Downsizing of supermassive black holes from the SDSS quasar survey

Starting from the ~50000 quasars of the Sloan Digital Sky Survey for which MgII line width and 3000 A monochromatic flux are available, we aim to study the dependence of the mass of active black holes on redshift. We focus on the observed distribution in the FWHM - nuclear luminosity plane, which can be reproduced at all redshifts assuming a limiting BH mass, a maximum Eddington ratio and a minimum luminosity (due to the survey flux limit). We study the z-dependence of the best fit parameters of assumed distributions at increasing redshift and find that the maximum mass of the quasar population evolves as log[M_BH(max)/M_Sun]~0.3z+9, while the maximum Eddington ratio (~0.45) is practically independent of cosmic time. These results are unaffected by the Malmquist bias.