August 27, 1998
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).
- end - Images and a caption associated with this release are available.
Other images from Hubble's Medium Deep Field catalog are available at http://astro.phys.cmu.edu/mds/
PHOTO CAPTION
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 expanding universe.
The images are each a combination of two exposures in yellow and deep red taken with Hubble's Wide Field and Planetary Camera 2. Each cluster's distance is inferred from the reddening of the starlight, which is due to the expansion of space. Astronomers assume these clusters all formed early in the history of the universe.
HST133617-00529 (left)
This collection of spiral and elliptical galaxies lies an estimated 4
to 6 billion light-years away. It is in the constellation of Virgo not
far from the 3rd magnitude star Zeta Virginis. The brighter galaxies in
this cluster have red magnitudes between 20 and 22 near the limit of the
Palomar Sky Survey. The bright blue galaxy (upper left) is probably a
foreground galaxy, and not a cluster member. The larger of the galaxies
in the cluster are probably about the size of our Milky Way Galaxy. The
diagonal line at lower right is an artificial satellite trail.
HST002013+28366 (upper right)
This cluster of galaxies lies in the constellation of Andromeda a few
degrees from the star Alpheratz in the northeast corner of the
constellation Pegasus. It is at an estimated distance of 4 billion
light-years, which means the light we are seeing from the cluster is as
it appeared when the universe was roughly 2/3 of its present age.
HST035528+09435 (lower right)
At an estimated distance of about 7 to 10 billion light-years (z=1),
this is one of the farthest clusters in the Hubble sample. The cluster
lies in the constellation of Taurus.
Credit: K. Ratnatunga, R. Griffiths (Carnegie Mellon University); and NASA.
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