Enhancement of AGN in a protocluster at z=1.6

Abstract: We investigate the prevalence of AGN in the high-redshift protocluster $\rm{Cl}\,0218.3$-$0510$ at $z=1.62$. Using imaging from the Chandra X-ray Telescope, we find a large overdensity of AGN in the protocluster; a factor of $23\pm9$ times the field density of AGN. Only half of this AGN overdensity is due to the overdensity of massive galaxies in the protocluster (a factor of $11\pm2$), as we find that $17^{+6}_{-5}\%$ of massive galaxies ($M_* > 10^{10}\,\rm{M}_{\odot}$) in the protocluster host an X-ray luminous AGN, compared to $8\pm1\%$ in the field. This corresponds to an enhancement of AGN activity in massive protocluster galaxies by a factor of $2.1\pm0.7$ at $1.6\sigma$ significance. We also find that the AGN overdensity is centrally concentrated, located within 3 arcmin and most pronounced within 1 arcmin of the centre of the protocluster. Our results confirm that there is a reversal in the local anti-correlation between galaxy density and AGN activity, so there is an enhancement of AGN in high-redshift protoclusters. We compare the properties of AGN in the protocluster to the field and find no significant differences in the distributions of their stellar mass, X-ray luminosity, or hardness ratio. We therefore suggest that triggering mechanisms are similar in both environments, and that the mechanisms simply occur more frequently in denser environments.