The Deep SWIRE Field. IV. First properties of the sub-mJy galaxy
  population: redshift distribution, AGN activity and star formation
  We present a study of a 20cm selected sample in the Deep SWIRE VLA Field,
reaching a limiting flux density of ~13.5 uJy at the image center. In a 0.6x0.6
square degrees field, we are able to assign an optical/IR counterpart to 97% of
the radio sources. Up to 11 passbands from the NUV to 4.5um are then used to
sample the spectral energy distribution (SED) of these counterparts in order to
investigate the nature of the host galaxies. By means of an SED template
library and stellar population synthesis models we estimate photometric
redshifts, stellar masses, and stellar population properties, dividing the
sample in three sub-classes of quiescent, intermediate and star-forming
galaxies. We focus on the radio sample in the redshift range 0.3<z<1.3 where we
estimate to have a redshift completeness higher than 90%, and study the
properties and redshift evolution of these sub-populations. We find that, as
expected, the relative contributions of AGN and star-forming galaxies to the
uJy population depend on the flux density limit of the sample. At all flux
levels a significant population of "green-valley" galaxies is observed. While
the actual nature of these sources is not definitely understood, the results of
this work may suggest that a significant fraction of faint radio sources might
be composite (and possibly transition) objects, thus a simple "AGN vs
star-forming" classification might not be appropriate to fully understand what
faint radio populations really are.
