Astronomers find the most distant star clusters hidden behind anearby cluster (Forwarded)

Public Information Office
University of California-Santa Cruz

Contact:
Tim Stephens, (831) 459-2495

January 10, 2007

Astronomers find the most distant star clusters hidden behind a nearby
cluster

Astronomers have discovered the most distant population of star clusters
ever seen, hidden behind one of the nearest such clusters to Earth. At a
distance of more than a billion light-years, the newly discovered star
clusters provide a unique probe of what similar systems in our own galaxy
once looked like.

"Given their distance, the light that we see today from these clusters was
emitted more than one billion years ago and may hold important clues for
understanding the evolution of globular clusters," said Jason Kalirai, a
postdoctoral fellow at the University of California, Santa Cruz, who will
present the findings in a talk at the American Astronomical Society
meeting in Seattle.

Kalirai and Harvey Richer of the University of British Columbia led the
study, which began as an investigation of a globular star cluster in the
Milky Way galaxy known as NGC 6397. The researchers acquired one of the
deepest optical images ever taken with the Hubble Space Telescope's
Advanced Camera for Surveys, focusing on a small field within NGC 6397.
This cluster, home to hundreds of thousands of stars, is 8,500 light-years
away, making it one of the closest globular clusters to Earth.

The new data from stars within NGC 6397 have yielded important insights
into the age, origin, and evolution of this cluster. Hidden in the
background, however, were findings that may hold even greater promise for
understanding the evolution of such clusters, Kalirai said. Within the
population of stars and galaxies behind NGC 6397, the Hubble image
revealed a large elliptical galaxy that contains several hundred globular
clusters.

Although each of these clusters probably contains hundreds of thousands of
stars, they are so far away from the Earth that each cluster appears as a
single faint point of light in the Hubble image. In fact, a single giant
star in NGC 6397 appears 10 million times brighter than one of the distant
globular clusters. Nevertheless, the faint light from these clusters could
yield valuable information, Kalirai said.

Kalirai and Richer followed up the Hubble imaging observations with
spectroscopic observations using the Gemini Multi-Object Spectrograph on
the Gemini South Telescope on Cerro Pachon in Chile. They were able to
determine the distance of the elliptical galaxy hosting the globular
clusters by measuring its redshift (a measure of how the expansion of the
universe shifts the wavelengths of light from a distant object). This
showed that the globular clusters are the most distant ever studied.

"The properties that we infer for these clusters may therefore represent
an important clue in understanding what our own Milky Way globulars, such
as NGC 6397, looked like in the past," Kalirai said.

Previous studies by other researchers of globular clusters in nearby
galaxies, including the Milky Way, have shown that these systems play a
very important role in understanding the formation and evolution of
galaxies. With a sample of almost 200 clusters in this one distant galaxy,
Kalirai's team will test whether the properties of these globulars are
consistent with the idea that elliptical galaxies formed the bulk of their
stars at early times. For the first time, the observations may also allow
astronomers to test for evolution in the properties of globular clusters
themselves, Kalirai said.

In addition to Kalirai and Richer, the team involved in this research
includes Jay Anderson of Rice University and Jay Strader and Kieran Forde
of UC Santa Cruz. This work was supported by NASA through a Hubble
Fellowship grant awarded by the Space Telescope Science Institute to
Kalirai. Support for this work was also provided by a grant from
NASA/STScI, the Natural Sciences and Engineering Research Council of
Canada, and the Canada-U.S. Fulbright Program through the award of a
Fulbright Fellowship to Richer.

Note to reporters: You may contact Kalirai at (831) 459-3804 and Richer at
(604) 822-4134.

An image of NGC 6397 can be obtained after the embargo expires at
http://www.ucolick.org/~jkalirai/globulars.jpg (683KB)