The Type II AGN-host galaxy connection

Abstract

We present a study of optically selected Type II active galactic nuclei (AGN) at 0.5 < z < 0.9 from the VIPERS and VVDS surveys, to investigate the connection between AGN activity and the physical properties of their host galaxies. The host stellar mass is estimated through spectral energy distribution fitting with the CIGALE code, and star formation rates are derived from the [OII]λ3727 Å line luminosity. We find that 49% of the AGN host galaxies are on or above the main sequence (MS), 40% lie in the sub-MS locus, and 11% in the quiescent locus. Using the [OIII]λ5007 Å line luminosity as a proxy of the AGN power, we find that at fixed AGN power Type II AGN host galaxies show a bimodal behaviour: systems with host galaxy stellar mass < 1010 M⊙ reside along the MS or in the starbursts locus (high-SF Type II AGN), while systems residing in massive host galaxies (> 1010 M⊙) show a lower level of star formation (low-SF Type II AGN). At all stellar masses the offset from the MS is positively correlated with the AGN power. We interpret this correlation as evidence of co-evolution between the AGN and the host, possibly due to the availability of cold gas. In the most powerful AGN with host galaxies below the MS we find a hint, though weak, of asymmetry in the [OIII] line profile, likely due to outflowing gas, consistent with a scenario in which AGN feedback removes the available gas and halts the star formation in the most massive hosts.