NASA's Hubble Space Telescope (HST) has revealed a new class of object in the universe — a grouping of gigantic star clusters produced by the collision of galaxies. Images of the core of the peculiar galaxy Arp 220 show that stars are produced at a furious rate from the dust and gas supplied by the interaction of two galaxies.
The discovery was made by Dr. Edward Shaya and graduate student Dan Dowling of the University of Maryland at College Park, MD, and the WFPC Team.
Astronomers have never before seen such a "starburst galaxy" in such detail. The core of Arp 220 promises to be a unique laboratory for studying supernovas (the self-detonation of massive stars) because they should explode frequently in gigantic young clusters. Over time, the core of this galaxy should resemble a string of firecrackers popping off. This will provide astronomers an unprecedented opportunity to study the late evolution of massive stars, as well as possibly improve techniques for measuring distances to galaxies, which utilize supernovae as "standard candle" distance indicators.
In the 1980s NASA's IRAS (Infra-Red Astronomy Satellite) observatory showed that Arp 220 is the brightest of a dozen or so "ultraluminous infra-red galaxies" which release 95% of their light in the infrared region of the spectrum.
Ground-based telescopic images show a dust lane down the center of Arp 220 which makes the galaxy appear double lobed. Astronomers suspected that Arp 220's dark lane hid a massive black hole which provided the energy for heating intervening dust which re-radiates in infrared light. These new observations show that much of the energy is provided by giant star clusters. These star clusters will drive gas into the accretion disk around the black hole.
Astronomers doubted that an incredibly swift burst of star formation could explain all of Arp 220's luminosity. It now seems that the dust is heated both by the nucleus and the giant star clusters.
The new Hubble observation seems to confirm a 1988 theory by David Sanders (University of Hawaii) which predicted that starburst activity is triggered in Arp 220 and other ultraluminous infrared galaxies as a result of mergers of two giant spiral galaxies. Additionally, Joshua Barnes (University of Hawaii) and Lars Hernquist (Lick Observatory, USC Santa Cruz) have calculated that when two spiral galaxies merge much of the gas and dust loses angular momentum and falls into the center. This high gas density would trigger a very high rate of star formation.
HST reveals for the first time six luminous knots which are super-bright star formation regions that lie within 2,000 light-years of the bright nucleus. These clusters are much brighter and ten times larger than any previously known star cluster. The HST observers speculate there may be even more super-clusters embedded deeper in the dust lane.
"We can now interpret previous microwave observations to estimate the clusters' age to be greater than ten million years," says Shaya. "This means that they are no longer in the starburst phase." Frictional forces, however, should draw these clusters toward the center of the galaxy where gravitational tidal forces should tear them apart within 100 million years. This limited age range adds further support for the galaxy collision scenario.
Since the clusters are young they must contain an abundance of massive short-lived stars. Shaya estimates that these should explode as supernovae several times per year.