Fueling QSOs: The Relevance of Mergers
  To study the relevance of mergers for the fueling of QSOs, we are currently conducting an HST imaging campaign of a sample of QSO host galaxies classified as ellipticals in the literature. Here, we present results from a study of the first five QSO host galaxies imaged with HST/ACS. For the majority of objects, strong signs of interactions such as tidal tails, shells, and other fine structure are revealed. We estimate the nature and age of the merger by comparing the images with numerical simulations. The merger ages range between a few hundred Myr up to a Gyr. These timescales are comparable to starburst ages in the QSO hosts previously inferred from Keck spectroscopy, but longer than theoretical estimates of AGN duty cycles. A possible scenario emerging from our results is that most QSO host galaxies experienced mergers with accompanying starbursts but that the activity is triggered with a delay of several hundreds Myr after the merger. To probe whether there is indeed a causal connection between the merger and the QSO activity, we study a control sample of inactive ellipticals. Our preliminary results do not reveal comparable fine structure. 
Nicola Bennert (1), Gabriela Canalizo (1), Bruno Jungwiert (2), Alan   Stockton (3), Francois Schweizer (4), Chien Y. Peng (...