Understanding the Effects of Dark Matter on Galaxies: Myths vs. Facts

[phinds]

It has occurred to me from the statements in various threads that I may be misunderstanding the effects of dark matter, so I'd like someone to either verify that I DO in fact have it right, or that I am mistaken.

MY understanding is that dark matter:

(1) has a gravitational effect on galaxies and causes the stars out from the center to rotate around the galactic center faster than they would otherwise do and that this effect becomes more pronounced the further from the center you get, such that while the otherwise expected speed of rotation trails off with a slope as you move away from the center, the actual measurements (attributed to the effects of dark matter) show that the speed is pretty much uniform all the way out
(2) dark matter is clumped up in an apparently uniform distribution from galactic centers out to somewhere beyond outer edges of each galaxy but does not extend out forever into the space between galaxies. That is, it is uniform in and near a galaxy, but clumped when looked at on much larger scales.
(3) dark matter is NOT what holds galaxies together. They would stay gravitationally bound without dark matter but the outer stars would rotate more slowly if the dark matter were not there.

#3 in particular seems to contradict some statements I have read here (I may have just misinterpreted the statements) so that's the one I'm wondering about the most but I want to make sure I have all of them right, or that I GET it them right eventually.

Thanks

 

[bcrowell]

(3) dark matter is NOT what holds galaxies together. They would stay gravitationally bound without dark matter but the outer stars would rotate more slowly if the dark matter were not there.

For this statement to be meaningful, you'd have to say what you're assuming stays constant. Are you saying you'd take an existing galaxy and (in a thought experiment) suddely remove all the dark matter? In that case, I think it would become unbound. Or are you talking about the hypothetical formation of galaxies in a universe that had never had dark matter? In that case, I think galaxies would exist and be bound, but they'd have different properties than the ones we actually observe (different sizes, different rotation curves,...).

 

[zhermes]

1 and 2 are exactly right---you've got it.
(There are a few nit-picky little details: e.g. we don't entirely know exactly how much DM clumps; it could clump to some degree within galaxies... we don't know)

3 (as bcrowell suggests) is a little more complicated; but if you just extraced (magically) all of the DM within a galaxy, most of the material would stay bound. More precisely, the disk/bulge material would stay bound, but some of the 'halo' material might escape. That being said, galaxies would look completely different if they formed/evolved without DM.

 

[phinds]

bcrowell and zhermes, thank you for the responses. Very helpful.

Yes, I was thinking of galaxy formation without DM and I did NOT realize, but clearly should have, that the resulting galactic characteristics would be significantly different. The rotation issue I DID think of but the fact that very likely less normal matter would become gravitationally bound did not occur to me and I really didn't think through the implications on size even aside from there being less matter, so your comments are most helpful.

 

[Darken-Sol]

i know the multiverse idea can't be tested so it gets shut down a lot. i was just wondering if dark matter has any chance of being another 'verses gravitational effects on ours. if so wouldn't it be away to test the multiverse theory? i understand we can rule it out by dismissing the multiverse crap. hypothetically is it at all possible if we factored in gravity from our neighbors?

 

[twofish-quant]

Yes, I was thinking of galaxy formation without DM and I did NOT realize, but clearly should have, that the resulting galactic characteristics would be significantly different.

One thing that you should know is that when it comes to galaxy formation we are currently at the "your guess is as good as mine" stage. The details of how dark matter impacts galaxy formation is something that we don't have a good handle on yet.

 

[twofish-quant]

i know the multiverse idea can't be tested so it gets shut down a lot. i was just wondering if dark matter has any chance of being another 'verses gravitational effects on ours.

The problem is that you get into the "too vague to be useful" issue. If you say "dark matter is made of weakly interacting particles" then it's not to hard to come up with ways of showing that it isn't true. If you say that dark matter is due to some gravitational interaction by some other universe, that's much too vague to shoot down.

i understand we can rule it out by dismissing the multiverse crap. hypothetically is it at all possible if we factored in gravity from our neighbors?

The immediate problem is that there is no obvious reason why the dark matter in a neighboring universe ought to track matter in our own galaxy.

There is a closely related idea from string theory that says that gravity is weak because much of the force "spills" into another dimension, and you can play with that idea, but for the purposes of dark matter, that's just another modified gravity theory.

Also most of the focus in dark matter/dark energy research isn't focused on what dark matter *is*, but it's focused more on figuring out what dark matter *isn't*. We've already made a lot of progress in figuring out what it *isn't*. It's not normal matter. It's also not hot.

 

[phinds]

One thing that you should know is that when it comes to galaxy formation we are currently at the "your guess is as good as mine" stage. The details of how dark matter impacts galaxy formation is something that we don't have a good handle on yet.

Thanks. That makes sense, since we don't know what it IS, we can't extrapolate that its currently observed effect of changing the rotational speed of outer stars tells us exactly what effect it would have as a galaxy is formed. What is the observational knowledge of how dark matter exists around the earliest galaxies that are within out range of possible study? That is, I doubt if we have the technology now to detect details of the dark matter (even assuming there is any) around galaxies in the Hubble Deep Field, but how about ones that are closer to us but still at least somewhat early in their formation?

 

[phinds]

What is the observational knowledge of how dark matter exists around the earliest galaxies that are within out range of possible study? That is, I doubt if we have the technology now to detect details of the dark matter (even assuming there is any) around galaxies in the Hubble Deep Field, but how about ones that are closer to us but still at least somewhat early in their formation?

I'm still wondering if anyone could shed any light on this.

Thanks

 

[twofish-quant]

What is the observational knowledge of how dark matter exists around the earliest galaxies that are within out range of possible study?

Very sketchy. Right now there is a lot of work on the "dark ages" which is the time between the time we see CMB and the time the first stars were formed. It's still mostly guesswork right now.

but how about ones that are closer to us but still at least somewhat early in their formation?

One of the consequences of the big bang model is that all of the galaxies started forming at roughly the same time so unlike stars there is no reason to think that there are any "young galaxies" near us. There are galaxies with young stars, but those are likely to be different than the first galaxies (or maybe not, we don't really know).

 

[phinds]

Thanks