A High Fraction of Heavily X-Ray-obscured Active Galactic Nuclei

Abstract

We present new estimates on the fraction of heavily X-ray-obscured, Compton-thick (CT) active galactic nuclei (AGNs) out to a redshift of z ≤ 0.8. From a sample of 540 AGNs selected by mid-infrared (MIR) properties in observed X-ray survey fields, we forward model the observed-to-intrinsic X-ray luminosity ratio () with a Markov Chain Monte Carlo simulation to estimate the total fraction of CT AGNs (f CT), many of which are missed in typical X-ray observations. We create model N H distributions and convert these to using a set of X-ray spectral models. We probe the posterior distribution of our models to infer the population of X-ray-nondetected sources. From our simulation we estimate a CT fraction of f CT = . We perform an X-ray stacking analysis for sources in Chandra X-ray Observatory fields and find that the expected soft (0.5–2 keV) and hard (2–7 keV) observed fluxes drawn from our model to be within 0.48 and 0.12 dex of our stacked fluxes, respectively. Our results suggests at least 50% of all MIR-selected AGNs, possibly more, are CT (N H ≳ 1024 cm−2), which is in excellent agreement with other recent work using independent methods. This work indicates that the total number of AGNs is higher than can be identified using X-ray observations alone, highlighting the importance of a multiwavelength approach. A high f CT also has implications for black hole (BH) accretion physics and supports models of BH and galaxy coevolution that include periods of heavy obscuration.