English: A team of astronomers using NASA's Hubble Space Telescope has discovered a "double nucleus" in the center of the neighboring spiral galaxy M31, located in the constellation Andromeda.

"Hubble shows that the M31 nucleus is much more complex than previously thought," says Dr. Tod R. Lauer of the National Optical Astronomy Observatories, Tuscon, Arizona.

A nucleus is a dense clustering of stars at the very center of a galaxy.

The astronomers report that the brighter member of the double nucleus might be the remnants of another galaxy cannibalized by M31. They say that an alternative possibility is that dust might dim the core to create the illusion of a pair of separate star clusters.

"The Hubble images intensify the mystery of what's happening in the center of this galaxy," says Lauer. "Neither interpretation offers a complete explanation of the M31 nuclear structure."

The double nucleus discovery is based on image analysis conducted by Lauer, Dr. Sandra M. Faber of the University of California, Santa Cruz, and other members of the HST Wide Field/Planetary Camera Imaging Team.

The HST pictures show two bright spots at the heart of the M31 galaxy. The dimmer of the two "light-peaks" appears to mark the exact center of the galaxy. The brighter peak is at least five light years away from the true center, but corresponds to what astronomers had previously thought was the nucleus of M31, based on ground-based observations.

Well-known as the Andromeda Galaxy, M31 (the 31st object in a catalog of non-stellar objects compiled by French astronomer Charles Messier in 1774) is located only 2.3 million light years away, making it the nearest major galaxy to our own Milky Way. M31 dominates the small group of galaxies (of which our own Milky Way is a member), and can be seen with the naked eye as a spindle-shaped "cloud" the width of the full moon.

Like the Milky Way, M31 is a giant spiral-shaped disk of stars, with a bulbous central hub of older stars. M31 has long been known to have a bright and extremely dense grouping of a few million stars clustered at the very center of its spherical hub. As seen from large ground-based telescopes, the starlight blends to resemble a single, bright, almost point-like source. Previous ground-based observations gave little hint of the true structure of the core, which is now revealed by Hubble.

In the 1960's the first high resolution photographs of M31's core were obtained by Stratoscope II, a balloon-borne observatory. The images were not as sensitive as Hubble's, and so only showed a single bright cluster of stars.

An important clue came with observations obtained in 1986 by the late Jean-Luc Nieto, then at the Pic du Midi Observatory in France. He found that the bright nucleus was offset by several light-years from the exact center of the galaxy's central bulge. The new HST images show that the dimmer peak instead is the true nucleus, and that the bright point of light evident from ground-based telescopes corresponds to the brighter of two peaks.

One possible explanation for the second cluster being offset from the exact center is that it is the remnant of a smaller galaxy that fell into M31 perhaps a billion or so years ago. The smaller galaxy's core is the only surviving fossil relic of the galactic collision.

A problem with the collision scenario is that the remnant core should be torn apart by the massive black hole hypothesized to dwell at the exact center of M31. The suspected black hole would be located in the middle of the dimmer peak uncovered by HST.

In 1988, the first evidence for a black hole at the exact center of M31 came from ground-based observations by Dr. John Kormendy (now at the University of Hawaii), and independently by Dr. Alan Dressler(Observatories of the Carnegie Institution of Washington), and Dr. Douglas O.Richstone (University of Michigan). Their data indicated an abrupt increase in the orbital velocities of stars in the center of the M31 nucleus.

This led the astronomers to conclude that M31 must have a strong but unseen concentration of mass at its center. A black hole at least ten million times the mass of the Sun is the most likely type of object matching these characteristics. A black hole is a theoretical object that is so dense that even light cannot escape its intense gravitational pull.

If such a black hole really exists, than the remnant core from the cannibalized galaxy would be torn apart in just a few hundred thousand years. "This is very short in cosmic time," says Lauer. "We would have to be looking at the galaxy at a very special time to see it now."

One way for the remnant to survive for a much longer time is if it has its own massive black hole. Gravity from a black hole in the remnant would hold it together against destruction from the other black hole at the M31 center.

"In retrospect, there may be evidence for this possibility in the spectra obtained by Kormendy, Dressler, and Richstone, says Lauer. "One problem with this picture, however, is if the black hole in the remnant were too big, it would distort even the true nucleus of M31."

Another interpretation of the "twin peaks" is that the bright spot is just the outer portion of a large nuclear star cluster, and that the central portions have been obscured by dust. A thick ring of dust might even cut across the nucleus, creating the illusion of two separate objects rather than one elongated structure.

Lauer explains that the problem with this idea is that normal galactic dust would scatter the light such that it would appear reddened. "But this is not the case, there are no color effects at all," he emphasizes. "This means that the dust grains would have to be much larger than average." In our own galaxy, however, the interstellar dust grains are roughly the same size. "We can only guess that earlier nuclear activity in M31 would have destroyed all the fine dust grains that would cause color effects," says Lauer.

M31 is an ideal target for Hubble once the telescopes's optics are improved during a space shuttle servicing mission in December. Spectrographs aboard Hubble will dissect the light from the two peaks of the double nucleus and determine if they are truly separate clusters. Astronomers will be able to measure the velocity of stars to pin down whether there is a black hole in either or both.

This is the latest in a series of Hubble observations that have uncovered unusual structures in the cores of galaxies. Some of them might be fossil evidence of galactic collisions. Hubble has also resolved very dense concentrations of stars in the cores of other galaxies that are circumstantial evidence that massive black holes are common among galaxies.