Astronomers peering deep into the core of the neighboring galaxy Andromeda are seeing double. Where there should be only one nucleus of densely clustered stars, they have detected two, and so are confronted with yet another puzzle over the violent forces that roil the central regions of galaxies.
The discovery of the apparent double nucleus in the M31 spiral galaxy in the Andromeda constellation was announced Monday by the Space Telescope Science Institute in Baltimore.In a detailed examination of new photographs taken by the orbiting Hubble space telescope, astronomers identified two bright spots of light at the heart of the galaxy, which is the nearest major aggregation of stars to our own Milky Way. The dimmer of the two spots appears to be at the center of the galaxy. The brighter one is at least five light-years away from the true center, but it corresponds to what astronomers had previously thought was the galactic nucleus, based on observations from the ground.
"Hubble shows that the M31 nucleus is much more complex than previously thought," said Dr. Tod R. Lauer of the National Optical Astronomy Observatories in Tucson, Ariz. He and Dr. Sandra M. Faber of the University of California at Santa Cruz directed the photographic analysis that led to the discovery.
It is not the first time a double nucleus has been found at the center of a galaxy, but astronomers said its occurrence in the Andromeda galaxy was particularly surprising and hard to explain. The few double-nucleus galaxies that have been discovered in recent years are generally attributed to the merger of two galaxies. Andromeda had appeared to be a single, relatively undisturbed spiral galaxy.
Scientists are not sure how common the double-nucleus phenomenon is. The centers of galaxies have yielded few of their secrets because they are obscured by dust and gas, although this is beginning to change with the advent of space-based observations.
Lauer and Faber suggested two possible interpretations of the Andromeda findings. One is that the brighter object might be the remnant of a smaller galaxy cannibalized by Andromeda. The other possibility is that dust might be dimming the core to create the illusion of a pair of star clusters when, in fact, they are two segments of a single elongated nucleus.
Dr. Lars Hernquist, an astrophysicist at the University of California at Santa Cruz, said in a telephone interview that the discoverers "seemed to be quite convinced that it's not dust." Normal galactic dust would scatter light in a way that would make it appear reddened, and this is not the case in the Hubble observations.