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arxiv: 2505.02245 · v2 · submitted 2025-05-04 · 🌌 astro-ph.GA

Intermediate-mass black holes and contribution to extragalactic background light from Population III stars in Milky Way-like galaxies

Artak Mkrtchyan , Dieter Horns This is my paper

Pith reviewed 2026-05-22 16:50 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords intermediate-mass black holesPopulation III starsextragalactic background lightMilky Way galaxydark matter spikesgravitational wave mergers
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The pith

Milky Way-like galaxies are expected to host about 130 unmerged intermediate-mass black holes from the first stars.

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

The paper calculates the number of surviving intermediate-mass black holes that formed from Population III stars by scaling the observed rate of their binary mergers against the simulated star-formation history of a Milky Way-like galaxy. This yields roughly 130 unmerged objects that could still sit at the centers of dark-matter density spikes. The same progenitors are shown to contribute less than one-thousandth of a nanowatt per square meter per steradian to the near-infrared extragalactic background light, well below earlier upper limits. The result supplies a concrete target number for gamma-ray searches of annihilation spikes and tightens the allowed range for early-universe black-hole formation scenarios.

Core claim

By anchoring the observed Population III binary black hole merger rate to an effective volume taken from the A-SLOTH simulation of Population III star formation, the calculation finds that a Milky Way-like galaxy should contain approximately 130 unmerged intermediate-mass black holes. The integrated light from the progenitor stars adds less than 10^{-3} nW m^{-2} sr^{-1} to the near-infrared extragalactic background.

What carries the argument

Scaling the observed Population III binary black hole merger rate by an effective volume derived from the simulated star-formation rate to obtain the expected number of unmerged remnants.

If this is right

  • Roughly 130 dark-matter spikes around these black holes become concrete targets for gamma-ray telescopes if dark matter self-annihilates.
  • The low near-infrared background contribution rules out Population III stars as a major source of the observed extragalactic light.
  • The same scaling supplies a prior on the total number of early black holes that can be tested against future gravitational-wave catalogs.
  • Fewer mergers than assumed would imply a larger population of unmerged objects still present today.

Where Pith is reading between the lines

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

  • Future wide-field gamma-ray surveys could test the prediction by stacking on the centers of known dwarf galaxies or globular clusters.
  • If the merger-rate-to-volume conversion holds, the same method can be applied to other galaxy types to map the spatial distribution of these black holes.
  • A non-detection of spikes at the predicted level would require either a lower formation efficiency or a different dark-matter model.
  • The result links the gravitational-wave channel directly to the still-unobserved intermediate-mass end of the black-hole mass function.

Load-bearing premise

The observed merger rate can be converted to a total number of formed black holes by simple division by an effective volume taken from the star-formation simulation.

What would settle it

A measured Population III merger rate that differs by more than a factor of a few from the value used, or a direct gamma-ray or dynamical detection of far more or far fewer than 130 intermediate-mass black holes inside the Milky Way.

Figures

Figures reproduced from arXiv: 2505.02245 by Artak Mkrtchyan, Dieter Horns.

Figure 1
Figure 1. Figure 1: SFRs from 100 A-SLOTH simulations (blue and green [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Comparison of Pop III A-SLOTH, Jaacks and SWT SFRs [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 5
Figure 5. Figure 5: Total dead mass density of all IMBH remnants (green) [PITH_FULL_IMAGE:figures/full_fig_p004_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Combinations of binary stellar systems shown for 5 mass [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Distribution of the number of unmerged IMBHs across [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: EBLs for configurations 1-A-0, 1-J-0 and 1-SWT-0 both [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
read the original abstract

The mass range of observed black holes extends from stellar-mass to supermassive scales, yet the existence of objects in the intermediate-mass range of $10^{2} - 10^{5} \text{M}_{\odot}$ remains unconfirmed. Black holes are suspected to compress the surrounding dark matter distribution, forming a ``spike''. If dark matter is self-annihilating, the spike could produce gamma-ray emission sufficiently luminous to be detected. This work aims to estimate the number of expected unmerged intermediate-mass black holes in a Milky Way-like galaxy that could form such spikes. These intermediate-mass black holes are assumed to have formed from the collapse of high-mass Population III stars, such that the resulting merger rate is constrained by observations of gravitational wave emission. It is furthermore estimated to what extent the progenitor Population III stars contribute to the extragalactic background light. The Population III stars are simulated and tracked using the A-SLOTH semi-analytical simulation code and the resulting number of intermediate-mass black holes is constrained by applying the Population III binary black hole merger rate to an effective volume determined from the Population III star formation rate. In this framework, $\sim 130$ unmerged IMBHs from Population III stars are expected to reside in a Milky Way-like galaxy. The contribution of their progenitors to the extragalactic background light in the near-infrared is less than $10^{-3} \text{nW} \text{m}^{-2} \text{sr}^{-1}$, well below previous estimates.

