Origin of the antihierarchical growth of black holes

Abstract

Observational studies have revealed a ‘downsizing’ trend in black hole (BH) growth: the number densities of luminous active galactic nuclei (AGN) peak at higher redshifts than those of faint AGN. This would seem to imply that massive BHs formed before low‐mass BHs, 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 BHs, set within the hierarchical paradigm for structure formation. In this model, BHs evolve from light seeds (∼100 M⊙) and their growth is merger driven. The original Somerville et al. 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). In order to solve these problems, we consider several modifications to the physical processes in the model: (1) a ‘heavy’ BH seeding scenario; (2) a sub‐Eddington accretion rate ceiling that depends on the cold gas fraction and (3) an additional BH accretion mode due to disc instabilities. With these three modifications, the models can explain the observed downsizing, successfully reproduce the bolometric AGN luminosity function and simultaneously reproduce galaxy and BH 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‐fitting model suggests a scenario in which disc instabilities are the main driver for moderately luminous Seyfert galaxies at low redshift, while major mergers are the main trigger for luminous AGN.

The interaction-driven starburst contribution to the cosmic star formation rate density

“…starburst driven by disk instabilities (e.g. Bower et al 2006;Lagos et al 2011b;Guo et al 2011;Fanidakis et al 2012Fanidakis et al , 2011Hirschmann et al 2012), could have an impact on the shape and normalization of the main sequence (see Lagos et al 2011b). While the latter constitutes an interesting complement to the merger-driven starbursts, its consistency with the observed fraction of starbursts still needs to be investigated.…”

Triggering active galactic nuclei in hierarchical galaxy formation: disk instability vs. interactions

“…At present, semi-analytical models only resort to ansatz tuned to maximize the model agreement with observations; in Hirschmann et al (2012) the mass accretion rate is then calculated considering all the mass exceeding the threshold to be added in the central bulge and a tiny fraction of it (∼10 −3 , motivated by the local BH-bulge mass relation) to be accreted in the central BH according to an exponential growth characterized by the Salpeter timescale. In the GALFORM model, Bower et al (2006) and Fanidakis et al (2012) assume that all the mass of the disk (not only the mass in excess) is added to the central bulge, leading to an enhanced effectiveness of the whole process.…”

On the dependence of galaxy morphologies on galaxy mergers

“…nium Simulation (Springel et al 2005, H0 = 73 km s −1 Mpc −1 , Ωm = 0.25, σ8 = 0.9, n = 1, ΩΛ = 0.75) and the "fiducial" SC-SAM model as presented in Porter et al (2014, including also the modifications discussed in Hirschmann et al 2012) applied to the Bolshoi simulation (Klypin et al 2011, H0 = 70.1 km s −1 Mpc −1 , Ωm = 0.279, σ8 = 0.817, n = 0.96, ΩΛ = 0.721). We do not expect the differences in the cosmological parameters to affect model predictions in a relevant way (see e.g Guo et al 2013;Wang et al 2008).…”