I recently posted about how cool it was on the Mythbusters when Adam and Jamie slammed a rocket sled into a car at 1000 km/hr. Now take that collision and multiply it by a million billion gazillion. What you get is this:
Holy Haleakala. That's quite an image. It's of an object 5.4 billion light years away called MACSJ0717.5+3745, which is really just a catalog number and a coordinate on the sky, so it's not nearly as awesome sounding as what it is: a massive collision between four separate galaxy clusters! Almost all galaxies reside in a group of some sort. The Milky Way is part of the local group, a few dozen galaxies, mostly dinky ones, of which we're actually the largest. And our little enclave is in the suburbs of the Virgo cluster, a much larger aggregation of hundreds of galaxies. These clusters move around, and sometimes they collide (check out the Bullet Cluster, for example, for a fantastic example of this kind of collision, and how it gave us proof positive of the existence of dark matter). But four clusters, colliding all at the same time? Wow! The image is a composite of Hubble (visible light image) and Chandra (which sees high-energy X-rays) data. So what are we looking at? First off, almost everything you see in that image is a galaxy (grab the big or the ginormous versions of the image for some fun). Foreground stars are the dots with the four spikes going through them (that's an artifact of the camera used by Hubble, and is most obvious in point sources, and almost non-existent in extended objects like galaxies). So right away you get an idea of what a cluster of galaxies is like... and what a disaster a collision between four of them can be! The clusters are all moving through space at incredibly high speed, many dozens of kilometers per second. In a vast collision like this one, the galaxies may physically smack into one another, though I suspect that's not as common as you might think; there is still way more empty space in a cluster than there are galaxies. Still, the gas between galaxies is spread out over hundreds of thousands or even millions of light years, well beyond the galaxies themselves. So this intercluster gas will indeed collide at high speed, causing it to heat up to millions of degrees and glow in X-rays. In the image, that's shown as the diffuse material colored purple-red (lowest energy gas) to blue (highest energy or hottest gas). In fact, by mapping the hottest parts of the gas -- where the light is brightest and bluest -- and comparing that to the positions of the galaxies, astronomers were able to see that there are four distinct clusters in this train wreck. Also, in the collisions the galaxies just keep moving, while the gas slows down as it collides, so comparing the positions of the gas and galaxies the direction of the clusters was found as well. The speed can be found by taking spectra of the galaxies, which was done using the giant Keck 10-meter telescope in Hawaii. That also helps track which galaxy belongs to which cluster, assuming the galaxies in a given cluster are moving at roughly the same speed. All in all, I can see that this cluster will keep astronomers busy for a long, long time. Studying it will tell us a lot about how intergalactic gas behaves, possibly about the nature of dark matter (which cannot be seen in this image -- hello, it's dark matter -- but certainly profoundly affects the way this system behaves), and also just plain old what happens when a few hundred galaxies slam into each other across the depths of space.
Hubble+Chandra+Keck image of MACSJ0717.5+3745, a collision of four galaxy clusters. Credit: NASA, ESA, CXC, C. Ma, H. Ebeling, and E. Barrett (University of Hawaii/IfA), et al., and STScI.