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 mu m silicate feature in absorption and have extreme MIR luminosities [ L( 6 mu m) similar or equal to 1046 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 mu m) > 10(12) L-circle dot and z = 1.3-3.0 to be about 17-22 and 11.7 deg(-2), respectively. Overall we find that similar to 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 x 10(12) L-circle dot, implying SFRs of 600-3000 M-circle dot yr(-1). In the whole sample, the average contribution from a starburst component to the bolometric luminosity, as estimated from the PAH 7.7 mu m luminosity in the composite IR spectra, is <= 20% of the total bolometric luminosity.