Weekly Science Update

Wednesday, August 06, 2008
Seven Billion Year-Old Galaxies
As modern telescopes peer more and more deeply into the universe, they are seeing older and older galaxies. The current record is a galaxy whose light has been traveling towards us for well over 12.5 billion years, 90% of the age of the universe. Astronomers are hard at work trying to understand what these objects are like, both because they are ancestors of our own Milky Way galaxy, and because they form from the seeds sown in the primordial big bang period and so offer clues into those primitive epochs.

Astronomical models use the observations of faint, distant galaxies to look for self-consistency and to refine the models' parameters. One of the key properties of galaxies is clustering -- the way in which hundreds or even thousands of galaxies can be bound together by their mutual gravitational attractions. Clustering not only influences how a particular member galaxy evolves, it also reflects the larger cosmic structures from which all the members form. The task of deciphering clustering in the early universe is difficult to do because distant galaxies are faint and only the brightest ones can be seen. This leaves one of the most crucial parameters of a cluster -- its total mass -- very uncertain indeed.

SAO astronomers Mark Birkinshaw and B. Maughan together, with eight colleagues, have used the Chandra X-ray Observatory and another X-ray satellite, XMM-Newton, to study in unprecedented depth a galaxy cluster that is about seven billion years old, far enough away to probe the cosmological nature of galaxy evolution yet close enough to be measurable. The very hot gas in the cluster emits at X-ray energies, and provides a measure of the total cluster mass because it traces the entire system of galaxies. In this case, the team finds that the total mass of the cluster is equivalent to about 5000 Milky Ways. The main significance of the research, however, is that it confirms for the first time the accuracy of simple models that predict how galaxy clusters evolve from the earliest times, at least for clusters out to a distance of about seven billion light years.

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