Post-starburst Tidal Tails in the Archetypical Ultra Luminous Infrared Galaxy Arp 220

TL;DR

This study reveals large-scale post-starburst regions and tidal tails in Arp 220, suggesting a complex multiple-merger history involving at least four galaxies within the last billion years.

Contribution

It provides new deep optical imaging and spectroscopy that identify post-starburst tidal tails and propose a multiple-merger scenario for Arp 220.

Findings

Detection of large-scale Ha absorption regions in tidal tails and bubbles.

Sizes of post-starburst regions are approximately 5-7.5 kpc by 5-20 kpc.

Evidence supporting a multiple-merger origin involving four or more galaxies.

Abstract

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.