Black hole growth and starburst activity at z = 0.6–4 in the Chandra Deep Field South - Host galaxies properties of obscured AGN

摘要

Aims. The co-evolution of host galaxies and the active blackholes which reside in their centre is one of the most important topicsin modern observational cosmology. Here we present a study of theproperties of obscured active galactic nuclei (AGN) detected in theCDFS 1Ms observation and their host galaxies. Methods. We limited the analysis to the MUSIC area, for which deep K-bandobservations obtained with ISAAC@VLT are available, ensuring accurateidentifications of the counterparts of the X-ray sources as well asreliable 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 of179sources in the MUSIC area detected in the Chandra observation;2)studied the host galaxies observed and rest frame colors andproperties. Results. We found that X-ray selected (ergs-1)AGN show Spitzer colors consistent with both AGN and starburstdominated infrared continuum; the latter would not have been selectedas AGN from infrared diagnostics. The host galaxies of X-ray selectedobscured AGN are all massive (10^{10}~M_odot$ -->)and, in50% 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 /M*ratio. Finally, we found that the X-ray selected AGNfraction increases with the stellar mass up to a value of % at z>1 and M*>3,a fraction significantly higher than in the local Universe for AGN of similar luminosities. Key words: galaxies: active - galaxies: starburst - X-rays: galaxies