University Communications
University of Arizona
Tucson, Arizona

Contact Information:
Karen Knierman, 520-621-6535
Patricia Knezek, 520-318-8442

January 11, 2006

Tidal Tales of Minor Mergers: Young Stars Where They Shouldn't Be
By Lori Stiles

Arizona astronomers have discovered a population of what appear to be
young star clusters where they aren't supposed to be. The newborn stars
appear to have formed in the debris of the NGC 2782 galaxy collision --
debris that lacks what astronomers believe are some important ingredients
needed to form stars.

A large, Milky Way-type galaxy collided with a much smaller galaxy in the
NGC 2782 collision. It's an example of the most common type of galaxy
collision in the universe. Scientists believe that such collisions played
an important role in the buildup of large galaxies in the early universe.

If confirmed, these newly discovered young star clusters and their
environment could help shed light on the process of star formation,
especially in the early universe in regions far from the crowded, active
centers of galaxies.

Karen Knierman, a graduate student and Arizona/NASA Space Grant Fellow at
The University of Arizona, and Patricia Knezek of the WIYN Consortium in
Tucson, Ariz., are reporting the research at the American Astronomical
Society meeting in Washington, D.C., today.

The astronomers found the star clusters by taking deep images of the
galaxy collision with the 4 Megapixel CCD camera of the 1.8 meter
(71-inch) Vatican Advanced Technology Telescope (VATT) at Mount Graham
International Observatory in Arizona.

NGC 2782 lies about 111 million light years away toward the Lynx
constellation. When the two galaxies of unequal mass collided about 200
million years ago, their gravitational pull ripped out two tails of debris
with very different properties.

Beverly Smith of Eastern Tennessee University and collaborators studied
the optical and gas properties of these two tails and published their
results in 1994 and 1999. Studying the gas properties tells astronomers
about neutral hydrogen gas and molecular gas -- both important ingredients
in star formation. Smith and collaborators found that the optically bright
eastern tail has some neutral hydrogen gas and molecular gas at the base
of the tail, and an optically bright, but gas-poor concentration at the
end of the tail. The optically faint western tail is rich in neutral
hydrogen gas, but has no molecular gas.

Knierman and Knezek found blue star clusters younger than 100 million
years along both tails, indicating that those stars formed within the
tails after the galaxy collision began.

"That's surprising because the western tail lacks molecular gas, one of
the key ingredients for star formation," Knierman said.

Star clusters are thought to form from the collapse of giant molecular gas
clouds. If this is the case, astronomers would expect to see remnants of
the molecular gas which helped give birth to the stars.

Given Smith's earlier observations of gas in the debris tails, Knierman
and Knezek expected they might see star formation in the eastern tail,
where molecular gas is clearly present. But they didn't expect to see star
formation in the western tail, where no molecular gas was detected.
Finding young star clusters in the western tail should prompt astronomers
to question their current models of star formation, the Arizona team said.

"Do we still need a model of giant molecular gas clouds?" Knierman asked.
"Or do we need a different model -- perhaps one with smaller clumps of
molecular gas that might have been destroyed or blown away when these
energetic young stars formed?"

Finding unexpected young star clusters in the western tail could help
explain why stars form in other places where there may be little molecular
gas, like the outer edges of the Milky Way galaxy or in the debris of
other galaxy collisions, Knierman and Knezek noted.

"This has important implications in how star formation proceeded when our
universe was young and galaxy collisions were much more common than they
are today," Knierman said.

"Only recently have we become aware of the importance of the merging of
small galaxies with larger systems in creating galaxies like our own Milky
Way," Knezek added.

PHOTO CAPTION:
[http://thorn.as.arizona.edu/~kak/NGC2782/Images/]
Optical image of the galaxy merger NGC 2782 showing the location of young
star clusters which formed in the sweeping tails of debris after the
collision. Even though the gas properties of the two tails are different,
both tails show unexpectedly similar populations of young star clusters.
This material was presented to the American Astronomical Society meeting
in Washington, D.C. on Jan. 11, 2006.

PHOTO CREDIT: Karen Knierman (Steward Observatory, University of Arizona)
and Patricia Knezek (WIYN)