A Joint Model of X-Ray and Infrared Backgrounds. II. Compton-thick Active Galactic Nucleus Abundance

Abstract

We estimate the abundance of Compton-thick (CT) active galactic nuclei (AGNs) based on our joint model of X-ray and infrared backgrounds. At L_(rest2-10keV) > 10^(42) erg s^(–1), the CT AGN density predicted by our model is a few ×10^(–4) Mpc^(–3) from z = 0 up to z = 3. CT AGNs with higher luminosity cuts (>10^(43), 10^(44), and 10^(45) erg s^(–1)) peak at higher redshift and show a rapid increase in number density from z = 0 to z ~ 2-3. The CT AGN to all AGN ratio appears to be low (2%-5%) at f_(2-10keV) > 10^(–15) erg s^(–1) cm^(–2) but rises rapidly toward fainter flux levels. The CT AGNs account for ~38% of the total accreted supermassive black hole mass and contribute ~25% of the cosmic X-ray background spectrum at 20 keV. Our model predicts that the majority (90%) of luminous and bright CT AGNs (L_(rest2-10keV) > 10^(44) erg s^(–1) or f_(2-10 keV) > 10^(–15) erg s^(–1) cm^(–2)) have detectable hot dust 5-10 μm emission, which we associate with a dusty torus. The fraction drops for fainter objects, to around 30% at L_(rest2-10keV) > 10^(42) erg s^(–1) or f_(2-10keV) > 10^(–17) erg s^(–1) cm^(–2). Our model confirms that heavily obscured AGNs (N_(HI) > 10^(23) cm^(–2)) can be separated from unobscured and mildly obscured ones (N_(HI) < 10^(23) cm^(–2)) in the plane of observed frame X-ray hardness versus mid-IR/X-ray ratio.

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