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 ~ Mpc) ³ comoving boxes, we also study how the BH feedback physical parameters affect the coevolution between SMBHs and their host galaxies. Starting as 10⁵ M_ seeds, we find that the BHs grow initially via BH mergers to 10⁷ 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 10² - 10³ times, causing their predominant growth from 10⁷ M_ to (10⁹ - 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. In terms of SMBH - host galaxy coevolution along the M ₁₇ - M_ 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.
Paramita Barai (Mon,) studied this question.