Some Globular Clusters May Be Leftovers From Snacking Galaxies (Forwarded)

Harvard-Smithsonian Center for Astrophysics

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For Release: August 12, 2004

Release No.: 04-26

Some Globular Clusters May Be Leftovers From Snacking Galaxies

Cambridge, MA -- Globular star clusters are like spherical cathedrals of light
-- collections of millions of stars lumped into a space only a few dozen
light-years across. If the Earth resided within a globular cluster, our night
sky would be alight with thousands of stars more brilliant than Sirius.

Our own Milky Way Galaxy currently holds about 200 globular clusters, but once
possessed many more. According to the hierarchical theory of galaxy formation,
galaxies have grown larger over time by consuming smaller dwarf galaxies and
star clusters. And sometimes, it seems that the unfortunate prey is not
swallowed whole but instead is munched like a peach, stripped of its outer
layers to leave behind only the pit. New research by Paul Martini
(Harvard-Smithsonian Center for Astrophysics) and Luis Ho (Observatories of the
Carnegie Institution of Washington) shows that some globular clusters may be
remnants of dwarf galaxies that were stripped of their outer stars, leaving only
the galaxy's nucleus behind.

Martini and Ho reported their results in the July 20, 2004, issue of The
Astrophysical Journal.

Their findings hint at an important yet puzzling connection between the largest
globular clusters and the smallest dwarf galaxies. "Star clusters and galaxies
are quite different from a structural standpoint -- star clusters are much more
centrally concentrated, for example -- and so the mechanisms that form them must
be quite different. Identification of star clusters in the same mass range as
galaxies is a very important step toward understanding how both types of objects
form," says Martini.

For their investigation, the team studied 14 globular clusters in the large
elliptical galaxy Centaurus A (NGC 5128) using the 6.5-meter-diameter Magellan
Clay telescope at Carnegie's Las Campanas Observatory, Chile. The clusters were
selected for their brightness, and since brighter clusters tend to contain more
stars and more mass, were expected to be massive. Yet their results surprised
even Martini and Ho, showing that the globular clusters of Centaurus A are much
more massive than most globulars in the Local Group of galaxies (which includes
the Milky Way and the Andromeda Galaxy).

"The essence of our findings is that these 14 globulars are 10 times more
massive than the smaller globulars in our neighborhood, and whatever process
makes them can produce some really huge objects - they begin to overlap with the
smallest galaxies," says Martini.

Martini also points out the recent discovery of a suspected intermediate-mass
black hole in the Andromeda Galaxy globular cluster known as G1, which offers
further evidence linking globular clusters to dwarf galaxies. The presence of a
moderate-sized black hole is more understandable if it once occupied the center
of a dwarf galaxy -- a galaxy that lost its outer stars to the pull of
Andromeda, leaving it only a shadow of its former self.

Ho, a co-discoverer of the intermediate-mass black hole in G1, adds, "One of the
most surprising findings is that the black hole in G1 obeys the same tight
correlation between black hole mass and host galaxy mass that has been well
established for supermassive black holes in the centers of big galaxies. This
puzzling result is more understandable if G1 was once the nucleus of a larger
galaxy. A very interesting question is whether some of the massive clusters in
Centaurus A also contain central black holes."

Although most of our Galaxy's globular clusters are much smaller than those of
Centaurus A, the largest Milky Way globulars (such as the omega Centauri star
cluster) rival those of the elliptical galaxy. The similarities between massive
globulars in both galaxies may point to similar formation mechanisms. Future
studies of these most massive globular clusters will explore connections between
the formation processes for star clusters and galaxies.

Centaurus A is located approximately 12.5 million light-years away. It is about
65,000 light-years across and is more massive than the Milky Way and Andromeda
galaxies put together. Centaurus A possesses a total of about 2000 globular
clusters -- more than all of the galaxies in the Local Group combined. Recent
Spitzer Space Telescope observations of Centaurus A uncovered evidence that it
merged with a spiral galaxy about 200 million years ago.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA scientists,
organized into six research divisions, study the origin, evolution and ultimate
fate of the universe.

The Magellan telescopes are operated by the Carnegie Institution of Washington,
the University of Arizona, Harvard University, the University of Michigan, and
the Massachusetts Institute of Technology.

Las Campanas Observatory is operated by the Carnegie Observatories, which was
founded in 1904 by George Ellery Hale. It is one of six departments of the
private, nonprofit Carnegie Institution of Washington.

The Carnegie Institution of Washington (www.CarnegieInstitution.org) has been a
pioneering force in basic scientific research since 1902. It is a private,
nonprofit organization with six research departments in the U.S.: Embryology, in
Baltimore, MD; the Department of Terrestrial Magnetism and the Geophysical
Laboratory in Washington, DC; Plant Biology and Global Ecology in Stanford, CA.;
and The Observatories in Pasadena, CA, and Chile (http://www.ociw.edu/).

Note to Editors: High-resolution artwork to illustrate this release is available
on line at
http://www.cfa.harvard.edu/press/pr0426image.html