We investigated the growth of central supermassive black holes (SMBHs) in galaxies, with the aim of distinguishing between gas accretion and black-hole (BH) merger-driven growth modes. We performed cosmological hydrodynamical simulations of (50 Mpc) ³ comoving volumes, and analyzed how the BH feedback parameters affect the coevolution between SMBHs and their host galaxies. Starting as 10⁵ M_⊙ seeds, we find that the BHs initially grow via BH mergers to ∼ 10⁷ M_⊙. Gas accretion onto the BHs is low soon after seeding, and then the accretion rate increases with time and reaches the Eddington rate after 7-9 Gyr. The BHs then undergo very fast growth via efficient gas accretion over a time period of 600 - 700 Myr; during this period 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, which is because of gas depletion from galaxy centers driven by BH feedback. In terms of the coevolution of SMBHs and their host galaxies along the M_ ̊m BH - M_⋆ relation, we find that they initially lie below the relation at earlier epochs and thereby move upward toward the relation. We made some physical implications about the growth of high-z little red dots recently observed by the 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 (Fri,) studied this question.