November 2013 • 2013ApJ...777....6S
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 rest2-10 keV > 1042 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 (>1043, 1044, and 1045 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 2-10 keV > 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 > 1044 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 rest2-10 keV > 1042 erg s-1 or f 2-10 keV > 10-17 erg s-1 cm-2. Our model confirms that heavily obscured AGNs (N H I > 1023 cm-2) can be separated from unobscured and mildly obscured ones (N H I < 1023 cm-2) in the plane of observed frame X-ray hardness versus mid-IR/X-ray ratio..