AAS #13: A History of (galactic) Violence

By Phil Plait | January 10, 2008 9:44 am

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.

Comments (9)

  1. Gary Ansorge

    Hubble is SOOOO informative. What a great investment! I look forward to the construction of really giant instruments in space, on the order of kilometers in diameter. We might eventually get them so large, we could see individual, rocky earths and their atmospheres, from several hundred light years distance. Wonder if we could spot the effluvia of a techno civilization, like high levels of ozone and CO in their air???

    Gary 7

  2. You should note, as some people may miss it, that these events, although catastrophic on a galactic scale, probably don’t much affect the people (if any) living in those galaxies. When we collide with Andromeda, the chance of a star from our galaxy and a star from Andromeda actually colliding is slim to none.

  3. Gnat

    I have a question regarding Dark Matter: If it is a mass (with gravity) but there is no light, would it be possible for a (hypothetical) spaceship to “get stuck” or “slam into it”?

    I know, silly question, but Dark Matter is harder for me to get my head around than Black Holes!

  4. Gnat writes:

    [[I have a question regarding Dark Matter: If it is a mass (with gravity) but there is no light, would it be possible for a (hypothetical) spaceship to “get stuck” or “slam into it”? ]]

    It would be a problem if dark matter turns out to be baryonic matter, like the matter we’re used to. There’s some thought that it’s not, though. If it’s nonzero-mass neutrinos, they won’t be much of a problem.

  5. Speaking of the universe being a dangerous place, this article is about a black hole that has the estimated mass of 18 BILLION suns (Holy Haleakala indeed!).


  6. “500 times the mass of our entire galaxy.”

    Is that 500 times the mass of the visible matter in our galaxy, or are you including the dark matter component of our galaxy in that figure?

  7. Jeffersonian

    Andromeda you say? Maybe we should become proactive now and either make friends or welcome our new masters.

  8. Eric TF Bat

    The Wikipedia article on dark matter is pretty dense (ha ha!) and suggests that the non-baryonic theory is the most popular: that is, dark matter isn’t composed of protons and neutrons, but of “something else”. So what happens if a large wodge of dark matter comes into contact with a large wodge of normal matter, say the planet Earth? Do they interact? Would we even notice? Or is it all too vague to tell yet?

    All the stuff about quarks and baryons and so on strikes me as insanely complicated, and that makes me suspicious. Nature, in general, isn’t complicated: the general theme is infinite complexity arising from extreme simplicity. Chucking quarks into the mix makes it look like a cake recipe that begins “two cups of flour, three eggs, the entire cockroach and daffodil population of Portugal sorted alphabetically, one cup water…”. I have a feeling our descendants a hundred years from now will be looking back on all this the way we do on ether and phlogiston.

  9. Speaking of Hubble, NASA is on the verge of postponing the August mission to upgrade the telescope. The delays with the current shuttle mission are having a domino effect down the line, and apparently there are a couple of higher priority missions to the ISS that need to take place before the Hubble fix.


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