SANTA CRUZ, CA — The discovery of a faint trail of stars in
the nearby Andromeda galaxy offers new evidence that large
spiral galaxies have grown by gobbling up smaller satellite
galaxies. The new findings are being presented on Monday,
January 6, by astronomers Puragra (Raja) GuhaThakurta of the
University of California, Santa Cruz, and David Reitzel of
UCLA at the American Astronomical Society meeting in Seattle.

Andromeda (also known as M31) is the nearest large galaxy to
our own Milky Way and is very similar to it in appearance.
Studying Andromeda gives astronomers an external perspective
on a galaxy much like our own — it’s like looking at a
bigger sibling of our galaxy, said GuhaThakurta, an
associate professor of astronomy and astrophysics at UCSC.
How such galaxies formed is a central question for
astronomers, he said.

In spiral galaxies like Andromeda and the Milky Way, most
of the prominent young stars lie in a flat disk with spiral
arms. In addition, a spherical halo of scattered stars
surrounds the disk. Last year, a team of European
astronomers reported a giant stream of stars threading
through the halo of Andromeda. It is thought to be a vast
trail of debris left over from an ancient merger of
Andromeda with a smaller galaxy.

The debris stream now being reported by GuhaThakurta and
Reitzel is well separated from the one reported last year.
It is also much fainter and was detected using a more
sensitive technique. The researchers focused on a narrow
section of Andromeda’s halo, and the fact that they found
a debris stream there suggests that there may be many
more faint streams in other parts of the galaxy,
GuhaThakurta said.

“We looked in just one narrow area of the halo and found
a debris stream, which tells us there are probably many
more lurking under the surface. Our ability to detect
such a faint stream also means that our technique is
very sensitive,” he said.

The European team, led by Rodrigo Ibata of the Strasbourg
Observatory in France, mapped the density of stars in
Andromeda’s halo, and the resulting image revealed a
prominent debris stream. This image shows no trace,
however, of the fainter stream found by GuhaThakurta and
Reitzel.

“We found another stream in a different part of the
galaxy, but it is much more subtle,” Reitzel said. “Only
about 10 percent of the stars in that part of the sky
belong to the stream.”

GuhaThakurta and Reitzel used the powerful LRIS and DEIMOS
spectrographs on the giant Keck Telescopes in Hawaii to
study stars in four fields along a line from the center
of the galaxy out through the halo. Spectrographs separate
the light from a star into a spectrum of different colors
or wavelengths. Astronomers can derive information about
a star’s motion and chemical composition from its spectrum.

GuhaThakurta and Reitzel discovered a coherent group of
chemically rich stars all traveling with the same velocity,
which stood out against a background of stars with a broad
range of chemical compositions and velocities.

Chemical elements heavier than hydrogen and helium, which
astronomers refer to as “metals,” are synthesized in very
massive stars, which then enrich future generations of
stars in these heavier elements. The stars in Andromeda’s
halo cover a broad range of chemical enrichment properties,
and their motions relative to one another are generally
random. So the researchers were surprised to find that
the most metal-rich stars in their sample all seem to be
moving together as a group.

“We think we are seeing the debris trail of a small,
chemically rich galaxy that fell into Andromeda,”
GuhaThakurta said. “Even as this galaxy was disrupted
by Andromeda, its stars would retain a memory of their
original velocity and continue to move as a coherent
group.”

Over time, debris streams mix together with the rest of
the galaxy and become less prominent. Using the
spectroscopic technique, which is sensitive enough to
detect a very subtle debris stream, astronomers can find
evidence of events that took place far back in time,
GuhaThakurta said. But it is hard to tell whether a
debris stream is faint because it is old or because the
galaxy it came from was small, he noted.

The researchers are continuing their survey of stars in
Andromeda’s halo, aiming to observe around 10,000 stars.
They are beginning to collaborate with Ibata’s group on
the survey. Eventually, their findings may enable the
researchers to piece together a detailed history of how
the Andromeda galaxy was assembled.

“These debris streams tell us something about how the
galaxy was assembled, but before we can really start to
describe the assembly process, we need to survey the
whole galaxy in this mode and do a statistical analysis
of the debris streams,” GuhaThakurta said.

GuhaThakurta is currently a Herzberg Fellow at the
Herzberg Institute of Astrophysics in Victoria, British
Columbia, Canada. Reitzel, who earned his Ph.D. with
GuhaThakurta at UCSC, is now a postdoctoral researcher
in the Department of Physics and Astronomy at UCLA.

Note to reporters: You may contact GuhaThakurta at
(250) 889-2498 or raja@ucolick.org and Reitzel at
(310) 825-0344 or reitzel@astro.ucla.edu .

Images can be downloaded from the web at
http://www.ucsc.edu/news_events/download/