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.

On R − W1 as a Diagnostic to Discover Obscured Active Galactic Nuclei in Wide-Area X-Ray Surveys

“…Similar results have been found in past X-ray surveys that used gas column density (N H ) as an indicator of obscuration (Ueda et al 2003;Hasinger 2008;Lanzuisi et al 2013;Merloni et al 2014;Ueda et al 2014;Buchner et al 2015).…”

The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample

“…Given that 80% of the sources are at z 1  and that 90% of the contribution to the high-luminosity bin comes from sources at z 0.3 1.1 » -, we can compare our results with recent determinations at similar redshifts. We find broad consistency with the estimated fractions for Compton-thin sources reported by Ueda et al (2014) and Buchner et al (2015) for z 0.1 1 = -and z 0.5 0.75 = -, respectively. To give an idea of the variance of our results on the adopted binning, we adjusted the bin width in order to include up to eight more sources (thus increasing the number of sources by ∼25%) and reported the range of the corresponding variations as gray regions.…”

“…-) AGN population and further shed light on the known decreasing trend between the numerical relevance of this population compared to all AGN (absorbed fraction) and the source luminosity (Lawrence & Elvis 1982;Gilli et al 2007;Burlon et al 2011;Buchner et al 2015) and its redshift evolution (La Franca et al 2005;Ballantyne et al 2006;Treister & Urry 2006;Aird et al 2015a;Buchner et al 2015;Liu et al 2017). They also allow exploration of the importance of the CT population, although with different constraining power and different non-negligible degrees of bias-especially at the highest column densities and lowest luminosities (e.g., Burlon et al 2011;Brightman et al 2014;Buchner et al 2015;Lanzuisi et al 2015;Ricci et al 2015).…”

“…Black indicates the absorbed fraction estimated from our sample. Blue, green, and orange are absorbed fractions obtained by the spectroscopic sample in Liu et al (2017), U14, and Buchner et al (2015), respectively (the latter reported at L log erg s 43.744.1…”

“…They also allow exploration of the importance of the CT population, although with different constraining power and different non-negligible degrees of bias-especially at the highest column densities and lowest luminosities (e.g., Burlon et al 2011;Brightman et al 2014;Buchner et al 2015;Lanzuisi et al 2015;Ricci et al 2015). Indeed, the large diversity in the spectral shapes, as well as poorly explored observational parameters in low counting regimes 26 such as the high energy cut-off and the reflection strength at high energies (Treister et al 2009;Ballantyne et al 2011, hereafter BA11), the scattered fractions at low energies (Brightman & Ueda 2012;Lanzuisi et al 2015), or physical parameters such as the Eddington ratio (Draper & Ballantyne 2010), may further introduce uncertainty or biases, enlarging the possible range of the fraction of CT sources to an order of magnitude (Akylas et al 2012) or even significantly reducing their importance (Gandhi et al 2007).…”