Anti-hierarchical evolution of the Active Galactic Nucleus space density in a hierarchical universe

摘要

Recent observations show that the space density of luminous active galacticnuclei (AGNs) peaks at higher redshifts than that of faint AGNs. Thisdownsizing trend in the AGN evolution seems to be contradictory to thehierarchical structure formation scenario. In this study, we present the AGNspace density evolution predicted by a semi-analytic model of galaxy and AGNformation based on the hierarchical structure formation scenario. Wedemonstrate that our model can reproduce the downsizing trend of the AGN spacedensity evolution. The reason for the downsizing trend in our model is acombination of the cold gas depletion as a consequence of star formation, thegas cooling suppression in massive halos and the AGN lifetime scaling with thedynamical timescale. We assume that a major merger of galaxies causes astarburst, spheroid formation, and cold gas accretion onto a supermassive blackhole (SMBH). We also assume that this cold gas accretion triggers AGN activity.Since the cold gas is mainly depleted by star formation and gas cooling issuppressed in massive dark halos, the amount of cold gas accreted onto SMBHsdecreases with cosmic time. Moreover, AGN lifetime increases with cosmic time.Thus, at low redshifts, major mergers do not always lead to luminous AGNs.Because the luminosity of AGNs is correlated with the mass of accreted gas ontoSMBHs, the space density of luminous AGNs decreases more quickly than that offaint AGNs. We conclude that the anti-hierarchical evolution of the AGN spacedensity is not contradictory to the hierarchical structure formation scenario.