AAS #13: A History of (galactic) Violence

Contributed by
Jan 10, 2008
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Space is a dangerous place. Stars explode, black holes gobble up matter... but some violent events are so huge they affect entire galaxies, mayhem on a scale so vast it numbs the mind.

Galaxies are island universes, cities of billions or even hundreds of billions of stars. Some galaxies, like our Milky Way, live pretty much on their own, but others live in vast complexes called clusters. These galaxy clusters may have hundreds or thousands of denizens, all orbiting each other due to their mutual gravity, looking something like bees buzzing around hive.

But there is more there than just the matter we see. Dark matter is there as well; invisible stuff that adds to the gravity of the cluster due to its mass, but gives off no light. However, it betrays its presence in two ways: its gravity changes the motion of the galaxies in the cluster, and it distorts the light from more distant galaxies due to gravitational lensing (see my last AAS post for info on that).

A team of astronomers has used the Hubble Space Telescope to examine the galaxy cluster Abell 901/902 (they call their project STAGES: Space Telescope A901/902 Galaxy Evolution Survey -- and you can surf their images yourself using their Skywalker software, which is totally cool). They wanted to very carefully map out many aspects of the cluster: how many galaxies it contains, what kinds of galaxies they are (spirals, ellipticals, etc.), and, using lensing, determine where the dark matter is. By making a map of all of these characteristics, they hoped to be able to understand the history of the cluster, since the present configuration of the cluster can provide clues to its past.

For the first time, these cosmic archaeologists were able to map out the dark matter of this cluster, and found four very large concentrations of it scattered throughout Abell 901/902. These clumps of invisible stuff are enormous: they total a stunning 100 trillion times the Sun's mass, or 500 times the mass of our entire galaxy.

Needless to say, that much mass exerts a powerful gravitational pull. Galaxies round the clumps are falling in toward them, inexorably drawn in by the clumps' gravity. And as they fall in from the suburbs to the downtown regions, they change. They slam into the thin gas between galaxies, which can blow out the gas inside the galaxies (like leaving you car window open on a highway can air out the inside of the car), for one. But as the galaxies fall in, the inevitably interact with one another, colliding and merging as the make the downhill slide. This distorts the galaxies' shapes, and that in turn allows the astronomers to determine the past history of the objects.

What's interesting is that they found that galaxies tend to be more distorted on their way in to the centers of the clusters than they are when they are actually at the center. It appears that as they fall, they have time to interact and merge, changing their shape, but once they aproach the center they are falling so quickly they simply don't have time to distort much as they pass each. Also, it takes time to settle in at the center, so the galaxies at the center appear to be very old, and have finished their transformation from being unsettled and twisted into more sedate, round, elliptical galaxies. The astronomers also determined that the galaxies at the edge of the cluster still produce stars, but by the time they reach the center that has mostly turned off. Their gas -- needed to make stars -- gets blown out of the galaxies on the way in, and the mergers trigger vast bursts of star formation, which also uses up the gas.

These discoveries were possible only through the use of Hubble, Spitzer, and other telescopes, each of which unpeeled another layer of the puzzle. I'll note that for Hubble's part, this represents the largest area of sky ever observed by the grand dame of space 'scopes; it took 80 separate pointings of Hubble to complete the survey of the cluster, and they mapped the locations and shape of 60,000 galaxies in all, a truly staggering amount.

One last thought: the Milky Way is more or less alone in space, being part of a loose collection of other galaxies. But we are headed toward the Andromeda galaxy, and in a couple of billion years we'll collide and merge with it. I hope that in this far flung future, some distant astronomers can use our own violent fate to learn a little more about the Universe, too. It only seems fair.