Hubble Spies Multiple Star Factories

Staring into the crowded, dusty core of two merging galaxies, NASA's Hubble Space Telescope has uncovered a region where star formation has gone wild. CREDIT: NASA, ESA, and C. Wilson

Astronomers have uncovered a frenzy of star forming activity in the dusty cores of two merging galaxies 250 million light-years away.

With the aid of NASA's Hubble Space Telescope, the researchers spied more than 200 mammoth star clusters in Arp 220, an uber-galaxy in the constellation Serpens that is forming as the result of a galactic collision that began about 700 million-years ago.

The star clusters are packed into a very small region only about 5,000 light-years across. The biggest cluster contains nearly 10 million suns worth of matter and is twice as massive as any star cluster ever discovered in the Milky Way Galaxy.

"This is star birth in the extreme," said study team member Christine Wilson of the McMaster University in Canada. "This is a nearby look at a phenomenon that was common in the early universe, when many galaxies were merging."

The clusters are so compact that even though they lie only a moderate distance away, they look like single stars. What gives them away as star clusters, however, is that they shine brighter than any single star would at that distance.

The findings, detailed in the April 20 issue of Astrophysical Journal, are based on 2002 observations with Hubble's Advanced Camera for Surveys and on previous data collected by the Near Infrared Camera and Multi-Object Spectrometer. The star clusters could yield insights about star evolution and about the early universe, when galaxy mergers were more common.

Created in fits or gradually?

The researchers measured the masses and ages for 14 of the clusters and then used this information to estimate the masses and ages for all of the clusters. The analysis revealed two different cluster populations: One that was less than 10 million years old and another that was 70 million to 500 million years old.

The younger clusters were more massive than the older ones.

It's still unclear if the clusters were created at two different epochs or whether they were created gradually over a long period of time and astronomers are just not seeing the intermediate-aged ones.

What is known, however, is that the starbursts were sparked by the collision of the two parent galaxies that make up Arp 220. Radio data show two objects 1,000 light-years apart which might be the cores of the original galaxies.

Powered by dust

Arp 220 is a so-called ultra-luminous infrared galaxy, or ULIRG, and glows brilliantly in the infrared because its dust has been superheated by starlight.

If not for the thick layer of dust that enshrouds the entire galaxy, Arp 220 would shine 50 times brighter than our galaxy. It's the gas, however, that fuels star birth in the clusters. It is estimated that there is as much gas in the tiny region where the star clusters were discovered as there is in the rest of the entire Milky Way.

Scientists think Arp 220 will continue to produce star clusters for about another 40 million years, at which time all of its gas will be exhausted. When the gas is gone, Arp 220 will look like a regular elliptical galaxy, which has little gas. However, some of newly discovered star clusters will still be visible.

Wilson recently announced a surprising finding involving Arp 220 and similar highly active galaxies: In terms of their mix of gas vs. dust, they are very similar to our more mature Milky Way.

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