Obscuration in Extremely Luminous Quasars
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Abstract
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.