AGN host galaxies from z=0.05-2.75: young stars and mergers?

Knud Jahnke (AIP)

In the context of AGN physics the active nucleus does not end at 1~pc or even 1~kpc: It is not known at present why of all galaxies with massive central black holes some have active nuclei and others not. On one hand side circumnuclear starburst have been observed for nearby AGN, on the other hand there are events like galaxy interactions that will influence both host galaxy and nucleus and that can be the cause for nuclear activity in at least one part of the AGN population. We studied the morphology and star formation activity of type quasar host galaxies from the local universe to redshifts of about 3 using ground based imaging data and HST ACS colour imaging data from the GEMS survey for z>0.5. We focus here on complete samples of AGN from the Hamburg/ESO survey at low redshifts, and COMBO-17 in the E-CDFS. At higher redshifts this is currently the largest existing sample of resolved quasar host galaxies. We found a) that consistent with the local universe, AGN host galaxies are almost always associated with unusual amounts of moderately young stellar components compared to inactive galaxies. This is independent of morphology, and out to high redshifts. b) About half of the studied AGN hosts shows signs of recent interaction, however the other half seems undisturbed. This suggests either the possibility of nuclear activity also in luminous AGN without strong tidal interaction, or is a sign of a delay between a merger event with a starburst and the onset of AGN activity.

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