Extreme Galaxies: Why are Some 300 Times Brighter than the Milky Way?:
Awealthofmol Arp 220 is the closest galaxy to the Milly Way with an extreme luminosity, defined as being more than about 300 times that of our own galaxy. Some dramatic galaxies have values of luminosity ten times brighter still. Astronomers are still piecing together the reasons for these huge energy outputs, while sorting out why our own galaxy is so modest.

The two primary suspects for the energetics are bursts of star formation that produce many hot young stars, and processes associated with accretion of material onto a supermassive black hole at a galaxy's nucleus. Arp 220 is the closest example, and one of the best places to probe these scenarios."