Dark Matter Mystery Deepens In Cosmic 'Train Wreck'

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Everything I need to know I learned through Googling.

Bad Astronomer take: BA Blog: A dark hole

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I think the B.A. queried correctly with the following:

Isn’t this what is happening in the Bullet Cluster, enabling the detection of weak lensing where there is no luminous matter or gas, implying that it must be dark matter doing the lensing?

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Everyone is entitled to his own opinion, but not his own facts.

The difference is, in the Bullet cluster, the Dark Matter appeared to avoid the collision: and in this case, Dark Matter appears to reside in the middle. You really cannot have it both ways; so something is not understood.

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jwj

It's a big universe out there...is it really unwinding, really burning out?

No. In the Bullet Cluster the galaxies and dark matter are still together, but they have separated from the X-ray emitting gas.

You might be able to have it both ways. The Bullet cluster represents a recent collision, and Abell 520 represents two or more clusters that collided long ago. The luminous matter in the Bullet cluster is still bound to one or the other of the dark matter clouds, and will in the long run redistribute itself as galaxies with too much orbital energy to ever approach the center again.

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Forming opinions as we speak

There are two threads on this topic. The other one is in the Universe Today Stories Forum.

Continuing Antoniseb's suggestion, if the dark matter is a little bit warm as has been suggested by at least one previous experiment, maybe it might slowly diffuse and gravitate closer to the center of the whole melange, just as the hot gas might.

Remember the surprises we got when we looked at the gas dynamics around recent supernova explosions? Nobody suggested gas dynamics was wrong, it was expected that some aspect of what is happening had not yet been identified. No new physics, just a more complete model-- and that's just what happened. I expect the same here, this doesn't seem like a fundamental challenge to the dark matter hypothesis.

This seems to be an nice forum, so I'll repost my comment from the BA blog here, in hope that someone who knows a lot will answer.

Regarding the galaxies in the Abel cluster that seems to be stripped of dark matter:

Doesn’t the dark matter theory also predict that there should exist galaxies devoid of dark matter? I mean, if u look at the famous picture of the bullet cluster, u see the heated gas separated from dark matter. Won’t that gas eventually cool down and form its own stars and galaxies, without dark matter? What kind of galaxies will be the result of that? Are people looking for such galaxies?

The Bullet Cluster's unusual dark matter distribution was discovered relatively recently. I expect that there are people who are looking to see the relative proportions of reactive matter and dark matter in various galaxies, but this is not an easy measurement to make for any but the closest of galaxies. Also, the cluster collision that the Bullet cluster experienced may be a fairly rare occurrence so we don't know how many such galaxies would be out there. One more thing. The collision of the two clusters is not a simple two body problem. It is very likely that the dark matter is separated from the central gas for now, but it may be coming back in some configuration or another.

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Forming opinions as we speak

Wow those things are amazing.

Sorry for the zero-intellectual prashe, but when i see this kind of space wonders i feel like a child.

Ditto.

With the recent Dark Matter Ring, if you were to look at that structure from the side I think the upper and lower parts of the ring would look very similar. In that situation the dark matter had been 'splashed' out of the central region dominated by the galaxy clusters, as they passed through each other.

In Abell520 we may be seeing the same thing except side on, if the galaxy clusters are colliding perpendicular to our line of sight.

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plenty of woo, at the hotel hoagaland...

I think the puzzle is, the galaxies and the dark matter are only affected by gravity, so one might expect them to follow the same dynamics. However, there is a concept of "kinematic temperature", which is a fancy way of saying 'velocity' of the dark matter particles, and the galaxies. As you all know, when only gravity matters, the only thing that can separate behavior is velocity, so maybe the galaxies had a very different average speed than did the dark matter particles, prior to the train wreck. That might explain why they 'splashed out' farther, if that is indeed what happened. But it's hard to speculate without knowing more of the numbers, like what those speeds are, and what actual simulations would show. I give it 3 months before some computer jock publishes simulations that get the galaxies and dark matter to behave that way, given a clever setup.