May 3, 1996 Contact: Robert Irion (408) 459-2495; irion@ua.ucsc.edu

COMPUTER MODELS OF GALAXY COLLISIONS AND "TIDAL TAILS" YIELD PUZZLING RESULTS ABOUT DARK MATTER AROUND GALAXIES

FOR IMMEDIATE RELEASE

SANTA CRUZ, CA--When galaxies collide, the results can be strikingly beautiful or deadly dull. Some pairs of galaxies fling sweeping arcs of stars into space; others just merge into shapeless blobs. The difference, it turns out, is a matter of some gravity.

Gravity dictates the overall shapes of galaxies during a collision, as well as the path of each star. However, the key forces come not from the stars themselves, but from invisible "halos" of dark matter theorized by astronomers to surround every galaxy. Those halos can create deep gravitational pits, making it less likely that stars will escape a collision to form graceful luminous arcs.

That delicate dance of physics allowed a team at the University of California, Santa Cruz, to use a supercomputer model of colliding galaxies as a novel way to probe how much dark matter is out there. When the researchers compared their results to images of real galaxies caught in the act of colliding, they unveiled a puzzle.

The model's collisions created dramatic "tidal tails" of stars only when the mass of each galaxy's dark-matter halo was fairly low--from four to eight times the mass of the visible stars. The simulated tidal tails became pathetically short and stubby when the team upped the dark-matter ratio to 15 to 30 times the mass of the stars alone.

Here's the dilemma: Popular theories of the evolution of the universe predict that galaxies should have huge cocoons of dark matter. Some astronomical data support this view. However, several well-known galaxy pairs have hurled tremendous tails of stars into space. This implies--according to the computer model--that their dark-matter halos are wimpy, not massive.

"It's not controversial that dark matter exists around galaxies," says John Dubinski, postdoctoral researcher in astronomy and astrophysics at UC Santa Cruz. "What's controversial is how much there is. We found that our simulations can't produce these beautiful long tidal tails if the galaxies have high-mass halos of dark matter."

Dubinski leads the team's report in the May 10 issue of the Astrophysical Journal. Coauthors are J. Christopher Mihos, formerly of UCSC and now Hubble Postdoctoral Fellow at Johns Hopkins University, and Lars Hernquist, associate professor of astronomy and astrophysics at UCSC.

At face value, the team's results suggest there is less dark matter around galaxies than astronomers believe. However, there are other interpretations. To date, telescopes have spotted less than a dozen good examples of systems with long tidal tails. Those collisions may have occurred in parts of the universe where the concentration of matter was less dense to begin with, so that the galaxies haven't had time to accumulate huge halos. In other regions, galactic halos may be more massive, so that most mergers create amorphous blobs that astronomers can't easily recognize as the products of colliding galaxies. Our own Milky Way and the nearby Andromeda galaxy appear headed for such a boring fate about three billion years from now.

Hernquist describes what he feels is the real power of his team's method: "There are ways to measure galaxy masses that people haven't thought about," he says. "Tidal tails sample the mass of a dark halo over large distances, which is something that other techniques don't do." Astronomers should collect detailed images of many colliding galaxies, he adds, for a more robust comparison with the results of these and other simulations.

Computer models let researchers explore all stages of a galaxy collision, which in the real universe can take a billion years or more to unfold. Although head-on smashes between stars are unlikely when two galaxies collide, the relentless tidal forces and slingshot effects of gravity render the original galaxies unrecognizable by the time the collision ends. Previous work by Hernquist and his colleagues, for instance, showed that two spiral galaxies can merge to form a featureless blob much like the elliptical galaxies scattered through the universe.

The latest model by Dubinski, Mihos, and Hernquist is the first to use high-mass halos of dark matter around galaxies. Earlier simulations had used halos with masses up to eight times that of the galaxy's visible material, not high enough to abort the birth of tidal tails. The team's model also tracked the fates of a large number of particles--48,000 in the disk, central bulge, and dark halo of each "galaxy," yielding excellent detail throughout the encounters.

The team designed its galaxies so that their orbits and their relative orientations would churn out the longest possible tails. Striking tails resulted only from collisions between galaxies with dark-matter halos either four or eight times the mass of the visible stars. When the ratio reached 15 to 30 times, short tails crept anemically into space before collapsing back onto the merging galaxies. "It's like launching a rocket--it only gets so far before it starts to fall back," Dubinski says. "If the earth was twice as massive, the rocket would not get as far and would fall back much more quickly."

Editor's notes: You may reach the scientists as follows: John Dubinski: (408) 459-5246 or dubinski@ucolick.org Lars Hernquist: (408) 459-3387 or lars@ucolick.org Christopher Mihos: (410) 516-5101 or hos@pha.jhu.edu

The study is available via the World Wide Web at http://www.ucolick.org/~dubinski/tails/tails.html

A diagram comparing snapshots in time from two of the simulations with a photograph of the "Antennae" colliding galaxies is available at http://www.ucolick.org/~dubinski/antennae/antennae.html

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