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FROM COMPUSERVE INFORMATION SERVICE, Space Forum, Library 17: Downloaded 6/27/92 Accesses: 913 Title : True-color Hubble image: Starburst Galaxy Arp 220 Keywords: STARBURST GALAXY HUBBLE ARP NASA True-color image of the center of Arp 220 taken by the Hubble Space Telescope. Reveals new class of object: gigantic young star clusters which seem to confirm David Sanders' 1988 theory predicting starburst activity in Arp 220 and other ultraluminous infrared galaxies from collisions of giant spiral galaxies. 640x480x256. ------------------------------------- This is an extended caption file describing images in STRBST.GIF. Photo courtesy E. Shaya and D. Dowling of the University of Maryland, the Hubble Space Telescope's Wide Field/Planetary Camera Team, and NASA. Caption file reprinted from Space News Roundup, the newsletter of Johnson Space Center. HUBBLE SPACE TELESCOPE IMAGE OF STARBURST GALAXY ARP 220 NASA's Hubble Space Telescope 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, University of Maryland, College Park, and the Wide Field/Planetary Camera Team. Astronomers have never before seen 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 use supernovae as "standard candle" distance indicators. In the 1980s, NASA's Infrared Astronomy Satellite observatory showed that ARP 220 is the brightest of a dozen or so "ultra luminous infra-red galaxies," which release 95 percent of their light in the infrared region of the spectrum. Ground-based telescopic images show a dust lane down the center of ARP 220 that 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 HST observation seems to confirm a 1988 theory by David Sanders of the University of Hawaii, which predicted that starburst activity is triggered in ARP 220 and other ultra luminous infrared galaxies as a result of mergers of two giant spiral galaxies. Additionally, Joshua Barnes, University of Hawaii, and Lars Hernquist, Lick Observatory, University of Southern California, Santa Cruz, have calculated that when two spiral galaxies merge much of the gas and dust lose angular momentum and fall 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 that are super-bright star formation regions that lie within 2,000 light-years of the bright nucleus. These clusters are much brighter and 10 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 10 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.


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