Chaotic collision. The Antennae galaxies, named for their insectlike appearance (left, from ground-based telescope) are two merging spiral galaxies that have spawned over 1000 young star clusters visible as bright blue spots from the Hubble Space Telescope image (right).

Spiral Galaxy Smash-Up

WASHINGTON, D.C.--Astronomers are watching a violent collision of the two spiral galaxies from a ringside seat. Images taken by the Hubble Space Telescope of the Antennae galaxies--a pair of colliding galaxies named for their long tails of stars--have revealed over 1000 bright, young star clusters triggered by the collision, astronomers said at a NASA press conference here today. This close-up view should help astronomers understand how collisions, which were once far more common than they are now, influenced star formation and the evolution of galaxies in the early universe.

When viewed through telescopes, most galaxies appear either spiral or elliptical. For decades astronomers have speculated that many elliptical galaxies formed when two spiral galaxies coalesced. This was partly based on observations that spiral galaxies tend to be isolated, whereas clusters of galaxies "are filled with ellipticals, " said Anne Kinney, of the Space Telescope Science Institute (STScI) in Baltimore. Although many pairs of merging galaxies have been observed, the theory remains uncertain, partly because average ellipticals have many more star clusters than spiral galaxies, and it wasn't clear where the extra clusters came from. One hypothesis was that the extra clusters form during the collision process.

The sharp-eyed Hubble is offering support for the fertile merger of spiral galaxies. Astronomers have detected clusters of stars, which flare to life as shock waves of collisions trigger clouds of gas to collapse into new stars. And since the color and brightness of young clusters gives their ages--and therefore, the time since a collision began--astronomers hope to put together a series of snapshots of the entire collision process by looking at many examples of merging galaxies. Since the Antennae galaxies are close to us, and began colliding relatively recently, they may offer a glimpse of the early stages of a collision. "It's like watching a car wreck as it happens, " says Steve Zepf of Yale University.

Preliminary attempts to create such a "fossil record" of merging galaxies using Hubble pictures support the theory that ellipticals can form from colliding spirals, according to Bradley Whitmore, of STScI, who will publish a five-snapshot sequence in November in the Astronomical Journal. But astronomer John Holtzman of New Mexico State University in Las Cruces--while he is basically supportive of the theory--thinks there is still uncertainty. "I think it's a hard argument to make either strongly in favor or strongly against the formation of ellipticals by mergers," since some clusters don't survive to the age of old ellipticals and the "infant mortality rate" is uncertain.

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