Bridging scales: Modeling suppressed Bondi accretion on black holes and its impact on galaxy growth

Antonio J. Porras-Valverde, Priyamvada Natarajan, Angelo Ricarte, Kung-Yi Su, Hyerin Cho, Ramesh Narayan, Ben S. Prather

First listed 2025-11-11 | Last updated 2025-11-11

Abstract

The accretion and feedback processes governing supermassive black hole (SMBH) growth span an enormous range of spatial scales, from the Event Horizon to the circumgalactic medium. Recent general relativistic magnetohydrodynamic (GRMHD) simulations demonstrate that strong magnetic fields can substantially suppress Bondi accretion by creating magnetically arrested disk (MAD) states, reducing inflow rates by up to two orders of magnitude relative to classical Bondi predictions. We incorporate this magnetic suppression prescription from Cho et al. (2023, 2024) into the Dark Sage semi-analytic model (SAM), which tracks SMBH and galaxy co-evolution within hierarchical merger trees derived from the IllustrisTNG cosmological simulation. Implementing the suppression across different Eddington-ratio regimes, we explore its impact on black hole mass functions (BHMFs), stellar mass functions (SMFs), and AGN luminosity functions. Restricting suppression to sub-Eddington accretors ($f_{\rm Edd} < 3 \times 10^{-3}$) and rescaling AGN feedback efficiencies gives simultaneous agreement with observed $z = 0$ SMFs and BHMFs, as illustrated by Case D in this work. At $z > 6$, super-Eddington growth episodes dominate in the SAM, reproducing JWST-inferred luminous AGN number densities. Our results highlight the critical sensitivity of galaxy assembly to the coupling between small-scale accretion physics and large-scale feedback regulation. Magnetic suppression of hot gas accretion can reconcile low-redshift constraints while preserving the rapid black hole growth required at early cosmic epochs, thereby providing a physically motivated bridge between horizon-scale GRMHD simulations and cosmological galaxy-formation models.

Short digest

The authors inject GRMHD-motivated magnetic suppression of Bondi inflow (MAD-driven, up to ~100× below classical Bondi) into the Dark Sage SAM built on IllustrisTNG trees and test Eddington-ratio–dependent implementations. Their best-performing setup applies suppression only to sub-Eddington accretors (f_Edd < 3×10^-3) while modestly rescaling AGN feedback, which simultaneously matches z=0 stellar and black hole mass functions (Case D). Crucially, super-Eddington episodes still dominate at z > 6, preserving the number densities of luminous AGN implied by JWST little red dots and other early quasars, thereby linking horizon-scale physics to galaxy-scale growth regulation.

Key figures to inspect

• Figure 1 (SMF and BHMF at z=0): Verify that the green Case D curves land within the grey observational bands for both functions, demonstrating that sub-Eddington-only suppression plus feedback rescaling reconciles local galaxy and BH demographics.
• Figure 2 (MBH–host relations at z=0): Compare model tracks to the compilation of Scott+2013, Reines & Volonteri 2015, Greene+2020, and Sturm & Reines 2024; note how Case B systematically underpredicts MBH at fixed host properties while Cases C/D recover the observed locus.
• Figure 3 (bolometric AGN luminosity functions vs. redshift): Check that Case D tracks Shen+2020 at low z yet still reaches the shaded high‑z region inferred if JWST little red dots are AGN, showing that early luminous growth survives despite sub-Eddington suppression at late times.
• Figure 4 (Eddington-ratio distributions): Inspect how models converge at early epochs but diverge at low redshift; in Cases C/D the sub‑Eddington tail is pruned, illustrating how MAD‑suppressed hot accretion reshapes duty cycles without quenching super‑Eddington bursts.