August 18, 2003
Astronomers capture a dwarf galaxy being torn
apart in the dark matter halo of a massive galaxy
By Tim Stephens
For the first time, astronomers have found direct evidence of a phenomenon
long thought to play an important role in the formation of giant galaxies:
the ongoing disruption of a small galaxy as it orbits within the dark
matter halo of a much larger galaxy.
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This enhanced Hubble Space Telescope
image shows the edge-on spiral galaxy and the dwarf galaxy above
it, at the center of the plumes. |
Images from the Hubble Space Telescope, confirmed by detailed observations
at the W. M. Keck Observatory in Hawaii, show a dwarf satellite galaxy
in the process of being torn apart by gravitational forces due to the
larger spiral galaxy and its halo of dark matter.
"Although long predicted, direct evidence for plumes of stars
being ripped from a dwarf galaxy as it is swallowed up by a giant galaxy
has remained elusive. This discovery provides the best evidence to date,"
said Duncan Forbes, an astronomer at Swinburne University of Technology
in Australia.
Forbes is first author of a report on the discovery published online
by Science Express on August 7. The paper will appear in print
in a later issue of the journal Science. Forbes's coauthors include
Jean Brodie, professor of astronomy and astrophysics at the University
of California, Santa Cruz, whose team examined the two galaxies during
an observing run at the powerful Keck I Telescope in March.
"In the Hubble images you can see a small blob close to the spiral
galaxy that seems to have two plumes coming from it, so you could speculate
that they are connected. But to prove it we had to show that they are
in fact in the same location in space and not just superimposed,"
Brodie said.
Spectroscopic analysis of the light from the two galaxies showed that
they are in fact physically associated, lying at a distance of about
2 billion light-years from Earth. Because the dwarf galaxy is so faint,
this type of analysis could only be achieved with a large telescope
like the Keck, Brodie said.
The two interacting galaxies are among thousands of faint galaxies
seen in the background of one of the first images obtained with the
new Advanced Camera for Surveys installed on the Hubble Space Telescope
last year. Dominating the image is the dramatic "Tadpole"
galaxy, but it was the backdrop of more distant galaxies that most excited
astronomers.
Michael Beasley, a postdoctoral researcher working with Forbes at Swinburne,
first noted the uncataloged spiral galaxy and its apparent satellite.
Beasley, Brodie, and Brodie's graduate student Jay Strader were at the
Keck I Telescope as part of an ongoing study of globular clusters in
distant galaxies when they decided to take a closer look at this intriguing
pair of objects.
Plumes of stars can be seen streaming away from the center of the dwarf
galaxy. The gravitational pull of the larger spiral galaxy and its associated
dark matter creates tidal forces that disrupt the smaller galaxy and
strip stars away from it. These stars eventually become part of the
spherical halo of stars around the flattened disk of the spiral galaxy.
With the observational data in hand, the researchers turned to Kenji
Bekki of the University of New South Wales, who performed computer simulations
to see how the interaction of the two galaxies would evolve over time.
The simulations suggest that the disruption process is transforming
the satellite from one type of dwarf galaxy into another, Forbes said.
"Our observations combined with our simulations lend support to
the idea of tidal disruption transforming dwarf galaxies," he said.
The new findings support the prevailing theory of a hierarchical universe
in which galaxies were formed within halos of dark matter, and massive
galaxies have grown by gobbling up smaller galaxies. Astronomers have
observed many cases of galaxy interactions involving two large galaxies,
but the evidence for giant galaxies absorbing less massive ones has
been largely circumstantial. For example, streams of stars in our own
Milky Way and in the nearby Andromeda galaxy have been identified as
relics of past mergers.
"We assume that mergers with minor galaxies are a very important
part of how massive galaxies build up their halos, but this is the first
time we've been able to see directly that it's happening, without making
any assumptions," Brodie said.
Simulations of dwarf satellites suggest why astronomers have not previously
witnessed such a clear example of ongoing disruption. Most dwarf galaxies
have highly elongated orbits that take them far away from their host
galaxies most of the time. In addition, the plumes or tails of stars
being stripped away from them are usually very faint and extremely hard
to detect.
"You can only see this type of interaction when the satellite
galaxy is very close to the more massive host galaxy, but dwarf galaxies
spend most of their time very far away," Brodie said.
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