AGN EVOLUTION FROM A GALAXY EVOLUTION VIEWPOINT

We explore the connections between the evolving galaxy and active galactic nucleus (AGN) populations. We present a simple phenomenological model that links the evolving galaxy mass function and the evolving quasar luminosity function, which makes specific and testable predictions for the distribution of host galaxy masses for AGNs of different luminosities. We show that the phi* normalizations of the galaxy mass function and of the AGN luminosity function closely track each other over a wide range of redshifts, implying a constant duty cycle of AGN activity. The strong redshift evolution in the AGN L* can be produced by either an evolution in the distribution of Eddington ratios, or in the m(bh)/m(*) mass ratio, or both. To try to break this degeneracy we look at the distribution of AGNs in the Sloan Digital Sky Survey (m(bh), L) plane, showing that an evolving ratio m(bh)/m(*) proportional to (1 + z)(2) reproduces the observed data and also reproduces the local relations that connect the black hole population with the host galaxies for both quenched and star-forming populations. We stress that observational studies that compare the masses of black holes in active galaxies at high redshift with those in quiescent galaxies locally will always see much weaker evolution. Evolution of this form would produce, or could be produced by, a redshift-independent m(bh)-sigma relation and could explain why the local m(bh)-sigma relation is tighter than m(bh)-m(*) even if sigma is not directly linked to black hole growth. Irrespective of the evolution of m(bh)/m(*), the model reproduces both the appearance of downsizing and the so-called sub-Eddington boundary without any mass-dependence in the evolution of black hole growth rates.