Orphaned Star Clusters Roam the Universe (Forwarded)

International Astronomical Union
Paris, France

Contacts:

Dr. Michael West
Department of Physics & Astronomy
University of Hawaii at Hilo

Telephone: 808-974-7744

Dr West will be attending the first week of the IAU General Assembly in Sydney,
Australia. From July 14 to 20, he can be contacted at the Novotel Darling
Harbour Hotel, Sydney, on telephone +61-2-9934-0000, or via the Media Room.

Helen Sim
Media Liaison
IAU General Assembly

Telephone: +61-419-635-905

July 17, 2003

Orphaned Star Clusters Roam the Universe

Sydney -– US and UK astronomers have discovered a population of previously
unknown star clusters in what was thought to be the empty space between
galaxies. The research is being presented today at the International
Astronomical Union's 25th General Assembly being held in Sydney, Australia, by
Dr. Michael West of the University of Hawaii.

Most galaxies are surrounded by tens, hundreds or even thousands of ancient star
clusters, which swarm around them like bees around a hive. Our own Milky Way
galaxy has about 150 of these "globular clusters", as they are called. Globular
clusters are systems of up to a million stars compacted together by gravity into
dense sphere-shaped groupings. Studies of globular clusters have provided many
important insights over the years into the formation of their parent galaxies.

The discovery of this new type of star cluster was made using images obtained
last year with the Hubble Space Telescope and the giant 10-meter Keck Telescope
on Mauna Kea, Hawaii. "We found a large number of ‘orphaned' globular clusters,"
said Dr West. "These clusters are no longer held within the gravitational grip
of galaxies, and seem to be wandering freely through intergalactic space like
cosmic vagabonds."

Although the lonely existence of such star clusters had been predicted for half
a century, it is only now that astronomers have finally been able to confirm
their existence. Dr West's team published preliminary findings about its
discovery in April this year, and is today presenting new results at the
International Astronomical Union's 25th General Assembly, being held in Sydney,
Australia.

"The new data from the Hubble Space Telescope and Keck Telescope confirm our
discovery, and are providing new insights to the origin of these objects," said
Dr West.

According to West, these globular star clusters probably once resided in
galaxies just like most of the normal globular clusters that we see in nearby
galaxies today. However, the pull of gravity from a passing galaxy can rip stars
and star clusters loose -- in some cases entire galaxies can be damaged or
destroyed by violent collisions or by the collective gravitational pull from
their galactic neighbors.

It is thought that the partial or complete destruction of their parent galaxies
spilled the globular star clusters into intergalactic space.

Finding these globular clusters hasn't been easy. With only one exception, all
of the intergalactic globular clusters the teams have detected are so far away
(millions of light-years) that they just look like tiny points of light in a
vast sea of blackness.

"Because they're so far away these objects are very faint, almost a billion
times fainter than the unaided human eye can see," said Dr West. "Detecting such
faint objects pushes the limits of even what the Hubble Space Telescope can do."

"By studying these intergalactic vagabonds in greater detail we hope to learn
more about the numbers and types of galaxies that may have been destroyed so far
during the life of the universe," said Dr West. "Some of these star clusters
might also eventually be ‘adopted' by other galaxies if they stray close enough
to be captured by their gravity."

The researchers are currently analyzing new Hubble Space Telescope images they
recently obtained, and are planning to obtain more at the end of this year.

Collaborators on this research (which involves two separate but complementary
teams) a

Dr. Michael West (University of Hawaii, USA)
Dr. Patrick Cote (Rutgers University, USA)
Dr. Ronald Marzke (San Francisco State University, USA)
Dr. Andres Jordan (Rutgers University, USA)
Dr. Michael Gregg (University of California, Davis, USA)
Dr. Henry Ferguson (Space Telescope Science Institute, USA)
Dr. Ted von Hippel (University of Texas, USA)
Dr. Nial Tanvir (University of Hertfordshire, England)

IMAGE CAPTIONS:

[Image 1:
http://www.astro.uhh.hawaii.edu/west...st-figure1.jpg (139KB)
http://www.astro.uhh.hawaii.edu/west...st-figure1.tif (759KB)]
This image taken by the Hubble Space Telescope shows a small region of space
within a large group of several thousand galaxies, about 400 million light-years
from Earth. The boxes indicate four of the numerous "orphaned" star clusters
that were recently discovered by a team of astronomers led by Dr Michael West of
the University of Hawaii.

Image credit: Dr. Michael J. West (University of Hawaii)

[Image 2:
http://www.astro.uhh.hawaii.edu/west...GC6093-HST.jpg (294KB)]
This Hubble Space Telescope image shows an example of a typical nearby globular
star cluster, called simply NGC6903, located in our own Milky Way galaxy. The
Milky Way has about 150 of these sphere-shaped star groups, each containing up
to a million stars. The team has been studying other globular star clusters that
are wandering in the otherwise empty space between distant galaxies, and are so
far away that they just look like tiny white dots in the black vastness of
intergalactic space.

Image credit: Hubble Heritage Team (AURA/STScI/NASA)