The assembly of supermassive black holes in hierarchical clustering cosmologies

Abstract

Barring any fine tuning of the initial mass function of metal‐free Population III stars, intermediate‐mass black holes (IMBHs) may be the inevitable endproduct of the first episode of pregalactic star formation. We review a scenario for the assembly of supermassive black holes (SMBHs) at the center of galaxies that traces their hierarchical build‐up far up in the dark halo ‘merger tree’, in (mini)halos collapsing at z ∼ 20 from the high‐σ peaks of the primordial density field. As pregalactic IMBHs become incorporated through a series of mergers into larger and larger halos, they sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, and form a binary system. Stellar dynamical processes drive the binary to harden and eventually coalesce. The binding energy liberated by shrinking SMBH pairs heats the surrounding stars, creating low‐density cores out of preexisting cuspy stellar profiles. A simple model, where quasar activity is driven by halo major mergers and SMBHs accrete at the Eddington rate a mass that scales with the fifth power of the circular velocity of the host halo, reproduces surprisingly well the observed luminosity function of optically‐selected quasars in the redshift range 1 < z < 6.