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Super-star clusters may be born small and grow by coalescing
James E. Kloeppel, Physical Sciences Editor
217 244-1073; kloeppel@illinois.edu
1/11/05
Editors: More images
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CHAMPAIGN,
Ill. — A trio of massive, young star clusters found embedded in
a star cloud may shed light on the formation of super-star clusters
and globular clusters.
The discovery, made with images taken with the Hubble Space Telescope,
is being presented today by You-Hua Chu and Rosie Chen of the University
of Illinois at
Urbana-Champaign and Kelsey Johnson of the University of Virginia to
the American Astronomical Society meeting in San Diego. This finding
indicates that super-star clusters may be formed by coalescence of smaller
clusters.
The tightly packed group of clusters was found in the core of the active
star formation region NGC 5461, within an arm of the giant spiral galaxy
M101. This galaxy is located about 23 million light-years away in the
constellation Ursa Major (the Big Dipper).
“NGC 5461 has such a high concentration of light in its core that
some astronomers have thought it might host a super-star cluster,”
said Chu, who is a professor of astronomy at Illinois and principal investigator of the project. Super-star clusters,
with a total mass of up to 1 million times that of the sun, are five
to 50 times more massive than the spectacular R136 cluster at the center
of the Tarantula Nebula in the Large Magellanic Cloud. They are believed
to be the young counterparts of the massive globular clusters in our
galaxy.
Hubble Space Telescope images of the core of NGC 5461 revealed a tight
group of three massive clusters surrounded by a cloud of stars within
a region about 100 light-years in diameter. Although each cluster is
comparable to the R136 cluster, the total mass within this small volume
is similar to that of a super-star cluster.
“If NGC 5461 were several times farther away, even the Hubble
Space Telescope would be unable to resolve this tight group of clusters,”
said Chen, a graduate student at Illinois. “It is possible that
some of the super-star clusters previously reported in distant galaxies
actually consist of groups of clusters similar to NGC 5461.”
The large amount of mass at the core of NGC 5461 produces a strong gravitational
field, causing the clusters and stars to move and interact dynamically.
The rapidly fluctuating gravitational field produced by this interaction
dissipates the relative motion of the clusters into random motions of
individual stars. Eventually, the clusters and surrounding star cloud
will merge into one single star cluster.
“The Hubble Space Telescope images of NGC 5461 provide a unique
glimpse of a super-star cluster in the making,” said Johnson,
a professor of astronomy at Virginia. “There is no super-star
cluster yet, but it is just a matter of time.”
The dynamical evolution of the clusters at the core of NGC 5461 is being
simulated by astronomy professor Paul Ricker at Illinois. Preliminary
results show that under optimal conditions these clusters may merge
within a few million years.
“Fortunately, NGC 5461 is near enough, and young enough for us
to resolve it with the Hubble Space Telescope,” Chu said. “We
were indeed lucky to catch it at such an opportune time.”
The work was supported by the National Aeronautics and Space Administration.
The researchers will report their findings in the Feb. 1 issue of the
Astrophysical Journal.
