Thread by @Astropartigirl

Galaxy clusters are giant structures of up to *thousands* of gravitationally bound galaxies. Of order 10¹⁵ times the mass of the sun, they curve spacetime so much that they act as gravitational lenses, producing arced images of galaxies that lie far more distant behind them. 1/

These beasts provide direct evidence of Einstein's Theory of General Relativity, revealing how mass curves spacetime by bending the trajectories of photons emitted by objects far behind them, producing the galaxy arcs you see in the above image. 2/

As you may have expected, the dominant form of matter in giant galaxy clusters is dark matter, making up ~80% of its matter content. What might come as a surprise, however, is that of the regular matter in galaxy clusters, a very small portion of it is stars: 3/

The intracluster medium (ICM) is gas that's spread throughout galaxy clusters, and the is at least 5 times more gas mass than stellar mass in galaxy clusters! And it's *extremely hot*, like 10⁷ to 10⁸ K hot! Not exactly the greatest vacation spot if you know what I mean 🔥🔥 4/

Here's the thing: stars are matter that doesn't "interact". There's so much space between them that they pass right by each other, even when galaxies, or clusters, collide! But gas does not -- it interacts and drags and sloshes when it encounters more gas! 5/

One example is the Bullet Cluster, two galaxies that have gone right past each other. The stars passed through, but the gas from each cluster (pink) has dragged behind. So the stars, along with the dark matter (blue) pass right through while the gas trails behind. 6/

We know where the dark matter is in the pic bc it doesn't interact, so it'll be where the stars are. And the fact that there's still *far more mass* in the region where the stars are than gas tells us dark matter exists, else the gas region would contain the most mass! 7/

Now, back to the fact that clusters have huge portions of interacting gas in them. In clusters, everything interacts and sloshes together. This is why you don't really find spiral galaxies in the largest galaxy clusters. 8/

Because when spiral galaxies enter a cluster, they shortly meet their demise by getting all their gas stripped away by the cluster, and then have to battle a slew of interacting stuff, both directly and gravitationally, that they probably lose their spiral shape shortly! 9/

So a pretty pinwheel like NGC 1232 (Dec 28 2017 APOD) would probably lose those beautiful arms if it landed itself in a giant galaxy cluster due to all the interactions it needs to battle in that environment. Additionally, losing its gas means it can no longer form stars! 10/

So basically what I'm saying is that galaxy clusters are giant spiral-galaxy-shredding structures in space that provide us with proof that mass curves spacetime, and show us that gas can truly be a beast. 11/11

PS: some of you might be thinking "hey you said dark matter doesn't interact, but you study Self-Interacting Dark Matter!" Good! Because galaxy clusters provide us with very good constraints on how large the interaction cross-section of dark matter can be in those systems.

In clusters, the interaction cross-section has to be very small. But in dwarf galaxies, it must be larger. This leads to a velocity-dependent cross-section for dark matter, since velocities in dwarf galaxies are much smaller (~10 km/s) than velocities in clusters (~1000 km/s).

So then the velocity dependence of the interaction cross-section of dark matter should be such that the cross-section decreases with increasing velocity. I wrote a moment about SIDM, if you're interested: twitter.com/i/moments/8657…

And that concludes my thread on galaxy clusters. Enjoy! 🙂