08	asserted 1.00
s3	The same conclusion holds for any population of dynamically subdominant early-forming seeds, unless their initial clustering is finely tuned or their activity is tied to massive host halos.	interpretation 0.57	0.67	0.08	asserted 0.99
s4	We conclude that the spatial clustering of JWST LRDs strongly favours the standard halo formation picture and disfavours PBHs, or similar early-forming seeds, as a randomly activated LRD population; PBHs remain viable as seeds only if their activity is governed by massive host halos, in which case clustering alone cannot distinguish them from astrophysical seeds.	interpretation 0.97	0.82	0.97	asserted 0.78

What kind of paper the classifier reports this is: theoretical result 0.85 · evidence from analysis of existing data 0.97.

What was examined, and what was not

sources read

arXiv listing abstract v1

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures

**Figure 1.** FIG. 1. Effective clustering bias

**Figure 2.** FIG. 2. Bias evolution of a tracer population formed at red

**Figure 3.** FIG. 3. Required PBH mass fraction

**Figure 5.** FIG. 5. Signal-to-noise ratio for the clustering amplitude as

**Figure 6.** shows the scales that dominate the Fisher integral I(k). For comparison, the nonlinearity scale kNL, defined implicitly by R kNL dk k2PL(k, z)/2π 2 = 1, gives kNL ≃ 10 h Mpc−1 at zeff = 5. As seen from the figure, most of the signal originates from linear or weakly nonlinear scales k ≪ kNL. [1] I. Labbe et al., Nature 616, 266 (2023), arXiv:2207.12446 [astro-ph.GA]. [2] M. Boylan-Kolchin, Nature Astron. 7, 731 (2023), arXiv:2208.01611 [astro-ph.CO]

Figures reproduced from arXiv:2610.23806 by the authors.

References

Discussion

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