The evolution of the AGN content in groups up to z! 1

Context. We explore the AGN content in groups in the two GOODS fields (North and South), exploiting the ultra-deep 2 and 4 Msec Chandra data and the deep multiwavelength observations from optical to mid IR available for both fields. Aims. Determining the AGN content in structures of di! erent mass/velocity dispersion and comparing them to higher mass/lower redshift analogs is important to understand how the AGN formation process is related to environmental properties. Methods. We use 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 identify 9 structures in GOODS-south (already presented in a previous paper) and 8 new structures in the GOODS-north field. We only consider structures where at least 2/3 of the members brighter than MR = " 20 have a spectroscopic redshift. We then check if any of the group members coincides with X-ray sources that belong to the 4 and 2 Msec source catalogs respectively, and with a simple classification based on total rest-frame hard luminosity and hardness ratio we determine if the X-ray emission originates from AGN activity or it is more probably related to the galaxies’ star-formation activity. Results. We find that the fraction of AGN with LogLH > 42ergs " 1 in galaxies with MR < " 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 preferentially 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, contrary to 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 the importance of galaxy encounters, not necessarily leading to mergers, as an e" cient AGN triggering mechanism.