The VLA-COSMOS 3 GHz Large Project: Star formation properties and radio luminosity functions of AGN with moderate-to-high radiative luminosities out to z ∼ 6

Abstract

We have studied a sample of 1604 moderate-to-high radiative luminosity active galactic nuclei (HLAGN) selected at 3 GHz within the VLA-COSMOS 3 GHz Large Project. These were classified by combining multiple AGN diagnostics: X-ray data, mid-infrared data and broadband spectral energy distribution fitting. We decomposed the total radio 1.4 GHz luminosity (L1:4 GHz, TOT) into the emission originating from star formation and AGN activity by measuring the excess in L1:4 GHz, TOT relative to the infrared-radio correlation of star-forming galaxies. To quantify the excess, for each source we calculated the AGN fraction ( fAGN) defined as the fractional contribution of AGN activity to L1:4 GHz, TOT. The majority of the HLAGN, (68:0 1:5)%, are dominated by star-forming processes ( fAGN 0:5), while (32:01:5)% are dominated by AGN-related radio emission (0:5 fAGN 1).We used the AGN-related 1.4 GHz emission to derive the 1.4 GHz AGN luminosity functions of HLAGN. By assuming pure density and pure luminosity evolution models we constrained their cosmic evolution out to z 6, finding (z) / (1+z)(2:640:10)+(0:610:04)z and L(z) / (1+z)(3:970:15)+(0:920:06)z. These evolutionary laws show that the number and luminosity density of HLAGN increased from higher redshifts (z 6) up to a maximum in the redshift range 1 z 2:5, followed by a decline toward local values. By scaling the 1.4 GHz AGN luminosity to kinetic luminosity using the standard conversion, we estimate the kinetic luminosity density as a function of redshift. We compare our result to the semi-analytic models of radio mode feedback, and find that this feedback could have played an important role in the context of AGN-host co-evolution in HLAGN which shows evidence of AGN-related radio emission ( fAGN 0).