AUSTIN, TEXAS -- Hubble Space Telescope imaging of the extended halo of a nearby spiral galaxy has turned up some intriguing ideas about the nature of the unseen "dark matter" that holds galaxies together.

Astronomers from the University of California, Berkeley, Yale University, Cambridge University in England and universities in France and the Netherlands took long-exposure infrared images of a small portion of the faint halo of the galaxy NGC 5907, an edge-on spiral galaxy a mere 39 million light years away.

What they did not see - lots of stars - has led them to conclude that the halo is composed of a weird population of stars, mostly dim dwarfs too faint to see from Earth.

Most galaxies contain a mix of bright giant stars and dim dwarf stars, with about half of the light coming from each group. If the halo of NGC 5907 contained a mix similar to that in our own galaxy, the team would have seen hundreds of bright giants in the field of view. Instead they saw only a handful of bright stars.

The best explanation of the team's observations is that at least 20 times more light comes from dwarfs than giants in the halo of NGC 5907.

"Our results force us to turn to more esoteric descriptions of the stellar content of NGC 5907's halo," said Michael Liu, a graduate student at UC Berkeley and lead author. "In particular, our data combined with the measured colors of the halo suggest a very metal-poor stellar population with an enormous excess of faint dwarfs.

"This is the first direct evidence of a substantial population of stars which is essentially all dwarf stars," Liu added. "Such a population has been invoked in the past as a constituent of the dark matter making up galaxy halos."

The results could mean that this faint or "dark" halo - as distinct from the normal stellar halo surrounding spiral galaxies - is the dark matter within NGC 5907, as suggested when it was first observed in 1994.

Even if the dwarfs don't have enough mass to account for the dark matter holding this galaxy together, the fact that the dwarfs have the same profile as the dark matter means some formation mechanism may well tie the two together, Liu said.

"What's exciting about this is that the stars in this faint halo may have the same kind of properties as inferred for the dark matter," said coauthor Stephen Zepf, an assistant professor in the astronomy department at Yale University.

Liu, Zepf and colleagues report their results in a poster session Jan. 6 at the annual meeting of the American Astronomical Society in Austin, Texas. Coauthors include UC Berkeley astronomy professor James Graham, Francine Marleau of the Institute of Astrophysics at Cambridge University, Stephane Charlot of the Institute of Astrophysics in Paris and Penny Sackett of the University of Groningen in the Netherlands.

Four years ago, Sackett and Heather Morrison, an astronomer then at the National Optical Astronomy Observatories in Tucson, took long-exposure optical pictures of NGC 5907 using the 0.9 meter telescope at Kitt Peak. The images of the galaxy, which covers an area of the sky about half the diameter of the moon in the constellation of Draco, revealed a faint red halo surrounding the disk.

Such measurements are difficult because the halo is about 1,000 times fainter than the glow of the night sky. This background glow must be subtracted from the image to view the halo.

Unlike the halo of normal stars surrounding many spiral galaxies, which drops off very quickly away from the center, this halo dropped off more slowly - at about the rate predicted for the mysterious dark matter. Dark matter, composed of either normal or exotic material, must fill the galaxy in order to explain why stars rotating around the center haven't flown away. Its mass must drop off as the square of the distance from the center, approximately. That is, the mass at twice the distance is only one fourth as great. (The matter in a normal stellar halo drops off as the fourth power of the distance from the center, so mass at twice the distance is only one sixteenth times as great.)

Other groups have since confirmed the existence around NGC 5907 of a halo of stars with a gradual fall-off, unlike, for example, the more rapid drop-off of the known stellar halo of our own Milky Way galaxy. A similar halo has since been observed around a second spiral galaxy.

Subsequent measurements suggested also that the halo stars must be metal-rich, which doesn't fit with the type of stars expected in a stellar halo. A normal stellar halo is thought to have little or no on-going star formation, so halo stars should be old and have formed out of metal-poor material. For this reason, NGC 5907's halo colors were hard to explain.

The team of American and European astronomers decided to use the Hubble Space Telescope's near infrared camera (NICMOS, or Near Infrared Camera and Multi-Object Spectrometer) to look for bright stars in a small area of this faint halo. They observed at a wavelength of 1.6 microns in the infrared. The team reasoned that if the faint red glow of the halo is from a normal population of stars, we should see hundreds of bright, giant stars in the field, which is about 20 arcseconds (1,000 light years) on a side.

The observations from the space telescope required 13 orbits totaling about nine and a half hours of viewing time. In comparison, recent deep field views from the Hubble Space Telescope required about 100 orbits. No one else has attempted such a close look at a galactic halo.

To their surprise, the researchers saw only one or two bright stars, hundreds of times fewer than expected.

The only way to explain the numbers and still account for the reddish color of the halo light is to assume that the mix of giant and dwarf stars is different from the normal galactic population, Liu said. That is, that almost all the light from the halo is coming from dwarf stars.

It is not clear how such a population could have arisen, though Zepf said that one speculation is that stars like our Sun formed differently billions of years ago, yielding different proportions of giant and dwarf stars.

A group of French astronomers suggested earlier this year that the faint halo around NGC 5907 is unrelated to the "dark matter," but rather resulted from a long-ago collision with a companion elliptical galaxy, which eventually merged with NGC 5907. The metal-rich stars of the companion would have permeated NGC 5907's halo, leading to the extended light observed. This scenario seemed plausible, Liu said, because elliptical galaxies are known to have metal-rich stars, small elliptical companions to big galaxies are common and galaxies often merge.

"Our HST NICMOS results argue against the merger scenario, since we don't see the hundreds of giant stars one should see for a normal metal-rich stellar population," he said.

The work was supported by the National Science Foundation and the National Aeronautics and Space Administration through grants awarded by the Space Telescope Science Institute.