Coevolution and downsizing of supermassive black holes and galactic bulges

Abstract

We consider recently reported “downsizing” of galaxies as well as SMBHs from a theoretical point of view. If the putative SMBH-to-bulge relation is incorporated, the downsizing of galaxies and SMBHs implies the downsizing of galactic bulges. We propose a physical mechanism to bring the downsizing of galactic bulges with the consideration of galaxy formation in UV background radiation. The star formation efficiency in primordial galaxies is basically regulated by the self-shielding againt UV background radiation. At higher redshift epochs, the self-shielding is stronger, and therefore galaxies form in a dissipationless fashion. As a result, earlier type (higher bulge-to-disk ratio) galaxies form at earlier epochs. Besides, a radiation-hydrodynamic mechanism is proposed to account for the SMBH-to-bulge relation. The growth of SMBHs can be promoted through the mass accretion driven by radiation drag which is exerted on dusty interstellar gas in radiation fields generated by bulge stars. It turns out that the resultant mass of a SMBH is predicted to be in proportion to the bulge mass, and the mass ratio is basically determined by the nuclear energy conversion efficiency from hydrogen to helium, ε = 0.007. In this scenario, the bulge luminosity overwhelms the BH accretion luminosity in the growing phase of SMBH. This phase corresponds to a “proto-QSO”, thereafter evolving to a QSO. Also, the proto-QSO phase is preceded by an optically-thick ultraluminous infrared galaxy (ULIRG) phase. This provides a coevolution scheme of SMBHs and bulges. If coupled with the downsizing of bulges, this coevolution scheme leads to the downsizing of SMBHs. The present scenario is a potential solution of the coevolution and downsizing of SMBHs and galactic bulges.