Obscured QSOs and the X-ray Background |
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| Institution: | 1. Department of Physics, University of Durham, South Road, Durham, DH1 3LE, England;1. Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany;2. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany;3. Centro Astronòmico Hispano Alemán de Calar Alto, E4004 Almeria, Spain;1. Department of Astronomy, University of Texas, Austin, TX 78712, USA;2. Laboratoire de l’Univers et de ses Théories, Observatoire de Paris, F-92195 Meudon Cedex, France;1. Landessternwarte Heidelberg, Königstuhl, D-69117 Heidelberg, Germany;2. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany;3. Universitätssternwarte Göttingen, Geismarlandstr. 11, D-37083 Göttingen, Germany;4. Department of Astronomy, The Ohio State University, 4055 McPherson Lab, 18th Avenue, Columbus, OH 43210, USA;1. Department of Physics and Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521, United States;2. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr. Honolulu, HI 96822, United States;3. Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, United States;4. Spitzer Science Center, Caltech, MS220-6, 1200 E. California Boulevard, Pasadena, CA 91125, United States;1. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany;2. Univesitätssternwarte, Scheinerstr. 1, 81679, München, Germany;3. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA;4. Department of Astronomy, University of Maryland, College Park, MD 20742, USA;5. Jansky Fellow, National Radio Astronomy Observatory, Department of Astronomy, University of Maryland, College Park, MD 20742, USA;1. Dipartimento di Fisica, Università di Roma “La Sapienza”, p.le A. Moro 5, I-00185 Rome, Italy;2. Dipartimento di Fisica, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Rome, Italy |
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| Abstract: | The question of whether there exists a large population of dust obscured QSOs is currently very controversial. In favour of this hypothesis are models for the origin of the X-ray Background (XRB) and also the Unified Model of AGN which both invoke large populations of obscured QSOs. For example, Madau et al. (1994) suggest a population of QSOs with NH ∼ 1024 cm−2 or AV = 1000m to improve the fit to the XRB between 1 < E < 100 keV. Arguments contradicting this theory include those of Boyle & di Matteo (1995) who claim that the tight X-ray/optical flux ratio relation for QSOs precludes the existence of a large population of objects obscured by significant amounts of intrinsic dust. Here, we follow Madau et al. (1994) and Comastri et al. (1995) to make fits to the XRB using obscured QSO populations and investigate whether selection effects may allow a tight distribution of X-ray/optical ratios to be maintained. We find that even for a flat distribution of absorbing columns, reasonable fits to the XRB can be obtained while both optical and X-ray absorption combine to produce the tight observed X-ray/optical correlation. |
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