The Environments of Active Galactic Nuclei within the zCOSMOS Density Field

The impact of environment on active galactic nucleus (AGN) activity up to z ~ 1 is assessed by utilizing a mass-selected sample of galaxies from the 10k catalog of the zCOSMOS spectroscopic redshift survey. We identify 147 AGN by their X-ray emission as detected by XMM-Newton from a parent sample of 7234 galaxies. We measure the fraction of galaxies with stellar mass M _* > 2.5 × 10¹⁰ M _sun that host an AGN as a function of local overdensity using the 5th, 10th, and 20th nearest neighbors that cover a range of physical scales (~1-4 Mpc). Overall, we find that AGNs prefer to reside in environments equivalent to massive galaxies with substantial levels of star formation. Specifically, AGNs with host masses between 0.25 and 1 × 10¹¹ M _sun span the full range of environments (i.e., field to group) exhibited by galaxies of the same mass and rest-frame color or specific star formation rate. Host galaxies having M _* > 10¹¹ M _sun clearly illustrate the association with star formation since they are predominantly bluer than the underlying galaxy population and exhibit a preference for lower-density regions analogous to Sloan Digital Sky Survey studies of narrow-line AGN. To probe the environment on smaller physical scales, we determine the fraction of galaxies (M _* > 2.5 × 10¹⁰ M _sun) hosting AGNs inside optically selected groups, and find no significant difference with field galaxies. We interpret our results as evidence that AGN activity requires a sufficient fuel supply; the probability of a massive galaxy to have retained some sufficient amount of gas, as evidence by its ongoing star formation, is higher in underdense regions where disruptive processes (i.e., galaxy harassment, tidal stripping) are lessened.