Cosmic evolution of radio selected active galactic nuclei in the COSMOS
  field
  We explore the cosmic evolution of radio AGN with low radio powers (L_1.4GHz
< 5\times10^25 W/Hz) out to z=1.3 using to-date the largest sample of ~600 low
luminosity radio AGN at intermediate redshift drawn from the VLA-COSMOS survey.
We derive the radio luminosity function for these AGN, and its evolution with
cosmic time assuming two extreme cases: i) pure luminosity and ii) pure density
evolution. The former and latter yield L_*\propto(1+z)^(0.8+/-0.1), and
Phi*\propto (1+z)^(1.1+/-0.1), respectively, both implying a fairly modest
change in properties of low radio-power AGN since z=1.3. We show that this is
in stark contrast with the evolution of powerful (L_1.4GHz > 5\times10^25 W/Hz)
radio AGN over the same cosmic time interval, constrained using the 3CRR, 6CE,
and 7CRS radio surveys by Willott et al. (2001). We demonstrate that this can
be explained through differences in black hole fueling and triggering
mechanisms, and a dichotomy in host galaxy properties of weak and powerful AGN.
Our findings suggest that high and low radio-power AGN activity is triggered in
different stages during the formation of massive red galaxies. We show that
weak radio AGN occur in the most massive galaxies already at z~1, and they may
significantly contribute to the heating of their surrounding medium and thus
inhibit gas accretion onto their host galaxies, as recently suggested for the
`radio mode' in cosmological models.
