Radio-faint AGN: a tale of two populations

Abstract

We study the Extended Chandra Deep Field-South Very Large Array sample, which reaches a flux density limit at 1.4 GHz of 32.5 μJy at the field centre and redshift ̃4, and covers ̃0.3 deg<SUP>2</SUP>. Number counts are presented for the whole sample while the evolutionary properties and luminosity functions are derived for active galactic nuclei (AGN). The faint radio sky contains two totally distinct AGN populations, characterized by very different evolutions, luminosity functions, and Eddington ratios: radio-quiet (RQ)/radiative-mode and radio-loud (RL)/jet-mode AGN. The radio power of RQ AGN evolves ̃ ∝(1+z)^{2.5}, similarly to star-forming galaxies, while the number density of RL ones has a peak at z ̃ 0.5 and then declines at higher redshifts. The number density of radio-selected RQ AGN is consistent with that of X-ray selected AGN, which shows that we are sampling the same population. The unbiased fraction of radiative-mode RL AGN, derived from our own and previously published data, is a strong function of radio power, decreasing from ̃0.5 at P<SUB>1.4 GHz</SUB> ̃ 10<SUP>24</SUP> W Hz<SUP>-1</SUP> to ̃0.04 at P<SUB>1.4 GHz</SUB> ̃ 10<SUP>22</SUP> W Hz<SUP>-1</SUP>. Thanks to our enlarged sample, which now includes ̃700 radio sources, we also confirm and strengthen our previous results on the source population of the faint radio sky: star-forming galaxies start to dominate the radio sky only below ̃0.1 mJy, which is also where RQ AGN overtake RL ones.