Differences in the AGN Populations of Groups and Clusters: Clues to AGN   Evolution
  We combine optical and X-ray data for eight low redshift ($z\sim 0.06$) poor groups of galaxies from the {\it XI}({\it XMM/IMACS}) Groups Project to study the AGN population in the group environment. Among $\sim 140$ group members, we identify five AGN based on their optical emission lines. None of these optically-selected AGN are detected by {\it XMM-Newton}. One additional AGN is discovered in the {\it XMM-Newton} observations. This X-ray detected AGN, which has no obvious AGN emission line signatures in its optical spectrum, is a member of the only X-ray luminous group in our sample. The lack of a significant population of X-ray bright, but optically dull AGN among less dynamically evolved groups is in stark contrast to the large fraction of such objects in rich clusters of galaxies (Martini et al. 2006). We suggest this result can be explained by a physical scenario for AGN accretion evolution: AGN activity is initially triggered by galaxy merging, leading to a high accretion rate and an optically dominant phase (via thin disk accretion). As the accretion rate drops in time, the AGN gradually enters an X-ray dominant low-accretion phase (via a radiative inefficient accretion flow). In this picture, optical- and X-ray-selected AGN are the same population of supermassive black holes observed at different epochs. Within the context of this scenario, the majority of AGN in poor groups are in the high-accretion optically dominant phase, while the AGN population in rich clusters is mostly in the low-accretion X-ray dominant phase. 
Yue Shen (1,2), John S. Mulchaey (2), Somak Raychaudhury (3), Jesper   Rasmussen (3), Trevor J. Ponman (3) ((1) Princeton Un...