2013A&A...552A.111P

We explore the AGN content in groups in the two GOODS fields (North and South) by exploiting the ultra-deep 2 and 4 Ms Chandra data and the deep multiwavelength observations from optical to mid-IR available for both fields. Determining the AGN content in structures of different mass/velocity dispersion and comparing them to higher mass/lower redshift analogs are important for understanding how the AGN formation process is related to environmental properties. We used our well-tested cluster finding algorithm to identify structures in the two GOODS fields, exploiting the available spectroscopic redshifts, as well as accurate photometric redshifts. We have identified 9 structures in GOODS-South (already presented in a previous paper) and 8 new structures in the GOODS-North field. We only considered structures where at least 2/3 of the members brighter than M_R=-20 have a spectroscopic redshift. We then checked whether any of the group members coincides with X-ray sources that belong to the 4 and 2Ms source catalogs. With a simple classification based on total rest-frame hard luminosity and hardness ratio we determine whether the X-ray emission originates in AGN activity or if it is more probably related to the galaxies' star-formation activity. We find that the fraction of AGN with Log L_H>42erg/s in galaxies with M_R←20 varies from less than 5% to 22%, with an average value of 6.3 ±1.3%, i.e., much higher than the value found for lower redshift groups of similar mass, which is just 1%. It is also more than double the fraction found for massive clusters at a similar high redshift (z∼1). We then explore the spatial distribution of AGN in the structures and find that they mainly populate the outer regions rather than the center. The colors of AGN host galaxies in structures tend to be confined to the green valley, thus avoiding the blue cloud and partially also the red sequence, unlike what happens in the field. We finally compare our results to the predictions of two sets of semi analytic models to investigate the evolution of AGN and evaluate potential triggering and fueling mechanisms. The outcome of this comparison attests to the importance of galaxy encounters, not necessarily leading to mergers, as an efficient AGN triggering mechanism.