Environment may influence how galaxies change,
says study

by Elizabeth Luciano, News Office staff

ostdoctoral research fellow Daniel H. McIntosh and other scientists have uncovered evidence supporting the hypothesis that spiral galaxies pulled into large galaxy clusters change drastically both in appearance and star formation. Furthermore, the team has found evidence of several galaxies in the midst of such transformations. The findings suggest a link between a galaxy's environment and the evolution of its appearance. The report was presented Jan. 8 at the meeting of the American Astronomical Society in Washington, D.C.

     In the nearby universe, galaxies reside in a variety of environments, McIntosh explained. These range from dense, cosmological "cities" called clusters containing hundreds of galaxies bound together by gravity, to more "rural," open regions of space with only a handful of galaxies. Astronomers have long known that the brightest cosmological "city" and "country" galaxies are usually quite different in appearance. The majority of luminous galaxies in nearby clusters, or "cities," appear red, smooth, and are spheroidal (football-like) in shape, and may have a modest disk of stars, he said.

     On the other hand, the brightest field, or "rural" galaxies, are most often spirals with bluer colors and large stellar disks. Astronomers also suspect that the visible differences indicate variations in the rates at which galaxies form stars. "Bluer galaxies are still forming significant numbers of stars while reddish galaxies have formed few stars over the past several billion years," explained McIntosh. "The color of a galaxy tells us something about the average age of its star population, thus, local clusters have very few blue, hence young, galaxies."

     The team sought to determine why galaxies found within dense galaxy clusters are so different from those in the surrounding non-cluster regions, said McIntosh.

     Collaborators were Hans-Walter Rix of the Max-Planck In-stitute for Astronomy in Heidelberg, Germany, and Nelson Caldwell of the Smithsonian Astrophysical Observatory, in Cambridge. The team observed three massive galaxy clusters using the wide-field MOSAIC camera on the 0.9-meter (36-inch) telescope at the National Science Foundation's Kitt Peak National Observatory, in Arizona. Although these huge clusters of galaxies are 320-530 million light years away (redshifts 0.03-0.06), they are considered quite nearby and thus their images provide astronomers with cosmologically recent information, McIntosh explained.

     Yet there is more to this story than the difference between nearby cluster and field galaxies, McIntosh said. More distant clusters, seen as they appeared further in the past, have more blue galaxies than nearby clusters, a phenomenon known as the Butcher-Oemler effect. In other words, the striking difference between the appearance of cluster and field galaxies is diminishing as we peer back in time, explained McIntosh.

     Scientists theorize the reason is two-fold: first, astronomers believe that galaxy clusters build through gravitational attraction over time, but just as the expansion of the universe has slowed down, so too has the process of galaxies being drawn into clusters, he said. "The rate at which clusters gather new members has steadily declined over the history of the universe," said McIntosh. Second, astronomers have hypothesized that as field spirals -- that is, galaxies from a more "rural" environment -- enter the denser cluster environment, they are transformed into red galaxies with a modest disk of stars. Each galaxy's passage into the denser environment ultimately puts an end to its formation of stars, which in turn dims and reddens its appearance.

     Additionally, their shapes are expected to become smoother as they move into the denser environment of a cluster. A new cluster galaxy is bumped and jostled by close encounters with increased numbers of other member galaxies, much like a person moving through a crowded room, an idea first put forth by a group led by Ben Moore of the University of Durham.

     The McIntosh team found many galaxies with blue colors in the outskirt regions of the three galaxy clusters they studied, suggesting that these galaxies were in the process of being pulled into a cluster. Significantly, the team found a distinct lack of structure in the recent cluster arrivals that were otherwise similar in color and brightness to typical spirals from the field. "These results suggest that spiral galaxies that are pulled into a cluster by gravity are affected by denser environments early on," said McIntosh. This finding supports earlier predictions.

     Previously, no direct evidence for cluster members caught in the act of transforming had been found in the nearby universe. It is particularly difficult to observe a galaxy as it changes, McIntosh noted. "Nearby cluster galaxies in the midst of this evolutionary scenario are quite uncommon," he said.

     The team's work was funded by NASA and the Max-Planck Institute for Astronomy.