The fundamental plane of black hole activity and the census of the local black holes' population

Studying a sample of both strongly and weakly active galactic nuclei with measured masses and 5 GHz and 2-10 keV core luminosities, together with a few galactic black holes simultaneously observed in the radio and X-ray bands, Merloni, Heinz, & Di Matteo (2003) showed that the sources are correlated through a ``fundamental plane'' relationship in the three-dimensional (log L_R, log L_X, log M) space. Here I elaborate on how such a relationship can be used to infer directly mass and accretion rate of any black hole given its radio and X-ray fluxes, complementing the information obtained from optical/UV surveys. As an example, I show how the local X-ray and radio luminosity functions, coupled with the black hole mass function derived from the M-\sigma relation, provide us with an accretion rate function. It is found that the typical X-ray Eddington ratio of an active black hole at redshift zero is about 5 \times 10^{-4}.