[Larger image] Astronomers have identified the vivid scar of a cosmic catastrophe: a blue
arc thousands of light years long produced when a galaxy pulled in a
smaller satellite galaxy and tore it apart.

The streak is composed of clusters of young blue stars that formed as the
larger galaxy, Centaurus A, absorbed the smaller galaxy about 200 million
to 400 million years ago. Researchers will report in the December
Astronomical Journal that their discovery suggests absorption of smaller
galaxies may be a significant contributor to the formation of galactic
halos, outer perimeters of galaxies where star populations are sparse.

“This adds a nice example in the local universe to the growing evidence
that galaxy halos are built up from the accretion of dwarf satellite
galaxies,” said Eric Peng, a graduate student in astronomy in the Krieger
School of Arts and Sciences at The Johns Hopkins University and lead author
of the new paper. “These halos are interesting partly because they’re hard
to study, but also because time scales for things to happen in halos are
very long, which means they may preserve conditions that reveal how a
galaxy formed and evolved.”

Peng and his colleagues found the streak in specially processed digital
images of Centaurus A taken at the National Science Foundation’s Blanco
4-meter telescope at Cerro Tololo Inter-American Observatory near La
Serena, Chile. The team made its observations in 2000.

At 10 million light years from our galaxy, Centaurus A (which is visible
with binoculars in the southern hemisphere’s night sky, but not visible at
all in the northern hemisphere) is quite close in galactic terms. The
galaxy’s most prominent features include a central lane of dust and debris,
and signs of violent activity on its perimeter that are suggestive of a
prior galactic merger.

Astronomers had previously noticed the arc that Peng and colleagues have
now identified as a galactic merger remnant, but without recognizing its
origin. That took evidence Peng and colleagues gathered with Mosaic II, a
new wide-field digital camera at the Blanco Telescope funded by the
National Optical Astronomy Observatory.

Peng and fellow researchers Holland Ford, a professor of astronomy at
Johns Hopkins; Ken Freeman, a professor at the Australian National
University; and Rick White, astronomer at the Space Telescope Science
Institute, used Mosaic II to create images of Centaurus A through several
different color filters. Comparing these images highlighted regions of the
galaxy that have different colors, and revealed the predominance of young
blue stars in the arc. This allowed Peng and his colleagues to identify the
arc as a remnant of a galactic merger and estimate the time when Centaurus
A absorbed it.

Astronomers have identified similar remnants of galactic mergers in
various stages of ingestion into the Milky Way, including the Sagittarius
Galaxy. Peng said the newly identified remnant in Centaurus A is unusual in
terms of both how recently the merger took place and how gas-rich the
satellite galaxy appears to have been.

Ford noted that the group hadn’t originally set out to find remnants of
dwarf galaxies.

“One of the joys of science is unexpected discoveries,” Ford said.
“Although our pictures were taken for another project, we decided to search
the data for evidence of ‘shredded’ dwarf galaxies. We were very excited
when the blue arc popped out of one of the images.”

Peng noted that the remnant cannot account for all the signs of prior
galactic merger activity seen in Centaurus A. As an elliptical (or roughly
football-shaped) galaxy, Centaurus A was likely produced by the merger of
two large galaxies.

“It’s possible that the small galaxy that was recently accreted by
Centaurus A was originally a satellite orbiting one of the large galaxies
involved in that larger merger,” says Peng. “Just like when our galaxy
merges with the Andromeda Galaxy in the distant future-the Milky Way’s
satellite galaxies, such as the Magellanic Clouds, will likely be involved
in that merger.”

Peng and other researchers are currently looking for other signs of
remnants elsewhere in Centaurus A, and planning for follow-up observations
on the arc they identified as a galactic remnant.

“If we can get the velocities of star clusters in the arc and map out the
orbit, that will allow us to place better constraints on how long ago the
merger occurred and on what the motions of the incoming galaxy were,” Peng
says.

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Note to editors: Digital images of Centaurus A and the arc identified by
Peng et. al as a merger remnant are available on request or online at the
following sites

JHU: http://www.jhu.edu/news_info/news/home02/oct02/galaxy.html

NOAO: http://www.noao.edu/outreach/press/pr02/pr02011.html

Online preprint of the Astronomical Journal
paper: http://xxx.lanl.gov/abs/astro-ph/0208422

Johns Hopkins University news releases can be found on the World Wide Web
at http://www.jhu.edu/news_info/news/