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. 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.

Images of the galaxies and a movie of the computer simulation are
available at the following web site:
http://astronomy.swin.edu.au/galaxy/dwarf.html

Reporters may contact Forbes at dforbes@swin.edu.au; 61-3-9214-4392
or 61-0404-171-540.