  We investigate the properties and the environment of radio sources with
optical counterpart from the combined VLA-COSMOS and zCOSMOS samples. The
advantage of this sample is the availability of optical spectroscopic
information, high quality redshifts, and accurate density determination.
  By comparing the star formation rates estimated from the optical spectral
energy distribution with those based on the radio luminosity, we divide the
radio sources in three families, passive AGN, non-passive AGN and star forming
galaxies. These families occupy specific regions of the 8.0-4.5 $\mu$m infrared
color--specific star formation plane, from which we extract the corresponding
control samples.
  Only the passive AGN have a significantly different environment distribution
from their control sample. The fraction of radio-loud passive AGN increases
from ~2% in underdense regions to ~15% for overdensities (1+delta) greater than
10. This trend is also present as a function of richness of the groups hosting
the radio sources. Passive AGN in overdensities tend to have higher radio
luminosities than those in lower density environments. Since the black hole
mass distribution is similar in both environments, we speculate that, for low
radio luminosities, the radio emission is controlled (through fuel
disponibility or confinement of radio jet by local gas pressure) by the
interstellar medium of the host galaxy, while in other cases it is determined
by the structure (group or cluster) in which the galaxy resides.
