Radio faint AGN: a tale of two populations
  We study the Extended Chandra Deep Field South (E-CDFS) Very Large Array
sample, which reaches a flux density limit at 1.4 GHz of 32.5 microJy at the
field centre and redshift ~ 4, and covers ~ 0.3 deg^2. 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, characterised by very different
evolutions, luminosity functions, and Eddington ratios: radio-quiet
(RQ)/radiative-mode, and radio-loud/jet-mode AGN. The radio power of RQ AGN
evolves ~ (1+z)^2.5, similarly to star-forming galaxies, while the number
density of radio-loud ones has a peak at ~ 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_1.4GHz ~ 10^24 W/Hz to ~ 0.04$ at P_1.4GHz ~ 10^22 W/Hz. 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 radio-quiet AGN overtake radio-loud ones.
