October 2016


Which galaxies host AGN?

This plot shows the fraction of AGN hosted by different morphological galaxy types in the 68Mpc/uhr simulation from the Magneticum Pathfinder simulation set, depending on the AGN luminosity (x-axis). We distinguish between pure spheroids (red solid line), pure discs (blue dashed line), and all remaining galaxies (intermediates, green dotted line). The arrows on the left show the fraction for all inactive galaxies (log(L)<43erg/s). We find that less luminous AGN are more often hosted by discs, although the fraction of discs and spheroids is roughly the same for inactive galaxies (arrows on the left). Towards higher luminosities, the fraction of AGN in spheroidal galaxies slightly increases.
This can be understood assuming the spheroidal galaxies in general have a lower gas fraction. AGN are thus often triggered by a particular event like a merger, which often leads to a very rapid BH growth and thus to a high AGN luminosity. In contrast, in discs there is often enough internal gas to trigger the AGN more continuously, for example via secular evolution.