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 ~ 0.06) poor groups of galaxies from the XI (XMM-IMACS) Groups Project to study the AGN population in the group environment. Among ~140 group members, we identify five AGNs based on their optical emission lines. None of these optically selected AGNs are detected by XMM-Newton. One additional AGN is discovered in the 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 AGNs among less dynamically evolved groups is in stark contrast to the large fraction of such objects in rich clusters of galaxies. 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, optically 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 AGNs 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.