Cluster Clash

August 28, 2008

Strange things can happen when galaxies collide. You'd expect the atoms and molecules to crash into each other with very high energy, but what if there was other types of matter in those galaxies that we couldn't see? What happens when so-called "dark matter" collides?

Thanks to the combined efforts of scientists with both the Hubble Space Telescope and the Chandra X-Ray Observatory, we have a pretty good idea.

What is this dark matter of which I speak? Our current model for how much "stuff" there is in the universe -- and yes, astrophysicists have figured out how to "weigh" it -- shows that only 4% of it is what one might call regular matter: the stuff that makes up everything we see in the visible universe, from galaxies and stars all the way down to quarks and leptons, and everything in between. A mysterious thing called dark energy accounts for around 73 percent of the "stuff." That leaves 23 percent, which is the dark matter.

Scientists aren't quite sure what it is just yet -- massive astrophysical compact halo objects (MACHOs) or weakly interacting massive particles (WIMPs) are two of the primary contenders -- but gosh darn it, they know it's there! And they know because in 2006, Hubble and Chandra joined forces to produce a stunning image of two galaxy clusters that collided and literally swept all the ordinary matter out of the way so scientists could hone in on the elusive dark matter and see it in all its weakly interacting glory. That image is known as the Bullet Cluster (or 1E 0657-56, if you want to be all formal about it).

Well, what we're actually seeing is indirect evidence of dark matter's existence through its gravitational impact. But it's pretty strong evidence, nonetheless, particularly if the dark matter turns out to be WIMPs.

MACHOs, at least, are technically "normal" matter: black holes, neutron stars, brown dwarfs, and other objects too dark for us to "see" via our usual imaging techniques.

But WIMPS? They would be an entirely new type of matter, one that almost never interacts with regular matter, except through gravity and the weak nuclear force. WIMPs are the monosyllabic wallflowers at parties that hardly anyone notices. Those that do, don't hang around too long to chat because frankly, WIMPs aren't ones for small talk.

That's why the Bullet Cluster image was so exciting: it dispersed the crowds that had been drowning out the dark matter so we could finally see it -- a feat scientists accomplished by being very clever. First, they took Hubble images and used them to map out the gravitational fields of the cluster by measuring how much they were distorted by gravitational lensing. Then they used that information to deduce what kind of mass concentration would have created that sort of gravitational lens.

Then it was Chadra's turn. The telescope took x-ray images which enabled scientists to map out where the hot gas was located -- the hot that comprised most of the regular matter in the cluster. Finally, they superimposed the two images and voila! You can see the distribution clear as day!

Now astrophysicists have done the same thing with a galaxy cluster known as MACS J0025.4-1222. It formed when two very large separate clusters -- each a quadrillion times larger than the mass of our sun -- collided at speeds of millions of miles per hour. Just as with the Bullet Cluster, the hot gas collided and slowed down, and the weakly interacting dark matter passed right through. Once again, overlaying the visible-light images from Hubble (measuring gravitational lensing) with the x-ray data from Chandra shows a very clear separation between normal matter and dark matter.

So, if the Bullet Cluster already provided strong evidence for the existence of dark matter, why are astrophysicists so excited over this new image? Because it proves the Bullet Cluster isn't just a fluke; this is actually how dark matter behaves.

Scientists love them some reproducibility, and now MACS J0025 (for short) provides that for the dark matter. Now they just need to come up with a cool nickname to rival "Bullet Cluster": "amorphous galactic blob" just won't cut it.  I'd suggest something hip, with a beat, so you can dance to it -- something like MAC-Daddy-J.

Photos: (top) Demotivational poster using Bullet Cluster image, created by Sean Carroll (Cosmic Variance). Bullet cluster image: NASA/ESA/CXC. (bottom) Image of MACS J0025.4-1222 (a.k.a., "MAC-Daddy-J"). Source: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University).

Comments

[IceBogan]

It would also be nice to see this analysis done for an ordinary, non-colliding, galaxy cluster. I assume the red and blue would overlap and be roughly circular. It's probably been done but I've never seen the results.

[Dave Mosher]

@IceBogan

I like that idea. When I think about it, though, it'd probably just look a homogeneous purplish blob (if they used the same false colors for the simulation, that is).