Santa Cruz, CA -- Astronomers have obtained new evidence of an extreme warp in the stellar disk of the Andromeda Galaxy (also known as M31), our nearest galactic neighbor. The findings are being presented by Puragra Guhathakurta and Philip Choi of the University of California, Santa Cruz, and David Reitzel of UC Irvine at the American Astronomical Society meeting in San Diego, CA.
Previous studies had suggested the presence of a warp in the outer parts of Andromeda's stellar disk. The new findings appear to confirm those observations and suggest that the warp in Andromeda may be the most extreme case of a warped stellar disk ever observed in a spiral galaxy. Possible causes of the warp include interactions between Andromeda and its smaller satellite galaxies.
Many spiral galaxies, including the Milky Way, appear to have warps in the outer reaches of their stellar disks. The rotating body of stars and gas that characterizes a spiral galaxy is generally flat, but the outer regions may deviate from the plane of the disk, like an old record album exposed to too much heat. The warp tends to occur at the outer edges, while the inner part of the stellar disk remains flat. These warps are very difficult to demonstrate conclusively, however, because the outer portions of the stellar disk are extremely faint compared to the bright central region.
"The faint outer parts of the galaxy are more susceptible to warping because they are less strongly bound by the gravitational forces that keep the disk in a plane, and they are also more susceptible to the influence of neighboring galaxies," Guhathakurta explained.
The Andromeda Galaxy is a good candidate for studying a warped stellar disk because the plane of its disk is inclined toward the Earth at an angle of about 77 degrees (an inclination angle of 90 degrees would give a perfect edge-on view of the disk). The inclination of Andromeda's disk gives it an elliptical shape in the sky.
The new findings are based on two distinct sets of observations by Guhathakurta and his coworkers. The first set of observations was obtained using a large-format digital camera on the 0.9-meter Burrell Schmidt telescope at Kitt Peak National Observatory in Arizona. The images showed that the inner part of Andromeda's stellar disk looks like a perfect ellipse, but in the extreme outer parts it starts to deviate. The warping gives the galaxy a slightly S-shaped appearance, with the outer edge tilted above the plane of the disk on one side and below it on the opposite side.
Older observations using photographic plates had suggested the same thing, but the features are so faint that they were at the limit of photographic detectability and were obscured by artifacts on the plates, Guhathakurta said. Modern digital cameras are far more sensitive than photographic plates, although they cannot yet cover as wide a field of view. In order to image the entire stellar disk of Andromeda, which covers an area of the sky several times the size of the full moon, the researchers had to take multiple images of adjacent areas of the sky and fit them together into a mosaic.
"The S-shaped pattern is very clear in the digital data. If you look at the digital images and the photographic work side by side, you can see the same thing, but you can also see why the photographic data was not very reliable," Guhathakurta said.
The researchers also obtained additional evidence suggesting that the degree of warping in Andromeda's stellar disk may be especially dramatic. This second set of observations was the unanticipated by-product of an unrelated study of stars outside the disk of the galaxy. Spiral galaxies consist of two components: the stellar disk and a ball-shaped collection of stars called the spheroid. The stars that populate the spheroid and the disk have very different properties. But Guhathakurta and Reitzel found stars with the properties of disk stars in a location far removed from the disk.
"We were trying to get as far away from the disk as possible to pick out stars belonging purely to the spheroid, and we had calculated the probability of finding a disk star where we were looking was much less than 1 percent," Guhathakurta said.
Much to their surprise, however, four of the 30 stars they observed had properties characteristic of disk stars. Reitzel, who conducted the study as a graduate student at UCSC, had set out to study the chemical compositions of red-giant stars in the spheroid component of Andromeda, using the LRIS spectrograph on the 10-meter Keck II Telescope in Hawaii. These stars would be expected to have relatively low abundances of metals. In addition, their random orbits (unlike the coherent motions of stars in the disk) should result in a broad distribution of velocities relative to the Sun. In both metallicity and velocity, the four stars stood out as having the properties of disk stars.
"This is only suggestive evidence, but it is exciting because, if confirmed, it could mean there is a very extreme warp in the stellar disk of Andromeda," Guhathakurta said.
The researchers hope to continue their investigation of Andromeda's warped stellar disk by conducting more detailed observations at Kitt Peak in fall 2001 as part of Choi's Ph.D. thesis work at UCSC. This research is supported by the Alfred P. Sloan Foundation and a grant from UC's California Space Institute.
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