Star-formation properties of AGN host galaxies

X-ray selected AGN are hosted by galaxies with a wide range of star-formation histories, both actively star-forming and quiescent systems.

The top plot shows the optical/near-infrared coloul-colour diagram (U-V vs V-J; UVJ diagram). The overall (non-AGN) galaxy population (black contours) cleary splits into two distinct clouds: passive and star-forming, separated by the dashed-lined wedge. X-ray selected AGN scatter on the UVJ diagram and appear in both passive hosts (red symbols) and star-forming systems (blue symbols). The different host galaxy colours are also associated with different morphologies. AGN in passive galaxies (red points) are mostly spheroids, while star-forming AGN hosts (blue points) inlcude a large fraction (~50%) of disks. These differences in the stellar populations and morphologies of AGN hosts may indicate different conditions of black hole growth in the two populations (passive vs star-forming).

This possibility is investigated in the bottom figure, which plots the Eddington-ratio distribution of X-ray AGN in passive and star-forming hosts. There is evidence, albeit weak, that the Eddington ratio distribuions of the two populations are different. AGN in star-forming hosts dominate at high Eddington-ratios (fast accretors) while AGN in passive hosts become important at low Eddington ratios (low accretors). Such differenes are consistent with differnt modes of black hole growth based on the available gas reservoirs in the galaxy. More details in Georgakakis et al. (2014).