June 10, 1998 Astronomers at Northwestern University have found newer, bluer galaxies than expected in nearby galaxy clusters, suggesting broods of young stars in these galaxies. The finding, presented at the American Astronomical Society annual meeting in San Diego, shows that star formation from galactic gas and dust in these clusters has occurred much more recently than astronomers had thought -- and may in fact be an ongoing process.
Clusters of galaxies are gravitationally bound groups of about 1000 galaxies about 3 million light years across, with a combined mass 10,000 times the mass of our own Milky Way galaxy.
"The model of galaxy evolution in clusters is that 4 billion to 5 billion years ago, clusters had many more blue galaxies than clusters do today," said Melville P. Ulmer, professor of physics and astronomy at Northwestern, who led the study. The blueness, he explained, is caused by younger, hotter stars.
"If you look at clusters of galaxies far away, which means a long time ago, on average they have a lot more blue galaxies than red galaxies," Ulmer said. "When you look nearby, you find clusters of galaxies have hardly any blue galaxies. They're almost all red galaxies."
But Ulmer and an undergraduate student, Anne J. Metevier, and postdoctoral researcher A. Katherine Romer focused on three clusters only 2 billion light years away. Although the universe was perhaps 80 percent as old as it is now when light left these clusters, these particular nearby clusters nevertheless appeared "young" because their X-ray pattern was still in the shape of two blobs rather than a fully formed sphere. Clusters emit X-rays from hot, ionized gas between their galaxies.
"The shape of the X-ray emissions led us to believe these clusters might still be forming, and the galaxies within them might be young as well," said Metevier, who is now in graduate study at the University of California, Santa Cruz. This turned out to be true, she said, as evidenced by the color of the clusters. In fact, one of the three clusters Ulmer's group studied is the bluest galaxy cluster ever.
"This cluster is remarkable not only because it is so blue but because until now astronomers believed that blue clusters could only exist at early times in the universe, yet this cluster is nearly a contemporary of our own," Ulmer said.
The belief that the galaxy population in clusters varies smoothly from blue (young) to old (red) as we change our view from distant to nearby clusters is called the Butcher-Oemler effect and may now have to be revised.
"What we think is happening is that these particular clusters we've looked at are just forming, and in the process of coalescing , some gas gets compressed," Ulmer said. "When the gas is compressed on the galaxies, that ignites star formation." The result is bluer galaxies within the cluster.
The optical observations were made by Romer on the Nation Science Foundation's National Optical Astronomy Observatory's 36-inch telescope on Kitt Peak in Arizona. The X-ray observations were made with NASA's Einstein Satellite. Romer is now at Carnegie Mellon University.
Ulmer said researchers now need to measure the fraction of blue galaxies in other clusters at a range of distances from 1 billion to 5 billion light years away, paying special attention to those clusters with a clumpy appearance in their X-ray images. Further insights into cluster formation and its link to galaxy formation, he said, may come from new, high quality X-ray images from NASA's Advanced X-ray Astrophysics Facility, scheduled for launch in December.
The reported work was supported in part by as NASA Space Grant to the State of Illinois, the NASA Astrophysics Data Program, Northwestern University, Carnegie Mellon University and the University of California, Santa Cruz.
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