The VLA Survey of the Chandra Deep Field South. V. Evolution and
  Luminosity Functions of sub-mJy radio sources and the issue of radio emission
  in radio-quiet AGN
  We present the evolutionary properties and luminosity functions of the radio
sources belonging to the Chandra Deep Field South VLA survey, which reaches a
flux density limit at 1.4 GHz of 43 microJy at the field center and redshift
~5, and which includes the first radio-selected complete sample of radio-quiet
active galactic nuclei (AGN). We use a new, comprehensive classification scheme
based on radio, far- and near-IR, optical, and X-ray data to disentangle
star-forming galaxies from AGN and radio-quiet from radio-loud AGN. We confirm
our previous result that star-forming galaxies become dominant only below 0.1
mJy. The sub-mJy radio sky turns out to be a complex mix of star-forming
galaxies and radio-quiet AGN evolving at a similar, strong rate; non-evolving
low-luminosity radio galaxies; and declining radio powerful (P > 3 10^24 W/Hz)
AGN. Our results suggest that radio emission from radio-quiet AGN is closely
related to star formation. The detection of compact, high brightness
temperature cores in several nearby radio-quiet AGN can be explained by the
co-existence of two components, one non-evolving and AGN-related and one
evolving and star-formation-related. Radio-quiet AGN are an important class of
sub-mJy sources, accounting for ~30% of the sample and ~60% of all AGN, and
outnumbering radio-loud AGN at < 0.1 mJy. This implies that future, large area
sub-mJy surveys, given the appropriate ancillary multi-wavelength data, have
the potential of being able to assemble vast samples of radio-quiet AGN
by-passing the problems of obscuration, which plague the optical and soft X-ray
bands.
