December 16, 1994 Contact: Robert Irion (408/459-2495)

RESEARCH WITH KECK TELESCOPE STRENGTHENS CASE FOR DARK MATTER IN TINY BUT COMMON GALAXIES

Study extends the classes of objects in which astronomers have detected dark matter

FOR IMMEDIATE RELEASE

SANTA CRUZ, CA--Astronomers have long suspected that dark matter, the mysterious material composing most of the mass in the universe, envelops galaxies and clusters of galaxies in tenuous shrouds. Now, they have found strong clues that dark matter also exists in a far smaller object: a dwarf galaxy in orbit around our Milky Way.

A team of researchers used the powerful W. M. Keck Telescope in Hawaii to chart the motions of individual stars in the tiny galaxy Leo II, some 720,000 light-years from earth. The rapid motions of the stars led the team to conclude that Leo II contains about six times more mass than meets the eye. Without the gravitational pull from the unseen mass, stars moving that quickly would escape the galaxy altogether.

Some astronomers believe that dark matter also lurks in other nearby dwarf galaxies, which dot space near the Milky Way like moths around a lamp. Others argue that the motions of stars in those galaxies might arise from tidal forces generated by the Milky Way's gravitational field. However, Leo II is so distant--more than twice as far from the Milky Way as most other dwarfs--that researchers have ruled out tidal forces as an explanation for the stars' motions.

"This is the first study to nail down the hypothesis that dwarf galaxies have large amounts of dark matter," says lead author Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz. "This result has significant implications for cosmology, because dwarfs are among the most common galaxies. If they all contain dark matter, they may contribute an unexpectedly large amount of total mass to the universe."

Vogt and three colleagues will publish their study in the January 1995 Astronomical Journal. His coauthors are Mario Mateo, assistant professor of astronomy at the University of Michigan; Edward Olszewski, associate astronomer at Steward Observatory, University of Arizona; and Michael Keane, graduate student at UC Santa Cruz. Mateo and Olszewski brought years of dwarf-galaxy research experience to their collaboration with Vogt and Keane.

Eight well-known dwarf galaxies, called "dwarf spheroidal galaxies" by astronomers, orbit around the Milky Way. A ninth was spotted close to the Milky Way's center earlier this year. The dwarfs are unspectacular conglomerations that barely register as smudges on old photographic plates; each contains perhaps one to ten million stars. Leo II, discovered in 1950, is one of the most remote of the dwarfs--fully one-third as far away as the gigantic Andromeda Galaxy but yet still bound to the Milky Way.

To search for signs of dark matter in the dwarfs, astronomers use spectrographs to examine how quickly their stars move. Dark lines in the spectra of many stars shift either toward the blue or the red end of the spectrum, revealing that each star is moving toward or away from earth. Many such observations yield an average picture of the motions in a swarm of stars. Just as cars can drive faster on a steeply banked curve, stars can orbit more quickly in a strong gravitational field. Thus, speedy motions point to a massive galaxy, while slower motions indicate less total mass. In this manner, astronomers including Mateo and Olszewski have found that most of the Milky Way's dwarf galaxies seem to contain much more mass than is apparent in their visible stars alone.

However, there are complications. Astronomers use red giant stars for this research because they are the brightest and hence easiest stars to see. The atmospheres of some red giants are unstable; their pulsations can create the illusion of rapid motions in space. To address that concern, the Leo II researchers chose only stable but much fainter red giants. Further, some stars revolve around other stars in binary systems, producing yet another kind of motion. The team used computer simulations and long-term observations of red giants in other dwarf galaxies to show that the small number of binaries they might have observed in Leo II would barely affect their results.

In addition, some researchers claim that the Milky Way inflicts serious tidal pulls on its nearby dwarf companions. Stars on the near side of each galaxy feel a stronger tug toward the Milky Way than those on the far side. Astronomers disagree about whether the resulting motions of stars would swamp a dark-matter signature. The minuscule tidal forces on Leo II, however, could not possibly account for the stellar motions seen by the team.

"With information about only one dwarf spheroidal, you can find ways to wiggle out of dark matter," says Olszewski. "With knowledge of each additional system, wiggling becomes harder. But Leo II provides a very clean test of the dark-matter hypothesis for dwarf spheroidals, because it's so far from the Milky Way."

Leo II is so distant, in fact, that its individual stars are beyond the pale of all telescopes except the ten-meter Keck. To get the precise measurements needed, the team used the High- Resolution Echelle Spectrograph (HIRES), designed by Vogt and built at the UCO/Lick Observatory shops at UC Santa Cruz. HIRES produces exquisitely detailed spectra, for it contains the largest lenses and gratings of any spectrograph. Thanks to the telescope's light- gathering power, difficult observations that require heroic efforts at other observatories are routine with HIRES.

"These observations were not even a challenge for Keck," says Mateo. "We were ecstatic that we got such a large sample of stars so fast. Keck and HIRES are letting astronomers do science that they had thought was unfeasible in their lifetimes."

The team obtained spectra of 31 red giants in Leo II during several hours of observations on the nights of March 19-21, 1994. Most stars needed only 10-minute exposures; a few exposures ran to 20 minutes.

The final result was a "velocity dispersion"--a statistical measure of how quickly stars move within a galaxy. If Leo II contained no dark matter, the astronomers would have expected a velocity dispersion of about 2.5 kilometers per second. Instead, they found a dispersion of 6.7 kilometers per second, with a margin of error of about 15 percent. That is not as high as the value seen in some other dwarf galaxies, but still solid evidence that dark matter composes about 85 percent of Leo II's mass.

The role that dwarf galaxies and their halos of dark matter might play in the universe is not clear. Because stars in dwarfs are spread out, the galaxies are quite faint and difficult to find. Some astronomers argue that dwarfs may outnumber all other galaxies in the universe by a factor of 10, 100, or even more. In addition, it appears that more dwarfs existed in the past. Over time, the Milky Way and other large galaxies may cannibalize these stragglers and absorb the dark matter into their own enormous halos of material.

"Dwarf spheroidals are an extreme class of galaxy, and many of them might not be visible at all," Mateo comments. "If they are a lot more common than we think, they could become an important mass component of the universe without contributing much light." So far, he notes, all dwarfs seem to contain about the same amount of dark matter: a mass equal to tens of millions of suns. Further, their dark matter must be of the "cold" variety--heavy and sluggishly moving material. Because of their relatively weak gravitational fields, dwarfs could not capture neutrinos or other fast-moving "hot" dark matter.

The team is now pondering whether to examine other distant dwarfs--such as those around the Andromeda Galaxy--in the same way. Meanwhile, Mateo and Olszewski are conducting more detailed studies of stellar motions in some of the Milky Way's dwarfs.

The W. M. Keck Observatory is operated by the California Association for Research in Astronomy, a partnership of the University of California and the California Institute of Technology. Both Mateo and Olszewski were supported by grants from the National Science Foundation.

Editor's note: For further comments, you may contact any of the following: Steven Vogt--(408) 459-2151 or vogt@lick.ucsc.edu Mario Mateo--(313) 936-1742 or mateo@ra.astro.lsa.umich.edu Edward Olszewski--(602) 621-1973 or edo@as.arizona.edu Michael Keane--(408) 459-2267 or mk@umbra.ucsc.edu

A color slide of the Leo I dwarf spheroidal galaxy, a twin of the Leo II dwarf, is available from the UC Santa Cruz Public Information Office. To request a copy, call (408) 459-2495.

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