EDP Sciences
Free Access
Volume 507, Number 3, December I 2009
Page(s) 1277 - 1289
Section Cosmology (including clusters of galaxies)
DOI https://doi.org/10.1051/0004-6361/200912261
Published online 01 October 2009
A&A 507, 1277-1289 (2009)
DOI: 10.1051/0004-6361/200912261

Black hole growth and starburst activity at z = 0.6–4 in the Chandra Deep Field South

Host galaxies properties of obscured AGN
M. Brusa1, F. Fiore2, P. Santini2, A. Grazian2, A. Comastri3, G. Zamorani3, G. Hasinger1, 4, A. Merloni1, 5, F. Civano6, A. Fontana2, and V. Mainieri7

1  Max-Planck-Institute for extraterrestrial Physik, Giessenbachstrasse, 1, 85748 Garching bei München, Germany
    e-mail: marcella@mpe.mpg.de
2  INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio-Catone (Roma) 00040, Italy
3  INAF – Osservatorio Astronomico di Bologna, via Ranzani 1 40127 Bologna, Italy
4  Max-Planck-Institute for Plasma Physics, Boltzmannstrasse 2, 85748 Garching bei München, Germany
5  Excellence Cluster Universe, Boltzmannstrasse 2, 85748 Garching bei München, Germany
6  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7  European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching bei München, Germany

Received 2 April 2009 / Accepted 22 September 2009

Aims. The co-evolution of host galaxies and the active black holes which reside in their centre is one of the most important topics in modern observational cosmology. Here we present a study of the properties of obscured active galactic nuclei (AGN) detected in the CDFS 1 Ms observation and their host galaxies.
Methods. We limited the analysis to the MUSIC area, for which deep K-band observations obtained with ISAAC@VLT are available, ensuring accurate identifications of the counterparts of the X-ray sources as well as reliable determination of photometric redshifts and galaxy parameters, such as stellar masses and star formation rates. In particular, we: 1) refined the X-ray/infrared/optical association of 179 sources in the MUSIC area detected in the Chandra observation; 2) studied the host galaxies observed and rest frame colors and properties.
Results. We found that X-ray selected (LX $\ga$ 1042 erg s-1) AGN show Spitzer colors consistent with both AGN and starburst dominated infrared continuum; the latter would not have been selected as AGN from infrared diagnostics. The host galaxies of X-ray selected obscured AGN are all massive ($M_{\ast}$ > 1010 $M_\odot$) and, in 50% of the cases, are also actively forming stars (1/SSFR < tHubble) in dusty environments. The median L/LEdd value of the active nucleus is between 2% and 10% depending on the assumed MBH/$M_{\ast}$ ratio. Finally, we found that the X-ray selected AGN fraction increases with the stellar mass up to a value of ~30% at z > 1 and $M_{\ast}$ > 3 $\times$ 1011 $M_\odot$, a fraction significantly higher than in the local Universe for AGN of similar luminosities.

Key words: galaxies: active -- galaxies: starburst -- X-rays: galaxies

© ESO 2009

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