The Mice at Play

Ancient constellations were named for figures of mythology and legend, but modern astronomers often use rather whimsical names for galaxies. To the Russian astronomer B.A. Vorontsov-Vel'Yaminov, the rounded bodies and long tail-like appendages of the two galaxies technically known as NGC 4676 suggested a pair of playing mice, and these galaxies have been called "The Mice" ever since.

The Mice came under renewed scrutiny as NASA chose targets to test the Advanced Camera for Surveys recently installed on the Hubble Space Telescope. To complement the spectacular images produced with this new camera, NASA turned to a computer simulation of The Mice created at the University of Hawaii's Institute for Astronomy (IfA).

Since the early 1970s, astronomers have known that the bizarre shapes of some "peculiar" galaxies could be explained as the results of galactic collisions. Astronomers Alar and Juri Toomre used The Mice to illustrate this idea, showing how the long tails noted by Vorontsov-Vel'Yaminov could be drawn out of ordinary spiral galaxies by gravitational forces. The Toomres predicted that The Mice are waltzing about each other; in 1974, IfA astronomer Alan Stockton measured their motion and confirmed this prediction.

These early calculations were incomplete; the mutual gravitational attraction of the two galaxies could only be roughly approximated with computers available in the 1970s. But two decades later, computers had improved so much that a proper calculation was possible. At that point, IfA astronomers Joshua Barnes and John Hibbard (now at National Radio Astronomy Observatory, Charlottesville, VA), formed a team. Barnes had created computer programs which could simulate galactic collisions. Hibbard had detailed observations and measurements of galactic velocities made using NRAO's Very Large Array. Together they decided to create a computer simulation of The Mice.

"Simulating colliding galaxies is a bit like investigating a car crash," says Barnes. "Suppose you had no witnesses, just a couple of wrecked cars. You might try different test crashes, varying things like speed and angle of impact, until you found a way to get the same damage as the original collision. That's basically what we did-try different collisions until we found one which would turn a couple of ordinary spiral galaxies into something like The Mice. It took about a month of trial and error-mostly error, of course!"

For their latest calculation, Barnes and Hibbard used six fairly small and inexpensive computers linked together. Calculations like this used to require supercomputers, but ordinary PCs or workstations are now so fast that the calculation took only three days. "The computing power needed for this research is essentially free", says Barnes. "That's a big plus-now we can go ahead and try things right away, instead of first asking for supercomputer time."

Presented in the form of an animation, the computer simulation shows two pinwheel galaxies falling together, swerving as they pass each other, and flinging out long tails of stars. At present the two galaxies have made one pass, and are coming back for a second and closer encounter. From their calculations, Barnes and Hibbard estimate that the galaxies first side-swiped each other about 150 million years ago. "That's why there weren't any witnesses to that first collision", says Barnes. "Dinosaurs didn't build telescopes."

The new pictures of The Mice, taken with the new camera on Space Telescope by Holland Ford (John Hopkins University, Baltimore, MD) and Garth Illingworth (Lick Observatory, Santa Cruz, CA), show details which match the simulations. For example, wisps of dusty gas from one galaxy block some of the light from its partner; this shows that the first galaxy, as expected, is slightly closer to us. And small clumps of stars and gas within the tails of The Mice, which are also seen in computer simulations, may eventually drift away to become small free-floating galaxies. But the computer model isn't perfect-it shows a long "bar" crossing the face of one galaxy. The Space Telescope images may also show a bar, but the angle of the bar is different. "We can probably improve our calculation", says Barnes. "But we're definitely on the right track."

The computer simulation predicts that the two galaxies of The Mice will merge with each other about 400 million years from now. That's a long time to wait to see if a calculation turns out right. But other collisions like this one have been happening for billions of years, and the Universe is littered with the wreckage of ancient crashes. When spiral galaxies merge, they form a disorganized pile of stars, which then settles into an "elliptical galaxy"-an oval blob of stars held together by gravity. The Mice are members of the Coma Cluster, a huge swarm of galaxies, and most of these galaxies are ellipticals. Barnes remarks: "It looks as if The Mice were a bit late getting together, but a billion years from now they'll make up one more elliptical galaxy in the Coma Cluster."

Images and animations of The Mice are available at http://www.ifa.hawaii.edu/~barnes/pressrel/mice/.

Space Telescope Science Institute press release on the Advanced Camera for Surveys images is available at
http://oposite.stsci.edu/pubinfo/pr/2002/11/.