Properties and environment of radio-emitting galaxies in the VLA-zCOSMOS survey

摘要

Aims. We investigate the properties and the environment of radio sources with optical counterparts from the combined VLA-COSMOS andudzCOSMOS samples. The advantage of this sample is the availability of optical spectroscopic informations, high quality redshifts, and accurateuddensity determination.udMethods. By comparing the star formation rates estimated from the optical spectral energy distribution with those based on the radio luminosity,udwe divide the radio sources in to three families passive AGN, non-passive AGN, and star-forming galaxies. These families occupy specific regionsudof the 8.0−4.5 μm infrared color- specific star-formation plane, from which we extract the corresponding control samples.udResults. Only the passive AGN have a significantly different environmental distribution from their control sample. The fraction of radio-loududpassive AGN increases from ~2% in underdense regions to ~15% for overdensities (1 + δ) greater than 10. This trend is also present as a functionudof richness of the groups hosting the radio sources. Passive AGN in overdensities tend to have higher radio luminosities than those in lower densityudenvironments. Since the black hole mass distribution is similar in both environments, we speculate that, for low radio luminosities, the radioudemission is controlled (by fuel availability or confinement of the radio jet by local gas pressure) by the interstellar medium of the host galaxy,udwhile in other cases it is determined by the structure (group or cluster) in which the galaxy resides.