Contact: Robert Irion (408/459-2495)

ASTRONOMERS COULD FIND SIGNS OF ENCOUNTERS BETWEEN DWARF GALAXIES AND THE MILKY WAY, SIMULATIONS PREDICT

Nearby dwarf galaxy in Sagittarius yields rare opportunity to study a strong tidal interaction in detail

NOTE: This release is embargoed until 9:20 a.m. MST Tuesday, January 10, 1995, when Kathryn Johnston will present her study in a poster (#51.01) at a meeting of the American Astronomical Society, Tucson Convention Center, Tucson, Arizona.

TUCSON, AZ--Dwarf galaxies spiraling into the gravitational maw of the Milky Way should leave visible trails of debris that persist for a billion years or more, according to new computer simulations by three astrophysicists.

Astronomers could learn much about the evolution of the Milky Way by finding these faint remains, say the creators of the simulations. Many such trails would bolster a theory that the Milky Way formed and continues to grow by capturing smaller galaxies and subsuming their stars, gases, and dark matter. On the other hand, an absence of trails might mean that our galaxy's appetite for stellar hors d'oeuvres is lower than many astronomers believe.

"When the Milky Way disrupts a dwarf galaxy, it takes some time for its stars to disperse," says Kathryn Johnston, a graduate student at the University of California, Santa Cruz. "We think that observations of these moving groups of stars are feasible. If they exist, they will offer an exciting window on the history of the galaxy."

Johnston discussed the research today (January 10) in a poster session at a meeting of the American Astronomical Society. Her coworkers were David Spergel, associate professor of astronomy at Princeton University, and Lars Hernquist, associate professor of astronomy and astrophysics at UC Santa Cruz.

The simulations examine the fate of a dwarf galaxy observed in the direction of the constellation Sagittarius. Astronomers from the University of Cambridge unveiled the dwarf last year beyond the Milky Way's dusty and crowded core; it is the closest known galaxy to our own. Its stretched shape suggests that the Milky Way is ripping it asunder, just as the gravitational tides of Jupiter broke apart comet Shoemaker-Levy 9. However, researchers do not yet know whether the dwarf is encountering the Milky Way for the first time or is in its death throes following many orbits.

Johnston, Spergel, and Hernquist realized that computer simulations were ideal for exploring the possible history of the dwarf. The team used techniques developed by Hernquist and others for modeling collisions between galaxies--spectacular events in which gravity flings enormous arcs of stars into space.

The team's most detailed model depicted the Sagittarius dwarf as a loose conglomeration of 5 million particles. A complex computer code calculated the results as the particles interacted with each other and with three components of the Milky Way: a central bulge, a saucer-shaped disk, and an extended halo of diffuse material harboring most of the Milky Way's mass. Dozens of hours of computer time were required to trace the dwarf through six orbits around the Milky Way--a process that would take about 13 billion years in the real universe.

The researchers found that when the dwarf passed near or through the disk of the Milky Way, its stars received a shock of energy. Here's why: Stars in the Milky Way invaded the boundaries of the dwarf each time it moved past. Those stars briefly increased the mass of the dwarf, squeezing it because of the additional gravitational force. When the dwarf left the disk, its stars rebounded and partially flew apart. Each passage thus created streamers of stars in distinct paths, both ahead of and behind the dwarf. (See attached diagram.) Meanwhile, a core of stars stayed closely packed together, even a billion years after the complete destruction of the dwarf.

Debris from these passages should be visible in the sky as groups of stars moving in the same direction, the simulations indicate. However, astronomers would need to conduct careful surveys of large swaths of sky to find the moving groups. Previous studies have shown that the Milky Way's coterie of dwarf galaxies-- nine in all--appears to lie along two great circles around our galaxy. "Those paths are logical places to look for evidence of previous encounters," Johnston suggests. Surveys could expose the remnants of a previous passage of the Sagittarius dwarf itself, or of encounters with other dwarfs during the last billion years.

Astronomers disagree over whether the Milky Way once hosted a much greater swarm of dwarf galaxies and other satellites. Johnston, Spergel, and Hernquist believe that the mere existence of the Sagittarius dwarf supports the view that the Milky Way gorges regularly on neighbors. The dwarf's close encounter now in progress, they observe, will last about 100 million years--less than 1 percent of the galaxy's age. "Clearly, accretions of dwarf galaxies must be common," they write, "or we must instead conclude that we are observing the galaxy during a special phase of its evolution." The latter alternative, they maintain, is unlikely.

The team's results also reveal that the Milky Way's halo of dark matter can disrupt dwarf galaxies just as readily as its disk of stars. As a result, many dwarfs need never approach the disk to disperse into unaffiliated stars. Some researchers claim that the Milky Way could not have consumed many small galaxies over time, because its disk appears too thin and undisturbed. However, the simulations show that the Milky Way could absorb a huge amount of material in small encounters--up to 10 billion times the mass of our sun--while still keeping a thin disk.

For its study, the team used the Connection Machine 5 (CM-5) at the National Center for Supercomputing Applications in Illinois. The CM-5, consisting of 512 computers running in parallel, is among the fastest machines available for this type of research. The project was part of the GC3 collaboration, a High-Performance Computing and Communications (HPCC) initiative funded by the National Science Foundation. The GC3 group includes scientists at the Massachusetts Institute of Technology, Princeton University, University of Illinois, University of Indiana, University of Pittsburgh, and UC Santa Cruz.

Editor's note: For further information, contact any of the researchers: Kathryn Johnston: (408) 459-2774 or kvj@lick.ucsc.edu David Spergel: (609) 258-3589 or dns@kocab.princeton.edu Lars Hernquist: (408) 459-3387 or lars@lick.ucsc.edu

This press release is also available on UC NewsWire, the University of California's electronic news service. To access by modem, dial (209) 244-6971.

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