Why Don't Galaxies Just Pass Through Each Other Instead Of Colliding?

When two galaxies collide they eventually coalesce to form a single (larger) galaxy. By far most of the mass of a galaxy is Dark Matter which feels no friction and suffers not from collisions. Why then do not the dark matter components of the galaxies simply pass through each other and continue going? To suggest that the movement of these dark components is simply governed by the behavior of the visible portions seem to be like the tail wagging the dog.
NGC 6621/2 (VV 247, Arp 81) is a strongly interacting pair of galaxies, seen about 100 million years after their closest approach. It consists of NGC 6621 (to the left) and NGC 6622 (to the right). NGC 6621 is the larger of the two, and is a very disturbed spiral galaxy. The encounter has pulled a long tail out of NGC 6621 that has now wrapped behind its body. Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and W. Keel (University of Alabama, Tuscaloosa)

NGC 6621/2 (VV 247, Arp 81) is a strongly interacting pair of galaxies, seen about 100 million years after their closest approach. It consists of NGC 6621 (to the left) and NGC 6622 (to the right). NGC 6621 is the larger of the two, and is a very disturbed spiral galaxy. The encounter has pulled a long tail out of NGC 6621 that has now wrapped behind its body. Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and W. Keel (University of Alabama, Tuscaloosa)

Originally posted on Forbes!

Your typical galaxy does indeed have much more mass in its dark matter reservoir than it has in its stars. Unfortunately for us observers, stars are the only easily visible part of a galaxy. With the exception of a few nearby galaxies, where we can measure how much of this dark matter is hanging around a galaxy more directly, we have to assume that there’s some extra mass, but it's hard to know exactly how much. In general, there’s something like two to five times as much dark matter surrounding a galaxy as there is matter in stars and gas.

You’re also completely correct that the dark matter component to a galaxy doesn’t seem to collide with anything, and doesn’t feel friction or any kind of aerodynamic drag. As far as we can tell, dark matter only interacts with other kinds of matter (and with itself) through the force of gravity. So with this setup, you would expect two blobs of dark matter to simply pass through each other if they are on a collision course. And yet, we see that galaxies do collide; how is it possible to pull two objects together without any friction?

To get a galactic collision, you need two things. The first is to have two objects moving along a path where they will eventually pass near each other. However, if these two objects are moving too quickly relative to each other, they may just slingshot past one another, never to encounter each other again. This is called a flyby encounter, and happens a lot in clusters of galaxies. Galaxy clusters contain thousands of galaxies, all moving very quickly, and so each galaxy may swing past a number of others, but is moving too quickly to stop and merge with any of them.

This Hubble image displays a beautiful pair of interacting spiral galaxies with swirling arms. The smaller of the two, dubbed LEDA 62867 and positioned to the left of the frame, seems to be safe for now, but will probably be swallowed by the larger spiral galaxy, NGC 6786 (to the right) eventually. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

This Hubble image displays a beautiful pair of interacting spiral galaxies with swirling arms. The smaller of the two, dubbed LEDA 62867 and positioned to the left of the frame, seems to be safe for now, but will probably be swallowed by the larger spiral galaxy, NGC 6786 (to the right) eventually. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

So let’s say you have both pieces; your galaxies are on a collision course, and moving slow enough so they won’t go zipping past each other. The speed is critical. The two galaxies need to have some time to influence each other - the longer the galaxies spend near each other, the longer they have to be influenced by the other galaxy. But how do you slow down the galaxy enough to capture it, and eventually merge it into a single galaxy? The slowing is due to a process called dynamical friction. Even if the individual particles which make up a galaxy (which is mostly dark matter and stars) never physically touch each other, they still influence each other through gravity.

Galaxies on a collision course will swing past each other - let’s say they’re moving past each other like two people on escalators going opposite directions (not a bad approximation). The nearest stars in one galaxy pull gravitationally on the nearest stars in the other galaxy, and both sets of stars wind up slowing down as a result. It would be like reaching across the barrier and clasping hands with the person on the escalator going the opposite direction as you; that brief pull between you would pull both of you off balance. Both of you would get pulled towards each other, against the direction of your escalator. (I don’t recommend trying this experiment.) Over time, as every star does this with every other star, and the dark matter particles do this with every other dark matter particle, the stars in both galaxies will come to rest relative to each other. You’ve created a new, single galaxy.

Arp 256 is a stunning system of two spiral galaxies in an early stage of merging. The Hubble image displays two galaxies with strongly disrupted shapes and an astonishing number of blue knots of star formation that look like exploding fireworks. The galaxy to the left has two extended ribbon-like tails of gas, dust and stars. The system is a luminous infrared system radiating more than a hundred billion times the luminosity of our Sun. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), and G. Ostlin (Stockholm University)

Arp 256 is a stunning system of two spiral galaxies in an early stage of merging. The Hubble image displays two galaxies with strongly disrupted shapes and an astonishing number of blue knots of star formation that look like exploding fireworks. The galaxy to the left has two extended ribbon-like tails of gas, dust and stars. The system is a luminous infrared system radiating more than a hundred billion times the luminosity of our Sun. Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), and G. Ostlin (Stockholm University)

This dynamical friction trick works great for individual galaxies, because both the dark matter and the stars are basically collisionless. Dark matter doesn’t interact with anything except by gravity, and the stars within a galaxy are so far apart that they’re incredibly unlikely to ever collide with another star. But galaxies aren’t uniquely stars and dark matter - there’s also gas: the stuff of nebulae. Gas is different from stars in that it can be compressed easily, and it collides with itself much more easily than the stars do. If you get enough galaxies together (as you do in a cluster), this gas can get pulled out of the galaxy, and heated to such a high temperature that it glows in X-rays.

So what happens if you take a cluster of galaxies, which is made of a huge mess of dark matter, several hundred galaxies made of stars, and a giant cloud of gas, permeating the whole cluster, and fling it at another cluster?

The Bullet Cluster. Dark purple indicates the location of the visible + dark matter mass. Red shows the X-ray emitting gas. Individual galaxies are seen in the optical image. Image credits: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al., Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al

The Bullet Cluster. Dark purple indicates the location of the visible + dark matter mass. Red shows the X-ray emitting gas. Individual galaxies are seen in the optical image. Image credits: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al., Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al

You get the Bullet Cluster, just above. This is one of the most famous clusters because it gave us really solid proof that dark matter was real, observable, and collisionless. The Bullet Cluster is actually two clusters in the early stages of merging together. We can see the two bundles of galaxies, on the left and right of the image. If you trace the amount of mass in each cluster, and then check where the X-ray gas is, you can see that the gas slammed into the gas from the other cluster like a brick wall - the stars and dark matter sailed right on through.

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