From the earliest seeds to today’s supermassive black holes

I review scenarios for the assembly of supermassive black holes (MBHs) at the center of galaxies that trace their hierarchical build-up far up in the dark matter halo “merger tree”. Monte Carlo realizations of the merger hierarchy in a ΛCDM cosmology, coupled to semi-analytical recipes, are a powerful tool to follow the merger history of halos and the dynamics and growth of the MBHs they host. X-ray photons from miniquasars powered by intermediate-mass “seed” holes may permeate the universe more uniformly than EUV radiation, make the low-density diffuse intergalactic medium warm and weakly ionized prior to the epoch of reionization breakthrough, and set an entropy floor. The spin distribution of MBHs is determined by gas accretion, and is predicted to be heavily skewed towards fast-rotating Kerr holes, already in place at early epochs, and not to change significantly below redshift 5. Decaying MBH binaries may shape the innermost central regions of galaxies and should be detected in significant numbers by LISA. 1 Massive black holes and galaxy formation The strong link observed between the masses of supermassive black holes (MBHs) at the center of most galaxies and the gravitational potential wells that host them suggests a fundamental mechanism for assembling black holes and forming spheroids in galaxy halos. The mBH-σ relation [12] [16] implies a rough proportionality between MBH mass and the mass of the baryonic component of the bulge. It is not yet understood whether this relation was set in primordial structures, and consequently how it is maintained throughout cosmic time with such a small dispersion, or indeed which physical processes established such a correlation in the first place [48][18][6]. In cosmologies dominated by cold dark matter (CDM) galaxy halos experience multiple mergers during their lifetime, with those between comparable-mass systems (“major mergers”) expected to result in the formation of elliptical galaxies [21]. Simple models in which MBHs are also assumed to grow during major mergers and to be present in every galaxy at any redshift – while only a fraction of them is “active” at any given time – have been shown to explain many aspects of the observed evolution of quasars [7][8][25]. The coevolution of MBHs and their host galaxies in hierarchical structure formation scenarios gives origin to a number of of important questions, most notably: • Did the first MBHs form in subgalactic units far up in the merger hierarchy, well before the bulk of the stars observed today? The seeds of the z ∼ 6 quasars