Post-starburst Tidal Tails in the Archetypical Ultra Luminous Infrared
  Galaxy Arp 220
  We present our new deep optical imaging and long-slit spectroscopy for Arp
220 that is the archetypical ULIRG in the local universe. Our sensitive Ha
imaging has newly revealed large-scale, Ha absorption, i.e., post-starburst
regions in this merger; one is found in the eastern superbubble and the other
is in the two tidal tails that are clearly reveled in our deep optical imaging.
The size of Ha absorption region in the eastern bubble is 5 kpc x 7.5 kpc and
the observed Ha equivalent widths are ~2 A +- 0.2 A. The sizes of the northern
and southern Ha-absorption tidal tails are ~5 kpc x 10 kpc and ~6 kpc x 20 kpc,
respectively. The observed Ha equivalent widths range from 4 A to 7 A. In order
to explain the presence of the two post-starburst tails, we suggest a possible
multiple-merger scenario for Arp 220 in which two post-starburst disk-like
structures merged into one, and then caused the two tails. This favors that Arp
220 is a multiple merging system composed of four or more galaxies, arising
from a compact group of galaxies. Taking our new results into account, we
discuss a star formation history in the last 1 Gyr in Arp 220.
