Spectacular shells in the host galaxy of the QSO MC2 1635+119

We present deep HSTACS 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 similar to 13 kpc. The light within the five shells comprises similar to 6% of the total galaxy light. Lower surface brightness ripples or tails and other debris extend out to a distance of similar to 65 kpc. Asimple N-body model for a merger reproduces the inner shell structure and gives an estimate for the age of the merger of between similar to 30 Myr and similar to 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 (similar to 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 aminor merger, or by debris from an older (similar to Gyr) major merger that is currently raining'' back into the central regions of the merger remnant.