Dark matter: galaxy vs. super-cluster evidence

[hkyriazi]

I was wondering if studies of individual galaxies predict the same amount and distribution of dark matter as do studies of galactic super-clusters.

 

[Chalnoth]

I was wondering if studies of individual galaxies predict the same amount and distribution of dark matter as do studies of galactic super-clusters.

Well, in reality, different galaxies have different amounts and distributions of dark matter among one another, so no, you're not going to get the same amount or distribution.

 

[hkyriazi]

Thanks, Chalnoth. What I meant was, do the distributions and amounts of hidden mass one must postulate to get individual galaxies right, translate immediately into the proper amounts to get the superclusters right? Or is there additional hidden mass, far outside individual galaxies, that must be postulated in order to get those same galaxies clustering together correctly?

 

[Chalnoth]

Thanks, Chalnoth. What I meant was, do the distributions and amounts of hidden mass one must postulate to get individual galaxies right, translate immediately into the proper amounts to get the superclusters right? Or is there additional hidden mass, far outside individual galaxies, that must be postulated in order to get those same galaxies clustering together correctly?

I don't think it's quite that simple. Rather, the larger the system is, the easier it is to simulate on the computer, and the more observations match those simulations. The basic problem here is that the properties of galaxies depend critically upon the behavior of normal matter, and the tight interactions of normal matter are exceedingly difficult to calculate. But larger systems like clusters are less dependent on those interactions, and better accord with the approximate calculations we are able to do.

 

[hkyriazi]

I thought dark matter was needed to explain why galaxy outer arms move at about the same velocity as the inner parts, when normally they'd have to move much slower to stay in "orbit." I don't know what the situation is with super-clusters. Are you saying that the motions of galaxies within super-clusters can be explained without the need to postulate dark matter?

 

[Chalnoth]

I thought dark matter was needed to explain why galaxy outer arms move at about the same velocity as the inner parts, when normally they'd have to move much slower to stay in "orbit." I don't know what the situation is with super-clusters. Are you saying that the motions of galaxies within super-clusters can be explained without the need to postulate dark matter?

Yes, but galaxies aren't terribly homogeneous: they tend to have rather different amounts and distributions of dark matter. The galaxy rotation curves, for instance, tend to be all over the place.

 

[hkyriazi]

Thanks, Chalnoth. I should've done more homework before posting. I took a look at some of the other dark matter posts, and found some helpful references. One was from Zwicky, back in the mid-1930s, where he first posited dark matter, and some of his evidence was from galactic clusters. I'm going to tentatively conclude that the two phenomena of individual galaxy rotation curves, and galactic clusters, are studied independently: the former being mostly close, to get a better look inside the galaxy, and the latter much further away, to get perspective on their interactions. Thus, the amounts of dark matter calculated for individual galaxies aren't generally applied to specific clusters. But, I'll keep reading.