Obscuration-dependent Evolution of Active Galactic Nuclei

Abstract

We aim to constrain the evolution of active galactic nuclei (AGNs) as a function of obscuration using an X-ray-selected sample of ~2000 AGNs from a multi-tiered survey including the CDFS, AEGIS-XD, COSMOS, and XMM-XXL fields. The spectra of individual X-ray sources are analyzed using a Bayesian methodology with a physically realistic model to infer the posterior distribution of the hydrogen column density and intrinsic X-ray luminosity. We develop a novel non-parametric method that allows us to robustly infer the distribution of the AGN population in X-ray luminosity, redshift, and obscuring column density, relying only on minimal smoothness assumptions. Our analysis properly incorporates uncertainties from low count spectra, photometric redshift measurements, association incompleteness, and the limited sample size. We find that obscured AGNs with N_H > 10^(22) cm^(–2) account for 77^(+4_(-5)% of the number density and luminosity density of the accretion supermassive black hole population with L_X > 10^(43) erg s^(–1), averaged over cosmic time. Compton-thick AGNs account for approximately half the number and luminosity density of the obscured population, and 38^(+8)_(-7)% of the total. We also find evidence that the evolution is obscuration dependent, with the strongest evolution around N_H ≈ 10^(23) cm^(–2). We highlight this by measuring the obscured fraction in Compton-thin AGNs, which increases toward z ~ 3, where it is 25% higher than the local value. In contrast, the fraction of Compton-thick AGNs is consistent with being constant at ≈35%, independent of redshift and accretion luminosity. We discuss our findings in the context of existing models and conclude that the observed evolution is, to first order, a side effect of anti-hierarchical growth.