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 1⁄4 1:3Y3 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 m silicate feature in absorption and have extremeMIR luminosities [L(6 m) ’ 10 ergs s ]. 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 QSOMIR luminosity function (Brown and coworkers) and on a color-selected sample of AGNs, we estimate the surface densities of obscured and unobscured QSOswith L(6 m) > 10 L and z 1⁄4 1:3Y3:0 to be about 17Y22 and 11.7 deg , respectively. Overall we find that 35%Y41% of luminous QSOs are unobscured, 37%Y40% are obscured by the torus, and 23%Y25% 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 L , implying SFRs of 600Y3000 M yr . In the whole sample, the average contribution from a starburst component to the bolometric luminosity, as estimated from the PAH 7.7 m luminosity in the composite IR spectra, is 20% of the total bolometric luminosity. Subject headinggs: galaxies: active — galaxies: high-redshift — infrared: galaxies — quasars: general Online material: color figures