Merger-Induced Quasars, Their Light Curves, and Their Host Halos

We revisit a basic model of quasar activation by major mergers of dark matter halos (with “galactic” masses of ≲10¹³ M_⊙ h^-1). This model usually consists of two main ingredients: the halo merger rate describing triggering, and a quasar light curve, which describes the evolution of individual quasars. We show how the matching between model predictions and a variety of new, independent data sets allows one to efficiently constrain several aspects of black hole growth and evolution that must be taken into account in future studies by more advanced models of galaxy formation. Our results can be summarized as follows: (1) A descending phase modelled such that quasars in more massive halos shut down faster than those in less massive ones allows a good description of the bright end of the AGN luminosity function at all epochs and is compatible with downsizing, with more massive galaxies shutting down star formation earlier. (2) We measure the average bias of type 2 AGNs in SDSS to be b = 1.233 ± 0.195, independent of luminosity in the range 42.5≤ log L(erg s^-1) ≤ 45.5. Such a value of the bias implies that faint AGNs at z<0.3 are mainly hosted by halos more massive than ~10^11.5-12 M_⊙ h^-1. The black hole mass function predicted by this model is flatter than previously found. (3) The high clustering signal measured at z>3 in SDSS forces successful models to be characterized by rather short delay times of t_delay≲10⁸ yr between the triggering and the shining epochs, implying massive “seed” BHs ≳ 10⁵M_⊙h^-1 and initial super-Eddington growth. (4) The low number counts of X-ray AGNs measured in recent deep surveys are better reproduced by models with a minimal post-peak phase and a higher minimum hosting halo mass at high redshifts. (5) Cross-correlating the feedback-constrained M_BH-M relation, with the redshift-dependent M_star-M relation obtained from the cumulative number-matching of the stellar and halo mass functions, we find a factor of ~2 larger BH-to-stellar mass ratio at high redshifts. We discuss the meaning of such trends in connection with the mild, positive evolution in the M_BH-σ_star relation, and the strong observed evolution in the sizes and velocity dispersions of their hosts.