2013ApJ...777....6S

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, the CT AGN density predicted by our model is a few x10^–4/pc3 from z = 0 up to z = 3. CT AGNs with higher luminosity cuts (>10⁴³, 10⁴⁴, and 10⁴⁵ erg/s) 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/cm2 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 or f_2-10 keV_> 10^–15 erg/s/cm2) 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 or f_2-10 keV_> 10^–17 erg/s/cm2. Our model confirms that heavily obscured AGNs (N_ H I_> 10²³/cm2) can be separated from unobscured and mildly obscured ones (N_ H I_< 10²³/cm2) in the plane of observed frame X-ray hardness versus mid-IR/X-ray ratio.