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Peering deep into space

Using the Hubble and other devices, astronomers hope to nail down the precise distance and mass of the farthest-off star clusters

Friday, August 28, 1998

By Byron Spice, Science Editor, Post-Gazette

A team of Carnegie Mellon University astronomers has used the Hubble Space Telescope to find some of the most distant galaxy clusters ever seen.

Using radio telescopes and other nonoptical devices, other astronomers have found even more distant clusters. The difference here, senior research scientist Kavan Ratnatunga said yesterday, is that the Carnegie Mellon team can also determine the relative abundance of those distant clusters.

With further study, astronomers hope to nail down the precise distance and mass of these clusters -- potentially important information for determining the fate of the universe.

"It opens up a lot of research," Ratnatunga said. Much of the follow-up work will need to be done with large, ground-based telescopes, and that's one of the reasons Carnegie Mellon recently pledged $6 million to help build one of the world's largest telescopes near Sutherland, South Africa.

The discovery of the galaxy clusters was part of a larger project on the Hubble called the Medium Deep Survey. One of three key projects planned for the Hubble before its launch in 1990, the survey is directed by Carnegie Mellon astrophysicist Richard Griffiths.

In contrast to the better known Deep Field Survey, which uses the Hubble to zero in on a tiny patch of sky and look as far into space as possible, the Medium Field Survey looks at lots of patches of sky. Over the past four years, the survey has cataloged about 300 parts of the sky, which together might be equal to half the size of the full moon.

The survey is piggybacked on other observations by the Hubble and works something like this: While the Hubble is locked on to some object that is being studied with the telescope's spectrograph or other instrument, the Medium Field Survey team uses the Hubble's wide-field camera to take a picture of whatever patch of sky is visible at the time.

The result is a collection of observations that are randomly scattered across the sky. Scientists can then use these random observations to make generalizations about the universe.

In the case of the galaxy clusters, for instance, what's most interesting isn't necessarily how distant they are, but how many of them they are, Griffiths said.

If there are an unusually large number of massive and distant clusters in the universe, he said, some theoretical models of the universe suggest that this is evidence that gravity will not be able to halt the expansion of the universe.

Though it seems paradoxical, Griffiths said, the presence of lots of massive clusters in the early universe could mean the universe lacks sufficient mass for gravity to act upon. The universe would thus continue to expand to infinity.

Earlier this month, discovery by Space Telescope Science Institute astronomer Megan Donahue of an ancient, massive cluster -- the weight of several thousand galaxies the size of our Milky Way -- was seen as some of the strongest evidence yet that this is a lightweight universe.

Theoretical models can change, however, and as yet the findings of the Medium Deep Survey are only preliminary.

For now, astronomers can only estimate the distance of the galaxy clusters on the basis on their brightness, Ratnatunga explained. It appears that many of the clusters detected might be 5 billion to 10 billion light-years away, perhaps half the age of the universe.

But this is just a rough estimate. To better determine the distance, and thus the age, of the clusters, astronomers will have to turn to large ground-based telescopes, such as the Keck Telescope in Hawaii, the Hobby-Eberly Telescope in Texas and -- if Carnegie Mellon scientists get their wish -- the South African Large Telescope, slated to be complete in 2003.

With those telescopes, astronomers can make the exacting observation necessary to calculate each cluster's red shift -- a measure of the speed at which an object is moving away from Earth. The larger the red shift, the faster it is moving and, thus, the farther away it is.

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