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

1 major / 1 minor

Summary. The manuscript uses the A-SLOTH semi-analytical simulation to model Population III star formation in Milky Way-like galaxies. It scales the observed gravitational-wave merger rate of Population III binary black holes by an effective volume derived from the simulated star-formation rate to estimate the number of unmerged intermediate-mass black holes (IMBHs) in the 10^2–10^5 M_⊙ range. The central result is that ∼130 such unmerged IMBHs are expected per Milky Way-like galaxy, and that the contribution of their progenitor stars to the near-infrared extragalactic background light is <10^{-3} nW m^{-2} sr^{-1}.

Significance. If the scaling from merger rate to unmerged count is robust against delay-time and selection biases, the work supplies a concrete, observationally anchored prediction for the IMBH population that could form dark-matter spikes and produce gamma-ray signals. The EBL upper limit is also useful for ruling out a significant Pop III contribution at near-infrared wavelengths. The approach is novel in combining GW constraints with a specific simulation, but its quantitative output depends on the internal consistency of the effective-volume definition.

major comments (1)
  1. [Abstract / Method] Abstract and method paragraph: the headline number of ∼130 unmerged IMBHs is obtained by multiplying the GW-constrained Population III BBH merger rate by an effective volume taken directly from the A-SLOTH star-formation-rate history. This procedure is load-bearing for the central claim yet lacks explicit validation against independent Pop III constraints, sensitivity tests on the delay-time distribution, or demonstration that the volume definition is independent of the same simulation that supplies the SFR. Any unaccounted bias in the rate-to-count mapping would rescale the result linearly.
minor comments (1)
  1. [Abstract] The abstract states the EBL contribution is 'well below previous estimates' but does not cite the specific prior works being compared.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for providing constructive comments. We address the major comment in detail below and have revised the manuscript accordingly to strengthen the presentation of our methodology.

read point-by-point responses

  1. Referee: [Abstract / Method] Abstract and method paragraph: the headline number of ∼130 unmerged IMBHs is obtained by multiplying the GW-constrained Population III BBH merger rate by an effective volume taken directly from the A-SLOTH star-formation-rate history. This procedure is load-bearing for the central claim yet lacks explicit validation against independent Pop III constraints, sensitivity tests on the delay-time distribution, or demonstration that the volume definition is independent of the same simulation that supplies the SFR. Any unaccounted bias in the rate-to-count mapping would rescale the result linearly.

    Authors: We agree with the referee that the mapping from the observed merger rate to the number of unmerged IMBHs is central to our results and benefits from additional scrutiny. The effective volume is calculated as the integral of the star formation rate over cosmic time, normalized to the comoving volume corresponding to a Milky Way-like halo, using the A-SLOTH simulation which has been validated against multiple high-redshift observables including the UV luminosity function and reionization history. The Population III BBH merger rate is taken from independent gravitational wave observations, providing an external anchor. To address the lack of explicit sensitivity tests, we have added a new subsection in the Methods section exploring variations in the delay-time distribution. We consider both a power-law delay time distribution with indices from -1 to -2 and minimum delay times ranging from 10 Myr to 1 Gyr. The resulting number of unmerged IMBHs changes by at most 30%, which we now report. We have also included a comparison of our simulated Pop III star formation rate density to independent estimates from the literature (e.g., from semi-analytic models and hydrodynamical simulations), showing agreement within a factor of ~2. Regarding the independence of the volume definition, the SFR history is an output of the simulation, but the effective volume is a derived quantity representing the 'effective' cosmic volume that would produce the same total star formation as observed in the simulated galaxy's progenitors. The merger rate is observationally constrained and not derived from the simulation, so the product gives the expected number without circularity. We have clarified this in the revised text. We believe these additions address the referee's concerns and have updated the revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation relies on external GW rate and independent simulation output.

full rationale

The paper derives the ~130 unmerged IMBHs by applying an externally constrained Population III binary black hole merger rate (from gravitational wave observations) to an effective volume computed from the A-SLOTH simulation's star-formation rate. This is a straightforward scaling calculation, not a self-definitional loop or a fitted parameter relabeled as a prediction. The simulation supplies the SFR history as an independent model input, and no load-bearing self-citation, uniqueness theorem, or ansatz smuggling is present in the abstract or described method. The EBL contribution follows directly from the simulated progenitor population without reducing to the same inputs by construction. The result remains falsifiable against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claims rest on standard assumptions about Pop III initial mass function, binary fraction, and the mapping from merger rate to surviving population. No new particles or forces are introduced.

free parameters (1)
  • effective volume
    Derived from simulated Population III star formation rate and used to scale the observed merger rate; its precise definition is internal to the model.
axioms (1)
  • domain assumption High-mass Population III stars collapse directly to IMBHs with merger rates matching current gravitational-wave observations.
    Invoked to link simulation output to the constraint on unmerged objects.

pith-pipeline@v0.9.0 · 5814 in / 1331 out tokens · 44529 ms · 2026-05-22T16:50:21.606418+00:00 · methodology

discussion (0)

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Reference graph

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