Many of you saw the pictures I posted Monday of colliding galaxy clusters. These pictures were spectacular, because they not only show galaxy clusters less than 200 million years after a collision (which is short, cosmically), they also show where the mass lies (traced in blue) and where the X-ray emitting areas are (pink). You get pictures like this one from the Bullet Cluster:
This one from MACS J0025:
And this one from Abell 520:
This is what happens when clusters collide, the normal matter gets separated from the dark matter! Let’s tell you how.
Some normal matter is packed together in tight, dense little clumps. Good examples of this are stars and galaxies. When you run two large clusters (spanning millions of light years) into each other, these little clumps hardly ever hit each other, and move with a lot of momentum. What does this mean? They tend to miss one another, and they hardly get slowed down by the friction of moving through the other cluster. In other words, they behave like the little metal balls in a game of “Crossfire”. They mostly just pass straight through to the other side.
That leaves us with the gas, which is where most of the normal matter is, and the dark matter. For all intents and purposes, these are distributed over the entire cluster, so they’re very diffuse, but also omnipresent. The gas is still made up of protons, neutrons, and electrons, and these tiny particles interact with one another very easily. When they run into each other, they behave similarly to running two jets of water into each other:
There is a lot of friction between them, which (if you remember) both slows them down and also heats them up. The slowing down is why the X-ray-emitting gas is always in the middle of these clusters (in all three cases), and the heating is why the gas becomes energetic enough to emit X-rays! In other words, the gas goes “SPLAT!”
But what of the dark matter? Although it obeys the same gravitational laws of physics, it’s missing the main source of friction — electric charge! In fact, we’re pretty sure that dark matter has practically no electromagnetic interactions at all. The friction between dark matter particles (as well as between dark matter and gas) is so small it might as well not even be there at all. Colliding dark matter with itself is as futile as colliding light beams with one another; they might as well not even be there!
But we’ve gone even further than that, and have simulated what would happen to both the normal, gaseous matter (in pink) and the dark matter that only has gravitational friction (in blue), when two clusters collide. The video is here, but I’ve taken some screen captures of the relevant parts.
Before the collision, you can see that the normal matter and dark matter line up very well with one another, and are both distributed in (roughly) spherical shapes over the entire cluster.
Until they start to collide, they both retain those shapes, and both the gas and dark matter remain in spheres. But all that changes once the collision starts…
You can see, right away, that the dark matter (blue) just keeps moving straight through, effectively unimpeded by anything. But the gas (pink) collides with itself, slowing down (losing kinetic energy) and heating up (gaining thermal energy). This gets more pronounced as we go forward another couple of million years.
And the separation increases as the dark matter just continues to coast, and the gas continues to simply heat up and slow down.
And as the collision progresses, the dark matter eventually reverses direction and comes back towards the cluster. You don’t see this in the Bullet cluster, which is in the early stages of the collision, but you can see it slightly in MACS J0025, which is in a slightly later stage, and it’s very pronounced in Abell 520, which is in quite a late stage of its merger.
And there you have it. That’s why colliding clusters do what they do, and that’s why there needs to be some kind of collisionless, uncharged stuff that interacts gravitationally. We call it dark matter.
It has gravity like matter, and it’s completely invisible, neither absorbing nor emitting light. So whether you love or hate the name, that’s what it does, and this explanation is the only sensible one we’ve found for this phenomenon. Dark matter. Remember it.