Obscuration in Extremely Luminous Quasars

The SEDs and IR spectra of a remarkable sample of obscured AGNs selected in the MIR are modeled with recent clumpy torus models. The sample contains 21 AGNs at z = 1. 3–3 discovered in the largest Spitzer surveys (SWIRE, NDWFS, and FLS) by means of their extremely red IR to optical colors. All sources show the 9. 7 μm silicate feature in absorption and have extreme MIR luminosities L (6 μ m) 10^ (46) ergs s^ (−1). The IR SEDs and spectra of 12 sources are well reproduced with a simple torus model, while the remaining nine sources require foreground extinction from a cold dust component to reproduce both the depth of the silicate feature and the NIR emission from hot dust. The best-fit torus models show a broad range of inclinations. Based on the unobscured QSO MIR luminosity function (Brown and coworkers) and on a color-selected sample of AGNs, we estimate the surface densities of obscured and unobscured QSOs with L (6 μ m) > 10^ (12) L☉ and z = 1. 3–3. 0 to be about 17-22 and 11. 7 deg^ (−2), respectively. Overall we find that ~35%-41% of luminous QSOs are unobscured, 37%-40% are obscured by the torus, and 23%-25% are obscured by a cold absorber detached from the torus. These fractions are consistent with a decrease of the torus covering fraction at large luminosities as predicted by receding torus models. An FIR component is observed in eight objects with luminosity greater than 3. 3 × 10^ (12) L☉, implying SFRs of 600–3000 M☉yr^ (−1). In the whole sample, the average contribution from a starburst component to the bolometric luminosity, as estimated from the PAH 7. 7 μm luminosity in the composite IR spectra, is ≤20% of the total bolometric luminosity.