AGN Host Galaxies
The co-evolution of central supermassive black holes and their surrounding host galaxies is still a strongly debated topic, but the morphological analysis of Active Galactic Nucleus (AGN) hosts usually is challenging due to the bright central point source.
Using a sample of X-ray selected distant AGN without a detectable optical nucleus, we studied the morphological properties of their hosts in unprecedented detail (Böhm & Wisotzki 2007). We found that the bimodal distribution in rest-frame color and light profile concentration, which is a well-known characteristic of non-AGN galaxies, is absent for AGN hosts. This is partly due to early-type hosts that have bluer colors than their quiescent counterparts, indicating recent or ongoing star formation potentially triggered by the same process as the nuclear activity (see figure).
Fig.1: Sersic index n versus rest-frame U-B color, comparing normal galaxies (open squares) and galaxies hosting AGN (solid circles). The samples are matched in luminosity and redshift (median z=0.7).
We also conducted a quantitative morphological comparison between type-1 AGN hosts (with prominent nuclei) and non-AGN galaxies at z~0.7, to test the suspected connection between major mergers and AGN phases. In contrast to many previous investigations, our approach fully accounted for the impact of an optical nucleus. Interestingly, the AGN hosts lacked signatures of violent gravitational interactions (Böhm et al. 2013). The central monster hence is most probably fed via minor mergers or secular processes like bar-driven gas inflows, at least in the intermediate AGN luminosity range covered by our survey.