ScienceDaily (June 29, 2012) — First discovered from the Infrared Astronomical Satellite's (IRAS) all-sky survey in the mid-1980s, ultraluminous infrared galaxies (ULIRGs) are the most luminous class of galaxies in the relatively near or local Universe. Most of their energy output is in the infrared, suggesting that they contain a large amount of dust, an indication of immense star formation.
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Astronomers have proposed different scenarios for the development of ULIRGs. Since ULIRGs' huge infrared luminosities (1012 Lsun), powered mostly by a large number of massive stars, are comparable to the high luminosity of quasars, the brightest class of active galactic nuclei, a 1988 scenario (Sanders, D. B., et al. 1988, ApJ, 325, 74) proposed that ULIRGs were an intermediate phase in the evolution of quasars after a merger. Another scenario proposed by Professor Taniguchi and his associate in 1998 (Taniguchi, Y., & Shioya, Y. 1998, ApJ, 501, L167) hypothesized that multiple mergers among several galaxies explained the observational properties.
However, a number of questions remained unanswered:
1) How many galaxies were merged into one? and
2) Which types of galaxies were merged into one? Since then, explanations for the origins of ULIRGs have remained controversial.
The current team conducted research to help answer these questions and to propose a plausible, data-based explanation for the origin of ULIRGs.

The team made detailed optical imaging observations of Arp 220 using FOCAS (Faint Object Camera and Spectrograph) on the Subaru Telescope and the LRIS (Low Resolution Imaging Spectometer) on the Keck II Telescope. The new imaging data revealed a spectacular pair of tidal tails extending more than 50,000 light years. Intermediate-mass stars (with masses a few to several times that of the Sun), the remains of intense star formation events called "starbursts," dominate the composition of the tidal tails. The presence of an H