Driving the growth of the earliest supermassive black holes with major mergers of host galaxies

Abstract

The formation mechanism of supermassive black holes (SMBHs) in general, and of SMBHs observed as luminous quasars at redshifts in particular, remains an open fundamental question. The presence of such massive BHs at such early times, when the Universe was less than a billion years old, implies that they grew via either super-Eddington accretion, or nearly uninterrupted gas accretion near the Eddington limit; the latter, at first glance, is at odds with empirical trends at lower redshifts, where quasar episodes associated with rapid BH growth are rare and brief. In this work, I examine whether and to what extent the growth of the quasar SMBHs can be explained within the standard quasar paradigm, in which major mergers of host galaxies trigger episodes of rapid gas accretion below or near the Eddington limit. Using a suite of Monte Carlo merger tree simulations of the assembly histories of 40 likely quasar host halos, I investigate (i) their growth and major merger rates out to , and (ii) how long the feeding episodes induced by host mergers must last in order to explain the observed quasar population without super-Eddington accretion. The halo major merger rate scales roughly as , consistent with cosmological simulations at lower redshifts, with quasar hosts typically experiencing major mergers between (), compared to ∼1 for typical massive galaxies at (). The high rate of major mergers allows for nearly continuous SMBH growth if (for example) a merger triggers feeding for a duration comparable to the halo dynamical time. These findings suggest that the growth mechanisms of the earliest quasar SMBHs need not have been drastically different from their counterparts at lower redshifts.