[The Desktop Project is my way of clearing all the pretty pictures off my computer's desktop, by posting one per day until they're gone. I think this week is it - I'm almost out!] Dark matter is funny stuff. We've known about its existence for many decades, and the more we look the better our evidence gets. We know it has mass, and therefore gravity, but we don't know what it is! We do, however, know what it isn't: normal matter of any kind, like cold gas, rogue planets, black holes, dead stars, or anything else made of protons, neutrons and the other types of particles we deal with in everyday life. Since careful observations have shown clearly it can't be any kind of normal matter, it therefore must be some sort of exotic flavor of matter, some kind of particle we haven't yet seen. One thing we're pretty sure about it, though, is that it doesn't interact with normal matter except through gravity. Dark matter can pass right through you and you'd never know it. But put enough of it in one spot, and its gravity will reveal its presence. Which is why the galaxy cluster Abell 520 is such a mystery. Here's the beauty shot:
Pretty, isn't it? Abell 520 is a galaxy cluster about 2.4 billion light years away, and a mass of several trillion times our Sun's -- it's made of galaxies, each with billions of stars in them. And a galaxy cluster is a collection of hundreds or even thousands of galaxies bound together by their gravity. In fact, Abell 520 is more than one cluster: it's actually a collision between two or more clusters! As they move through space, clusters can collide, and actually quite a few of these cosmic train wrecks are known. When clusters collide, a lot of things happen. The gas clouds in between galaxies in the two cluster slams into each other, heating up to millions of degrees and glowing in X-rays. In the picture above, that gas has been colored green so you can see it (invisible to the eye, the X-rays were detected by the Chandra Observatory). The orange glow is from stars in galaxies (as seen by the Canada-France-Hawaii and Subaru telescopes). The blue is actually a map of the dark matter made using Hubble observations. The gravity of dark matter distorts the light passing through from more distant galaxies, making it possible to map out the location of the otherwise invisible stuff (you can read about how that's done here and here).
Since dark matter doesn't interact with normal matter, we expect it to simply pass through the collision point, sailing on as if nothing had happened. That's been seen in a half dozen other galaxy cluster collisions, including the Bullet Cluster -- hailed as definitive proof of the existence of dark matter -- as well as Abell 2744 aka Pandora's cluster (seen here on the right), and the newly found Musketball cluster. But Abell 520 isn't like those others. The problem is, there's a clear peak in the dark matter right in the middle of the cluster, not off to the sides as you might expect. It looks as if the dark matter slammed to halt in the middle of the collision instead of sailing on. Here's the thing: this does not mean dark matter doesn't exist, or we're wrong about it. The other clusters I mentioned above make it clear we do have a pretty good grip -- so to speak -- on the behavior of dark matter. What these new observations almost certainly do mean is that there's more going on here than we can easily see. For example, it's possible there's a lot of normal matter in the middle of the cluster that we can't see for some reason. Or maybe it's a geometric effect; it could be we're seeing this collision right down the barrel, and not from the side. In that case, the dark matter did indeed keep moving, but directly toward and away from us, so it still looks like it's in the middle. The astronomers who made these observations have proposed several ideas about what's going on here, in fact. The thing is, they all are either unlikely, or invoke special circumstances (like our viewing angle). That doesn't mean they're impossible, it's just that as scientists, we don't like doing that. We'd prefer not to have to invoke mitigating circumstances to explain phenomena -- especially when you only have a handful of such phenomena to study -- because once you start doing that, you start finding excuses to explain away every little thing you see that way. It's far better to have a more general explanation for things! The problem is we just don't have enough examples of cluster collisions to know how weird Abell 520 is. It may be that when we observe more cluster collisions they'll all behave the way we expect, and Abell 520 will be seen as the rare exception. When that happens, special pleading is more acceptable, since it's only used rarely and when things really do look weird (but you still have to back it up with good evidence if you want folks to believe you). And when you observe something lots of time, eventually, statistically you're bound to see something weird happen (like flipping a coin hundreds of times, and having it land on its side once). Hopefully, as time goes on, we'll get lots more observations of cluster collisions and be able to crack this nut. In the meantime, dark matter will do whatever it is dark matter does, and we'll keep looking and trying to figure it out. Image credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)
Related Posts: - The Top Ten Astronomy Images of 2006 (scroll down to #4 for the Bullet Cluster description) - Galaxies swarm and light bends under dark matter’s sway - Dark matter, apparently, is midichlorians - Opening the lid on Pandora’s Cluster