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Published November 2011 | Published
Journal Article Open

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


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 × 10^(45) 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_⋆ > 10^(10)M_⊙) 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).

Additional Information

© 2011 ESO. Received 14 May 2011. Accepted 14 September 2011. Published online: 11 November 2011. We are grateful to Hagai Netzer for inspiring discussions and to Giulia Rodighiero for providing the Herschel curves in Fig. 9 in electronic format. We thank the referee, Montse Villar-Martin, for a careful reading of the manuscript and useful comments. This work is based on observations made with ESO Telescopes at the La Silla/Paranal Observatories under the zCOSMOS Large Programme 175.A-0839; the XMM-Newton satellite, an ESA science mission with instruments and contributions directly funded by ESA Member States and the US (NASA); the Magellan Telescope, operated by the Carnegie Observatorie and the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution; the Subaru Telescope, operated by the National Astronomical Observatory of Japan; and the NASA/ESA Hubble Space Telescope, operated at the Space Telescope Science Institute, which is operated by AURA Inc, under NASA contract NAS 5-26555. Support from the Italian Space Agency (ASI) under the contracts ASI-INAF I/088/06/0 and I/009/10/0 is acknowledged. G.H. and M.S. acknowledge support by the German Deutsche Forschungsgemeinschaft, DFG Leibniz Prize (FKZ HA 1850/28-1).

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