Black hole accretion and host galaxies of obscured quasars in XMM-COSMOS^⋆

¹ ESO, Karl-Schwarschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: vmainier@eso.org
² Max-Planck-Institute für Extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
³ Excellence Cluster Universe, TUM, Boltzmannstr. 2, 85748 Garching bei München, Germany
⁴ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁵ ICREA and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
⁶ Space Telescope Science Institute, Baltimore, Maryland 21218, USA
⁷ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁸ Institute for the Physics and Mathematics of the Universe (IPMU), University of Tokyo, Kashiwanoha 5-1-5, Kashiwa-shi, Chiba 277-8568, Japan
⁹ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹⁰ Laboratoire d’Astrophysique de Marseille, Marseille, France
¹¹ Dipartimento di Astronomia, Universita‘ di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹² Max-Planck-Institute für Plasma Physis, Boltzmann Strasse 2, 85748 Garching bei München, Germany
¹³ Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
¹⁴ Dipartimento di Astronomia, Universita di Padova, Padova, Italy
¹⁵ INAF–IASF Milano, Milan, Italy
¹⁶ SUPA, Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
¹⁷ INAF – Osservatorio Astronomico di Brera, Milan, Italy
¹⁸ Max-Planck-Institute für Astrophysik, 85748 Garching bei München, Germany
¹⁹ Dipartimento di Astronomia, Universita di Padova, Padova, Italy
²⁰ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
²¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
²² Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
²³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio-Catone, Italy
²⁴ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
²⁵ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
²⁶ AIM, Unité Mixte deRecherche CEA CNRS, Université Paris VII, UMR n158, Paris France
²⁷ CEA-Saclay, Service d’Astrophysique, Orme des Merisiers, Bât.709, 91191 Gif-sur-Yvette, France

Received: 14 May 2011
Accepted: 14 September 2011

Abstract

Aims. We explore the connection between black hole growth at the center of obscured quasars selected from the XMM-COSMOS survey and the physical properties of their host galaxies. We study a bolometric regime ( ⟨ L_bol ⟩  = 8  ×  10⁴⁵ erg s^-1) where several theoretical models invoke major galaxy mergers as the main fueling channel for black hole accretion.

Methods. To derive robust estimates of the host galaxy properties, we use an SED fitting technique to distinguish the AGN and host galaxy emission. We evaluate the effect on galaxy properties estimates of being unable to remove the nuclear emission from the SED. The superb multi-wavelength coverage of the COSMOS field allows us to obtain reliable estimates of the total stellar masses and star formation rates (SFRs) of the hosts. We supplement this information with a morphological analysis of the ACS/HST images, optical spectroscopy, and an X-ray spectral analysis.

Results. We confirm that obscured quasars mainly reside in massive galaxies (M _⋆  > 10¹⁰M_⊙) and that the fraction of galaxies hosting such powerful quasars monotonically increases with the stellar mass. We stress the limitation of the use of rest-frame color − magnitude diagrams as a diagnostic tool for studying galaxy evolution and inferring the influence that AGN activity can have on such a process. We instead use the correlation between SFR and stellar mass found for star-forming galaxies to discuss the physical properties of the hosts. We find that at z ~ 1,  ≈62% of Type-2 QSOs hosts are actively forming stars and that their rates are comparable to those measured for normal star-forming galaxies. The fraction of star-forming hosts increases with redshift:  ≈ 71% at z ~ 2, and 100% at z ~ 3. We also find that the evolution from z ~ 1 to z ~ 3 of the specific SFR of the Type-2 QSO hosts is in excellent agreement with that measured for star-forming galaxies. From the morphological analysis, we conclude that most of the objects are bulge-dominated galaxies, and that only a few of them exhibit signs of recent mergers or disks. Finally, bulge-dominated galaxies tend to host Type-2 QSOs with low Eddington ratios (λ < 0.1), while disk-dominated or merging galaxies have at their centers BHs accreting at high Eddington ratios (λ > 0.1).