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The Mice at Play
May 16, 2002
The Mice at Play
image: NASA and the ACS Science Team

Located 300 million light-years away, these two colliding galaxies have been nicknamed "The Mice" because of the long tails of stars and gas emanating from each galaxy (see photo above). From calculations, it's estimated that the galaxies first side-swiped each other about 150 million years ago. 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) (see simulation below).

Calculations and observations made in the early 1970s demonstrated that The Mice are waltzing about each other, but the early calculations were incomplete because computers of the 1970s could only roughly approximate the gravitational attraction of the two galaxies. Two decades later, computers had improved so much that a proper calculation was possible.

Computer simulation of "The Mice"
animation: Joshua E. Barnes, Institute for Astronomy, University of Hawaii and John E. Hibbard, National Radio Astronomy Observatory, Charlottesville, VA
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!"

The computer simulation shows two pinwheel galaxies falling together, swerving as they pass each other, and flinging out long tails of stars. The video pauses to show how it matches the image above and then resumes. At present the two galaxies have made one pass, and are coming back for a second and closer encounter. Otherwise known as NGC 4676, the pair will eventually merge into a single giant galaxy.

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