Simulations on NCSA's SGI Origin2000 supercomputer are giving researchers insight into the nature of dark matter and how galaxies form clusters. They're also disproving a common method of measuring the mass of those galaxy clusters.

by Aries Keck
A biologist studying frogs can just pick one up by its clammy skin and flip it over to get a good look. But studying galaxies takes more than touch. Cosmologists can't grab a few galaxies and hold them up to the light. And they can't slice a galaxy open to see what's inside.

Cosmologists are limited to only one point of view of their star-filled subjects, that of the earth and relatively near-earth space. Often they know only three pieces of information about location of the galaxies—a vertical coordinate, a horizontal coordinate, and, if the stars move in any of these directions over time, speed. At best, cosmologists are working with flat pictures of galaxies that may be faintly embossed with mere hints of depth.

John Dubinski is hoping to change all this. An astrophysicist and numerical theorist at the University of Toronto, he's created a massive computer simulation that lets scientists manhandle not only individual galaxies, but whole clusters of them. It even crashes clusters into each other—simulating the collisions that produce merged galaxies. His nine simulations of individual galaxy clusters contain nine to twelve million particles each, and creating them required 60,000 CPU hours on the Alliance's SGI Origin2000 supercomputer at NCSA.

Then, the make-believe galaxies were compared to observations of the real thing. "We brought the two together. The reality and the simulations," says cosmologist Margaret Geller of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The goal is to create the most realistic simulations possible—exposing the secrets that are obscured from our limited point of view.

Comparing simulations to observations is usually like comparing apples to oranges because most simulations are vastly oversimplified, according to Geller. With huge amounts of data and the computing power available at NCSA, however, the results and the comparisons that can be made to them are vastly more complex. Because the models are detailed enough to include information such as an inordinate number of individual galaxies, Dubinski says they are now operating in an environment that's much closer to the true physics of reality.

Access Online | Posted 9-5-2000