Arp 220

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**Arp 220**
Observation data (Epoch J2000)
Galaxy	List of galaxies

Constellation	Serpens
Right ascension	15^h 34^m 57.1^s^[1]
Declination	+23° 30′ 11″^[1]
Redshift	5434 ± 7 km/s^[1]
Distance	250 Mly (77 Mpc)^{[citation needed]}
Type	S?^[1]
Apparent dimensions (V)	1′.5 x 1′.2^[1]
Apparent magnitude (V)	13.9^[1]
Notable features
Other designations
IC 1127,^[1] IC 4553,^[1] UGC 9913,^[1] PGC 55497^[1]

Arp 220 is the result of a collision between two galaxies which are now in the process of merging. Located 250 million light-years away in the constellation Serpens, it is the 220th object in Halton Arp's Atlas of Peculiar Galaxies. It is the closest Ultraluminous Infrared Galaxy (ULIRG) to Earth, with its energy output dominated by the far-infrared part of the spectrum.It is often regarded as the prototypical ULIRG and is the subject of much study as a result. Recent (2002 and 1997) observations of Arp 220, taken in visible light with Advanced Camera for Surveys and in infrared light with Near Infrared Camera and Multi-Object Spectrometer (both instruments on Hubble Space Telescope), revealed more than 200 huge star clusters in central part of the galaxy. The heftiest Arp 220 clusters contains enough material to equal about 10 million suns.^[2]. Observations in the X-rays with the Chandra and XMM-Newton satellites have shown that Arp 220 probably includes an Active Galactic Nucleus at its core. This raises interesting questions about the link between galaxy mergers and AGN. Another nearby galaxy merger, NGC 6240, has been shown to contain two AGN that will presumably merge at some point in the future. However, despite the presence of an AGN in Arp 220, most of its energy output is thought to be the result of a massive burst of star formation, a starburst, thought to have been triggered by the galaxy merger.

Far-infrared luminous objects like Arp 220 have been found in surprisingly large numbers by surveys of the submillimetre sky using instruments such as SCUBA at the JCMT. Arp 220 and other local ULIRGs are being studied as local equivalents of this kind of object.

Arp 220 contains two nuclei, an OH megamaser, and a water maser.^[3]