2007ApJ...669..801C

We present deep HST ACS images and Keck spectroscopy of MC2 1635+119, a QSO hosted by a galaxy previously classified as an undisturbed elliptical galaxy. Our new images reveal dramatic shell structure indicative of a merger event in the relatively recent past. The brightest shells in the central regions of the host are distributed alternately in radius, with at least two distinct shells on one side of the nucleus and three on the other, out to a distance of ∼13 kpc. The light within the five shells comprises ∼6% of the total galaxy light. Lower surface brightness ripples or tails and other debris extend out to a distance of ∼65 kpc. A simple N-body model for a merger reproduces the inner shell structure and gives an estimate for the age of the merger of between ∼30 Myr and ∼1.7 Gyr, depending on a range of reasonable assumptions. While the inner shell structure is suggestive of a minor merger, the total light contribution from the shells and extended structures is more indicative of a major merger. The spectrum of the host galaxy is dominated by a population of intermediate age (∼1.4 Gyr), indicating a strong starburst episode that may have occurred at the time of the merger event. We speculate that the current QSO activity may have been triggered in the recent past by either a minor merger, or by debris from an older (∼Gyr) major merger that is currently ``raining'' back into the central regions of the merger remnant.