Large-scale clustering of buried X-ray AGN: Trends in AGN obscuration and redshift evolution

Abstract

In order to test active galactic nucleus (AGN) unification and evolutionary models, we measured the AGN clustering properties as a function of AGN obscuration defined in terms of hydrogen column density, $N_{\rm H}$. In addition to measuring the clustering of unobscured ($N_{\rm H} < 10^{22}\,{\rm cm}^{-2}$) and moderately obscured ($10^{22} \leq N_{\rm H} < 10^{23.5}$) AGNs, we also targeted highly obscured sources ($N_{\rm H}\geq 10^{23.5}$) up to redshifts of $z=3$. We have compiled one of the largest samples of X-ray-selected AGNs from a total of eight deep XMM/Chandra surveys. We measured the clustering as a function of both AGN obscuration and redshift using the projected two-point correlation function, $w_{\rm p}(r_{\rm p})$. We modeled the large-scale clustering signal, measured the AGN bias, $b(z, N_{\rm H})$, and interpreted it in terms of the typical AGN host dark matter halo, $M_{\rm halo}(z, N_{\rm H}$). We find no significant dependence of AGN clustering on obscuration, suggesting similar typical masses of the hosting halos as a function of $N_{\rm H}$. This result matches expectations of AGN unification models, in which AGN obscuration depends mainly on the viewing angle of the obscuring torus. We measured, for the first time, the clustering of highly obscured AGNs and find that these objects reside in halos with typical mass $\log M_{\rm halo} = 12.98_{-0.22}^{+0.17} [h^{-1} M_\odot]$ ($12.28_{-0.19}^{+0.13}$) at low $z \sim 0.7$ (high $z \sim 1.8$) redshifts. We find that irrespective of obscuration, an increase in AGN bias with redshift is slower than the expectation for a constant halo mass and instead follows the growth rate of halos, known as the passive evolution track. This implies that for those AGNs the clustering is mainly driven by the mass growth rate of the hosting halos and galaxies across cosmic time.