Origin of the anti-hierarchical growth of black holes
  Observational studies have revealed a "downsizing" trend in black hole (BH)
growth: the number densities of luminous AGN peak at higher redshifts than
those of faint AGN. This would seem to imply that massive black holes formed
before low mass black holes, in apparent contradiction to hierarchical
clustering scenarios. We investigate whether this observed "downsizing" in BH
growth is reproduced in a semi-analytic model for the formation and evolution
of galaxies and black holes, set within the hierarchical paradigm for structure
formation (Somerville et al. 2008; S08). In this model, black holes evolve from
light seeds (\sim100M\odot) and their growth is merger-driven. The original S08
model (baseline model) reproduces the number density of AGN at intermediate
redshifts and luminosities, but underproduces luminous AGN at very high
redshift (z > 3) and overproduces them at low redshift (z < 1). In addition,
the baseline model underproduces low-luminosity AGN at low redshift (z < 1). To
solve these problems we consider several modifications to the physical
processes in the model: (1) a 'heavy' black hole seeding scenario (2) a
sub-Eddington accretion rate ceiling that depends on the cold gas fraction, and
(3) an additional black hole accretion mode due to disk instabilities. With
these three modifications, the models can explain the observed downsizing,
successfully reproduce the bolometric AGN luminosity function and
simultaneously reproduce galaxy and black hole properties in the local
Universe. We also perform a comparison with the observed soft and hard X-ray
luminosity functions of AGN, including an empirical correction for torus-level
obscuration, and reach similar conclusions. Our best-fit model suggests a
scenario in which disk instabilities are the main driver for moderately
luminous Seyfert galaxies at low redshift, while major mergers are the main
trigger for luminous AGN.
