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Inside Iowa State
January 8, 1999

Astronomers find galactic collisions more common than previously thought

by Skip Derra

Collisions between galaxies that include billions of stars, like our Sun, have happened more frequently than previously thought, according to a team of Iowa State astronomers. In addition, these galactic collisions apparently have helped shape the properties of the current universe, said Russ Lavery, assistant professor of physics and astronomy and leader of the research team.

Lavery and graduate students Michael Reed and Anthony Remijan (now at the University of Illinois), examined random images from the Hubble Space Telescope and looked for ring galaxies, a type of galaxy that is the product of galactic collisions. Collisional ring galaxies have a distinctive ring-like structure of intense star formation at the outer edges. A well-known example is the "Cartwheel" galaxy, about 500 million light-years from Earth in the constellation of Sculptor.

"We have looked at about 100 Hubble images and we expected to find maybe one ring galaxy among them," Lavery said. "Instead we've identified 20 ring galaxies."

Lavery presented the research results this week at the annual meeting of the American Astronomical Society.

Lavery plans to analyze more than 500 images from the Hubble to provide a more representative survey of the sky. But already the team is finding some surprising results.

"The images we examined were random, suggesting that galactic collisions have happened all around the universe," Lavery explained. "This, coupled with the fact that we see more of these types of galaxies the farther we look into space, and thus back into time, suggest that collisional galaxies have played a major role in determining the types of galaxies we observe around us today."

The Iowa State team's results support the earlier work of Alar Toomre of the Massachusetts Institute of Technology, Cambridge, and Francois Schweizer of the Carnegie Institution of Washington, D.C. Both have stressed the importance of mergers of galaxies in forming large elliptical galaxies.

If interactions between galaxies were more frequent in the past, it is quite likely that the rate at which galaxies merged also was more frequent, and these mergers could have produced many of the large elliptical galaxies in the present day universe, Lavery explained.

Lavery said the powerful Hubble Space Telescope played a key role in obtaining images that are not possible from Earth- based telescopes. The telescope is operated for NASA by the Association of Universities for Research in Astronomy. Iowa State is a member of AURA.

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