Faint radio-loud quasars: clues to their evolution

Abstract

The quasar sample selected by cross-correlating the FIRST and the 2dF Quasar Redshift Surveys allows us to explore, for the first time, the faint end of the radio and optical luminosity functions up to z = 2.2. We find indications (~3 \sigma) of a negative evolution for these faint sources at z > 1.8, both in radio and optical bands. This corresponds to a decrement in the space density of faint quasars of about a factor 2 at z=2.2 and confirms the presence of a differential evolution for the population of radio-active quasars. The faint end of both luminosity functions flattens and the comparison with the (optical) number density of the whole quasar population supports a dependence of the fraction of radio detected quasars on the optical luminosity. A progressive decrease in the fraction of quasars in the whole radio source population can be consistently accounted for within the `receding torus' scenario. The population of low luminosity quasars, which the FIRST-2dF detects, appears to depart from the `classical' scheme for radio-loud quasars.