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

Host galaxies properties of obscured AGN

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

Abstract

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 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 ( > 10¹⁰ ) and, in 50% of the cases, are also actively forming stars (1/SSFR < ) in dusty environments. The median  value of the active nucleus is between 2% and 10% depending on the assumed / ratio. Finally, we found that the X-ray selected AGN fraction increases with the stellar mass up to a value of ~30% at and > 3  10¹¹ , a fraction significantly higher than in the local Universe for AGN of similar luminosities.