Jan. 23, 1998
A team of astronomers probing the activities of a pair of colliding galaxies has been startled to discover that the merger has resulted in a nest of a dozen or more extremely powerful supernova explosions, comparable to the most powerful ever observed.
Analysis of images captured by a network of 17 radio telescopes scattered in Europe and the United States suggest that about two of these massive stars are exploding each year in the merging galaxy, known as Arp 220. By comparison, our own Milky Way generates only about one supernova every century.
The study further indicates that each of the Arp 220 supernovae triggers hundreds of times more radio wave energy than a "typical" supernova. To put this in perspective, supernovae normally produce more energy in one explosion than our sun will generate in its entire lifetime and glows with such intensity that it outshines an entire galaxy of hundreds of billions of stars.
The results, published in the January 20 issue of the Astrophysical Journal Letters, surprised the astronomers who previously suggested that such fury was likely unleashed by an enormously bright quasar-considered the ultimate fireworks display in the universe.
Instead, the new images suggest that Arp 220, located about 250 million light years from Earth, is a "starburst galaxy" whose tremendous energy and giant supernovae explosions are powered by the rapid spawning of very hot, fast-burning stars.
"I would say that within the group of people working on star formation and infrared emission of galaxies, this is a startling result," said Harding Smith, a professor of physics in the Center for Astrophysics and Space Sciences at the University of California, San Diego. "I know I was stunned."
Other members of the astronomer team included Colin Lonsdale of the Massachusetts Institute of Technology's Haystack Observatory, Carol Lonsdale of Caltech's Infrared Processing and Analysis Center, and Philip Diamond of the National Radio Astronomy Observatory. A sort of family project, Carol Lonsdale is Smith's wife and Colin Lonsdale's sister.
The tremendous energy released when galaxies collide has been the subject of intense debate among astronomers for about a decade since these so-called "ultraluminous infrared galaxies" were first discovered during NASA's Infrared Astronomical Satellite mission.
On one side of the debate are those who believe that colliding galaxies might awaken a massive black hole lurking in the galactic core. Once activated, huge quantities of gas and dust stirred up by the collision would start falling into the center, feeding the black hole and allowing it to grow and shine with the power of a billion suns. Thus, a quasar is born.
The competing theory suggests that these ultra-radiant galaxies are illuminated by young, hot stars and powerful supernovae that are born from shockwaves moving through large masses of gas in colliding galaxies.
Arp 220, the 220th object in Halton Arp's Atlas of Peculiar Galaxies, is considered a prototype ultraluminous colliding galaxy whose "double nucleus" of two bright compact star clusters represents the remnant cores of spiral galaxies before they collided, probably several hundred million years ago. In the aftermath of the collision, huge clouds of dust and gas were kicked up, effectively obscuring the twin nuclei from optical telescopes even as powerful as the Hubble Space Telescope.
To cut through these dense clouds, the astronomers tuned into radio waves capable of passing through the thick dust without obstruction. By using radio telescopes that span the globe and a technique known as Very Long Baseline Interferometry (VLBI), the researchers were allowed to peer into the heart of Arp 220. To translate the resulting information, signals from the 17 radio telescopes in the observational net were combined using special equipment at Haystack Observatory to create startling new images.
Among other things, the images reveal about a dozen extremely powerful supernovae exploding in the vicinity of one of the nuclei, located in the northeast sector of Arp 220; only two relatively faint supernovae were seen in the southeast nucleus.
Combined, the radio emission produced by the supernovae and their associated starburst account for all the radio emissions associated with the galaxy, the astronomers said, suggesting that a quasar isn't needed.
"It's proof positive that you can produce these kinds of energies through star formation," said Smith. "These findings are consistent with the 'starburst model,' with the inference that one of the nuclei is far more active for star formation than the other."
Though the results represent a victory for those supporting the "starburst" model over the quasar theory, Smith and his co-authors acknowledge that similar studies of other ultraluminous infrared galaxies have revealed huge quasars at their cores.
Smith wonders if these seemingly contradictory results mean the existence of two separate classes of intensely lit galaxies, or if there might be some evolutionary relationship between the two.
"If there is an evolutionary relationship, then Arp 220 could be in its very early stages, where these two galaxies have collided and star formation is going on very actively, but the quasar has not really had a chance to turn on," he said. "So we still wonder if there isn't, at some very low level, a nascent quasar at the center of Arp 220."
Future observations will follow the life cycles of the dozen or so supernovae in Arp 220 to see if, as predicted, they grow fainter with time. Theoretically, these studies should also reveal new massive stellar explosions in the galaxy. The team also expects a new infrared camera and spectrometer developed at UCSD for the Keck telescope will assist in future observations of Arp 220 and other similar colliding galaxies.
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