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 ~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 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 are 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.Comment: 14 pages, 5 figures. Accepted for publication in the Astrophysical
Journa
