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January 13, 2003

Astronomers detect a faint debris trail in the Andromeda galaxy, more evidence of galactic cannibalism

By Tim Stephens

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 were presented last week by astronomers Puragra (Raja) GuhaThakurta of UCSC and David Reitzel of UCLA at the American Astronomical Society meeting in Seattle.

Adromeda galaxy photo

The nearby Andromeda galaxy (M31) is a spiral galaxy very similar to our own Milky Way. The "debris trails" astronomers have discovered in the galaxy's halo appear to be remnants of smaller galaxies that merged with Andromeda. The findings support the idea that large galaxies have grown by gobbling up smaller ones.
Photo: P. Choi and P. GuhaThakurta

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. 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. Reitzel, who earned his Ph.D. with GuhaThakurta at UCSC, is now a postdoctoral researcher in the Department of Physics and Astronomy at UCLA.

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