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

TL;DR

This study uses cosmological simulations to analyze SMBH growth modes, revealing initial merger-driven growth followed by rapid gas accretion, with implications for observed high-redshift black holes.

Contribution

It distinguishes between merger-driven and gas accretion-driven SMBH growth modes and explores their impact on SMBH and galaxy coevolution.

Findings

SMBHs grow via mergers to about 10^7 M_sun initially.

Gas accretion increases after 7-9 Gyrs, reaching Eddington rates.

SMBHs do not exceed 10^10 M_sun by z=0 due to gas depletion.

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.