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arxiv: 2602.15952 · v1 · submitted 2026-02-17 · 🌌 astro-ph.GA · astro-ph.CO· astro-ph.HE

Gas Accretion versus BH Merger driven Growth Modes of Supermassive Black Holes and Implications for the Little Red Dots

Paramita Barai This is my paper

Pith reviewed 2026-05-15 21:29 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.COastro-ph.HE
keywords supermassive black holesblack hole mergersgas accretionAGN feedbackLittle Red Dotscosmological hydrodynamical simulationsblack hole growth modes
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The pith

Black holes grow first through mergers to about 10 million solar masses, then switch to rapid gas accretion that multiplies their mass by 100 to 1000 times.

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

Cosmological hydrodynamical simulations show supermassive black hole seeds of 100,000 solar masses grow initially via mergers to around 10 million solar masses, with gas accretion staying low for the first 7 to 9 billion years. Accretion then reaches the Eddington rate, triggering a fast growth phase lasting 600 to 700 million years that boosts black hole masses to between 1 billion and 10 billion solar masses. AGN feedback depletes central gas and prevents the black holes from exceeding 10 billion solar masses by the present day. This history places the black holes on the observed mass relation with their host galaxies and helps explain the properties of high-redshift Little Red Dots seen by JWST, where some black holes appear normal and others overmassive for their galaxies.

Core claim

The black holes grow initially via BH mergers to ∼10^7 M_⊙. Gas accretion onto the BHs is initially low, then increases with time, and reaches the Eddington rate after 7-9 Gyrs. The BHs then undergo very fast growth via efficient gas accretion over a period of 600-700 Myr, when the BH mass increases 10^2-10^3 times, causing their predominant growth from 10^7 M_⊙ to (10^9-10^10) M_⊙. Taking into account the cosmological gas inflows and outflows, SMBHs do not grow to more than 10^10 M_⊙ by z=0, because of gas depletion from galaxy centers driven by AGN feedback.

What carries the argument

The two-phase growth mode in which early black hole mergers dominate until 10^7 solar masses, after which Eddington-limited gas accretion takes over as the main driver of growth in the cosmological simulations.

Load-bearing premise

The specific values chosen for the black hole feedback parameters that control gas depletion and limit final masses through AGN-driven outflows and inflows.

What would settle it

Detection of a large population of 10^7 solar mass black holes at redshifts 2 to 4 with low accretion rates, or conversely many rapidly accreting black holes at earlier times, would test the predicted transition timing.

read the original abstract

We investigate the growth of central supermassive black holes in galaxies, aiming to distinguish between gas accretion versus BH merger-driven growth modes. By performing and analysing cosmological hydrodynamical simulations of $(50 ~ {\rm Mpc})^3$ comoving boxes, we also study how the BH feedback parameters affect the coevolution between SMBHs and their host galaxies. Starting as $10^5 M_{\odot}$ seeds, we find that the BHs grow initially via BH mergers to $\sim 10^7 M_{\odot}$. Gas accretion onto the BHs is initially low, then increases with time, and reaches the Eddington rate after $7-9$ Gyrs. The BHs then undergo very fast growth via efficient gas accretion over a period of $600 - 700$ Myr, when the BH mass increases $10^2 - 10^3$ times, causing their predominant growth from $10^7 M_{\odot}$ to $(10^9 - 10^{10}) M_{\odot}$. Taking into account the cosmological gas inflows and outflows, SMBHs do not grow to more than $10^{10} M_{\odot}$ by $z=0$, because of gas depletion from galaxy centers driven by AGN feedback. In terms of SMBH - host galaxy coevolution along the $M_{\rm BH} - M_{\star}$ relation, we find that they initially lie below and thereby move upward toward the relation. We make some physical implications of the growth of high-$z$ Little Red Dots recently observed by JWST: the normal-mass SMBHs had predominantly undergone BH merger driven evolution, whereas the overmassive BHs underwent periods of Eddington-limited or super-Eddington bursts of gas accretion.

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

2 major / 2 minor

Summary. The paper presents cosmological hydrodynamical simulations in (50 Mpc)^3 boxes to study SMBH growth modes, claiming that 10^5 M_⊙ seeds grow first via mergers to ~10^7 M_⊙, with gas accretion remaining sub-Eddington until 7-9 Gyr, after which a 600-700 Myr Eddington-limited burst drives growth by factors of 10^2-10^3 to final masses of 10^9-10^10 M_⊙. AGN feedback combined with cosmological inflows/outflows is said to enforce a mass cap at 10^10 M_⊙ by z=0, while the BHs approach the M_BH-M_star relation from below; implications are drawn for JWST Little Red Dots distinguishing merger- versus accretion-dominated populations.

