Anti-hierarchical growth of supermassive black holes and QSO lifetimes

I present a new method to unveil the history of cosmic accretion and the build-up of Supermassive Black Holes (SMBH) in the nuclei of galaxies, based on observations of the evolving radio and (hard) X-ray luminosity functions of AGN. The fundamental plane of black hole activity discovered by Merloni, Heinz & Di Matteo (2003) is used as a mass and accretion rate estimator. I adopt the local BH mass function as a boundary condition to integrate backwards in time the continuity equation for the SMBH evolution, neglecting the role of mergers. Under the most general assumption that accretion proceeds in a radiatively efficient way above a certain rate, and in a radiatively inefficient way below, the redshift evolution of the mass and accretion rate functions are calculated self-consistently. The only tunable parameters are the accretion efficiency and the critical ratio of the X-ray to Eddington luminosity at which the transition between accretion modes takes place. The evolution of the BH mass function between z=0 and z ~ 3 shows clear signs of an anti-hierarchical behaviour: while the majority of the most massive objects (M > 10^9) were already in place at z ~ 3, lower mass ones mainly grew at progressively lower redshift. As an example, I will discuss the consequences of these results for the lifetimes of accreting black holes.