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 didtry 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
errormostly 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 plusnow 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 perfectit 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/.