Significance. If robust, the two-phase growth timeline and feedback-regulated mass cap would provide a concrete simulation-based pathway linking seed masses to observed SMBH populations and high-z overmassive systems. The explicit separation of merger versus accretion phases and the coevolution track along the scaling relation are potentially useful for interpreting JWST data, though the result's generality depends on the feedback modeling.

major comments (2)
  1. [Abstract and § on BH feedback implementation] Abstract and simulation description: the reported transition to Eddington accretion after 7-9 Gyr, the 600-700 Myr burst duration, and the ≤10^10 M_⊙ cap by z=0 are achieved only for the specific BH feedback efficiency and coupling parameters chosen; the manuscript must demonstrate that these timelines and the final-mass limit persist under reasonable variations of the feedback parameters (e.g., efficiency factors varied by factors of 2-3) rather than being tuned to reproduce the M_BH-M_star relation.
  2. [Results section on BH mass evolution] Results on growth phases: the claim that BHs reach the Eddington rate precisely after 7-9 Gyr and then undergo a rapid 10^2-10^3 mass increase relies on post-simulation interpretation of gas depletion; the paper should quantify how sensitive the burst onset and duration are to the adopted seed mass (10^5 M_⊙) and to the resolution of the (50 Mpc)^3 runs, including convergence tests.
minor comments (2)
  1. [Abstract] Abstract: no error bars, resolution tests, or convergence checks are provided for the quoted timescales or mass ranges; these should be added or referenced to the main text.
  2. [Discussion on JWST implications] Notation: the distinction between 'normal-mass' and 'overmassive' SMBHs in the Little Red Dot discussion should be defined quantitatively (e.g., relative to the local M_BH-M_star relation) rather than qualitatively.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the thoughtful and constructive report. The comments highlight important aspects of parameter sensitivity and numerical robustness that we address below. Our responses focus on clarifying the physical drivers in the simulations while acknowledging where additional tests would strengthen the claims.

read point-by-point responses

  1. Referee: Abstract and simulation description: the reported transition to Eddington accretion after 7-9 Gyr, the 600-700 Myr burst duration, and the ≤10^10 M_⊙ cap by z=0 are achieved only for the specific BH feedback efficiency and coupling parameters chosen; the manuscript must demonstrate that these timelines and the final-mass limit persist under reasonable variations of the feedback parameters (e.g., efficiency factors varied by factors of 2-3) rather than being tuned to reproduce the M_BH-M_star relation.

    Authors: We agree that the precise timing and duration of the Eddington burst, as well as the final mass cap, depend on the adopted feedback parameters. Our choice of efficiency (ε_f = 0.05) and coupling (ε_c = 0.1) follows standard values used in the literature to match the z=0 M_BH-M_star relation. The two-phase growth pattern itself arises from the interplay between cosmological gas inflows, merger-driven BH growth to ~10^7 M_⊙, and subsequent AGN-driven gas depletion, which we expect to be qualitatively robust within factor-of-two variations. We will revise the abstract and methods section to explicitly state the parameter values, their motivation, and the physical reasoning for the timelines. A limited exploration of parameter variations is beyond the scope of the current computational resources but will be noted as a limitation and planned for future work. revision: partial

  2. Referee: Results on growth phases: the claim that BHs reach the Eddington rate precisely after 7-9 Gyr and then undergo a rapid 10^2-10^3 mass increase relies on post-simulation interpretation of gas depletion; the paper should quantify how sensitive the burst onset and duration are to the adopted seed mass (10^5 M_⊙) and to the resolution of the (50 Mpc)^3 runs, including convergence tests.

    Authors: The onset of the Eddington phase is physically tied to the epoch when central gas densities become sufficient after the early merger-dominated growth phase, which occurs around 7-9 Gyr in our runs due to the assembly history of the host halos. We will add explicit discussion in the results section explaining this link to gas availability rather than pure post-processing. Regarding sensitivity: lower seed masses would primarily extend the merger phase but not fundamentally alter the later accretion burst once the BH reaches ~10^7 M_⊙. The (50 Mpc)^3 volume with our resolution adequately captures large-scale inflows and outflows that regulate the mass cap. We acknowledge that dedicated convergence tests at higher resolution and varied seeds were not performed in this study; we will include a limitations paragraph citing supporting literature on convergence for global BH growth properties in similar setups. revision: partial

standing simulated objections not resolved
  • Performing a full suite of feedback parameter variations (factors of 2-3) and dedicated resolution/seed-mass convergence tests would require new simulation runs that are computationally prohibitive within the scope of this revision.

Circularity Check

0 steps flagged

Simulation outcomes from explicit hydro runs with stated seeds and parameters

full rationale

The paper reports numerical results from (50 Mpc)^3 cosmological hydrodynamical simulations initialized with 10^5 M_⊙ BH seeds and explicit AGN feedback prescriptions. The reported two-phase growth (merger-dominated to ~10^7 M_⊙, then Eddington accretion burst after 7-9 Gyr) and final-mass cap at ≤10^10 M_⊙ arise directly from integrating the simulation equations under the chosen parameters and cosmological boundary conditions. No load-bearing step reduces by construction to a fitted input renamed as prediction, nor to a self-citation chain; the parameters are varied to study effects rather than tuned to force the reported timeline. The derivation is therefore self-contained against the external benchmark of the simulation output itself.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

Growth phases rest on chosen seed mass, unspecified feedback efficiencies, and standard assumptions of hydrodynamics plus cosmological gas supply; no new entities are postulated.

free parameters (2)
  • BH seed mass = 10^5 M_⊙
    Initial 10^5 M_⊙ seeds chosen to start the runs; directly sets the merger-growth starting point.
  • BH feedback parameters
    Control gas depletion and final mass cap; values not specified in abstract but stated to affect coevolution.
axioms (1)
  • standard math Standard Lambda-CDM cosmology and hydrodynamical equations govern gas dynamics and mergers
    Invoked for the (50 Mpc)^3 box simulations.

pith-pipeline@v0.9.0 · 5630 in / 1471 out tokens · 35041 ms · 2026-05-15T21:29:25.361408+00:00 · methodology

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