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 .
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 \sim 13 kpc .
The light within the five shells comprises \sim 6 % of the total galaxy light .
Lower surface brightness ripples or tails and other debris extend out to a distance of \sim 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 between \sim 30 Myr and \sim 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 are more indicative of a major merger .
The spectrum of the host galaxy is dominated by a population of intermediate age ( \sim 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 ( \sim Gyr ) major merger that is currently “ raining ” back into the central regions of the merger remnant .