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Published March 10, 2008 | Published
Journal Article Open

Obscuration in extremely luminous quasars

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.

Additional Information

© 2008 The American Astronomical Society. Received 2007 April 11; accepted 2007 September 26. We gratefully acknowledge the anonymous referee for a careful reading of the manuscript and for useful comments and suggestions that improved the paper. M. P. kindly thanks R.Maiolino, N. Levenson, D. J. Axon, and E. Treister for stimulating discussions, B. Siana for providing the optical spectral classification, L. Hao for providing the data used in Figure 13 in machinereadable format, J. Surace for help with the IRAC measurements, M. Lacy for providing information on the latest FLS catalog, S. Croom for his advice on estimating the surface density of unobscured type 1 QSOs, and A. Afonso-Luis for providing information on the SWIREMIPS 70 and 160 m observations. M. P. acknowledges financial support from the Marie-Curie Fellowship grant MEIF-CT-2007-042111. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided byNASA through an award issued by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract1407. Support for this work by the IRS GTO team at Cornell University was provided by NASA through contract 1257184 issued by JPL/Caltech. This work is dedicated to the memory of Harding Eugene (Gene) Smith.

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