Extreme Galaxies –Some 3000 Times Brighter Than Milky Way

Arp220_hst

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.


Arp 220 is the brightest object in the local universe. In the late 1980s, it was discovered to be an ultraluminous infrared galaxy and headed a list compiled from observations with the now-defunct IRAS satellite. New observations with the Hubble Space Telescope revealed the two colliding spiral galaxies at the center of Arp 220.

A result of this spiral collision are fantastic knots of new star formation visible as the bright spots on the above photograph. Below the "half-moon" shaped knot on the right in the image above is a massive disk of dust possibly hiding a dying spiral's central black hole. The bright knot to the left is the center of the other broken spiral galaxy. The galaxy cores are about 1200 light years apart and are orbiting each other.

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.

Several million regions of activity are localized within a relatively small volume (a few thousand light-years) around the nucleus. The new results are an important improvement in our understanding of what powers extreme galaxies, and how they differ from the Milky Way.

The Daily Galaxy via Harvard-Smithsonian Center for Astrophysics

Image credit: Credit: R. Thompson (U. Arizona) et al., NICMOS, HST, NASA

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