AMES, Iowa -- 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 University astronomers. In addition, these galactic collisions apparently have helped shape the properties of the current universe, said Russ Lavery, an 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 specific type of galaxy that is the product of galactic collisions. Collisional ring galaxies result when a small galaxy passes almost directly through the center of a spiral galaxy.
The result is a galaxy with a distinctive ring-like structure of intense star formation at its outer edges. A well-known example is the "Cartwheel" galaxy, located about 500-million light-years from Earth in the constellation of Sculptor. (A light year is the distance light travels in a year, about 6 trillion miles).
"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 team's research results today (Jan. 6) at the annual meeting of the American Astronomical Society in Austin, Tex., (Jan. 5-9, 1999).
The ISU-lead project tries to identify distant ring galaxies in deep space images obtained with the fixed Wide Field/Planetary Camera (WFPC2) on the Hubble Space Telescope. These images are available through the Space Telescope Science Institute Archives program. The Hubble Space Telescope orbits Earth providing unprecedented views of astronomical objects. Hubble images have allowed these astronomers to identify ring galaxies out to a redshift of 1, equivalent to a distance of about 8-billion light-years.
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 much more frequent, it is quite likely that the rate at which galaxies merged was also much more frequent and could have produced many of the large elliptical galaxies in the present day universe, Lavery explained.
The Iowa State astronomers focused on the distinctive ring galaxies for their studies because these galaxies are relatively easy to identify. Lavery adds that the powerful Hubble Space Telescope played a key role in obtaining images that are not possible from Earth-based telescopes.
The study of ring galaxies is also being undertaken by other Iowa State astronomical teams. Astronomers Phil Appleton and Curt Struck have for several years examined the Cartwheel galaxy and other ring galaxies searching for clues about how stars are born, how they evolve and how they die. The Hubble Space Telescope is operated for NASA by the Association of Universities for Research in Astronomy. Iowa State University is a member of AURA.
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Editors: Some of the images Russ Lavery has obtained of collisional ring galaxies can be viewed at http://www.public.iastate.edu/~lavery/Researchstuff/dirRings/ringshst.html
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