A survey of galaxy clusters by NASA's Hubble Space Telescope has found what could be some of the most distant clusters ever seen. If the distances and masses of the clusters are confirmed by ground-based telescopes, the survey may hold clues to how galaxies quickly formed into massive large-scale structures after the Big Bang, and what that may mean for the eventual fate of the universe.
According to theoretical models, if the clusters turn out to be massive and very distant, it could imply that the cosmos does not contain enough matter for gravity to stop the expansion of the universe. These models predict that such a low-density universe would have built most of its galaxy clusters long ago.
About 10 to 20 of the farthest clusters in the Hubble survey may be over seven billion light years away, which means that the clusters, and their populations of tens or perhaps hundreds of galaxies each, were fully assembled early in the history of the universe.
Present distance estimates are based on the colors of the galaxies in each cluster. The redder the overall cluster appears, the more distant it is, an assumption based on the apparent reddening of light known as red shift as stars and galaxies move away from us at high speeds. The distances can be more accurately measured using a spectrograph attached to a ground-based telescope.
The Hubble survey contains 92 new clusters uncovered during a six-year sky survey known as the Medium Deep Survey, led by a team of astronomers now at Carnegie Mellon University, Pittsburgh, PA.
The project has been led by Professor Richard Griffiths and senior scientist Dr. Kavan Ratnatunga. The catalog samples an area of the sky that is small, but scattered over 300 random directions.
The clusters were found using an automated procedure developed by the Carnegie Mellon team. They first identified large elliptical galaxies in random fields taken by Hubble. Next, an automated procedure was used to search statistically for an over-abundance of galaxies around the large elliptical galaxies. The assumption is that the excess galaxies all belong to the same cluster. This procedure helped to discriminate clusters against the field galaxy population which is smoothly distributed across the sky
Major new telescopes must be used to study these clusters to measure their distances.
The whole HST catalog of galaxies can be searched on the web at: http://astro.phys.cmu.edu/mds/
The Hubble observations will be published in the Astronomical Journal. The research team members are: E. J. Ostrander; K. U. Ratnatunga; and R. E. Griffiths, Department of Physics, Carnegie Mellon University.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
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