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 archetyp- ical ULIRG in the local universe. Our sensitive Himaging has newly revealed large-scale, Habsorp- tion, 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 Habsorption region in the eastern bubble is 5 kpc × 7.5 kpc and the observed Hequivalent widths are � 2 u ± 0.2 u The sizes of the northern and southern H�-absorption tidal tails are � 5 kpc × 10 kpc and � 6 kpc × 20 kpc, respectively. The observed Hequivalent widths range from 4 u to 7 u 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. Subject headings: galaxies: individual (Arp 220) — galaxies: interactions — galaxies: starburst