Starless Galaxies: BBC Report on Dark Matter Discovery

[tdunc]

I don't follow you on the luminous matter thing. So are you saying matter that is equally massive yet does not emit light per say would not achieve the same effect?
Also the rotation curve around a black hole Would Absolutly be Flatter at larger distances...

 

[SpaceTiger]

I don't follow you on the luminous matter thing. So are you saying matter that is equally massive yet does not emit light per say would not achieve the same effect?

The rotation curve depends on the mass as a function of radius. MOND is designed to make it so that we can simply say that the amount of mass is proportional to the amount of light we see and still reproduce the shapes of observed rotation curves. Black holes have basically a delta function mass profile (it's all at the center), so this isn't the same thing.

Also the rotation curve around a black hole Would Absolutly be Flatter at larger distances...

In Newtonian/Einsteinian gravity, the rotation curve would go as:

v=\sqrt{\frac{GM}{r}}

a large distance from the black hole. This isn't flat.

 

[tdunc]

I read the paper a second time, what they are really talking about is the discovery of a Dark Matter Halo, not a galaxy per say which would be contained within that halo. A halo is supposedly around every galaxy to explain for the flat rotation curve. Basically the rotation of stars are not what they think they should be according to gravitational tides from a central black hole and visible stars alone, so they introduce a halo a very large distance from the edge of the galaxy itself to explain why the stars near the outter edges are rotating so fast - effectivly giving a flat rotation curve. Personally I still don't understand the dynamics of how that would be.

First of all what is a rotation curve? Here is a diagram Fig.2 with a short description.
http://en.wikipedia.org/wiki/MOND

Fig.3 is shows where the Dark matter halo would be.

MOND attempts to do away with the need for a Dark matter halo and explain the flat rotation curve by a modification to Newtons 2nd law of motion which gives a change to acceleration in situations where very large distances between bodies are present therefore the effects of gravitational pull are lower overall than say here on Earth or around our Solar system.

Have I got this right?

SpaceTiger
"a large distance from the black hole. This isn't flat."
I still do not know why you would say this. Every galaxy has a black hole at the center, it is known the rotation curve a "large distance from the black hole" remains flat to the outter edges. What do you consider a large distance from center? I would say a 3rd maybe 4th from the center, that is well within the range of the observed flat rotation curve. In MOND theory "Consequently, the velocity of stars on a circular orbit far from the center is a constant, and doesn't depend on the distance r: the rotation curve is flat."

But that's not quite relevant now, also I don't recall anything said in the paper about such rotation curvature, whether is would be flat or not the point is mute because the paper is about the exsistence of the halo not the exsistence of the galaxy which is what you need to have in order to observe said rotation curve. Of course I might have missed that if they mentioned it with regards to the hydrogen that is located where the galaxy should be, and its rotation, but that doesn't make much sense because rotation of stars and rotation of an isotropic hydrogen are two different dynamics.

In any case I guess I'm still not convinced a black hole is not or could not be there for the sake of argument. Or even that what they have discovered is a Dark Matter halo which are up to this point as it would seem somewhat theoretical.

 

[SpaceTiger]

Personally I still don't understand the dynamics of how that would be.

One simple way to produce a flat rotation curve is with a simple isothermal sphere mass profile:

\rho \propto \frac{1}{r^2}

We used this as a fit to galaxy dark matter profiles for a long time, but recent evidence indicates that it's not quite as simple as that. We need another parameter to fit the observed velocity profiles.

Have I got this right?

Everything you've said up to this point looks right.

"a large distance from the black hole. This isn't flat."
I still do not know why you would say this. Every galaxy has a black hole at the center, it is known the rotation curve a "large distance from the black hole" remains flat to the outter edges.

Your theory, as I understood it, was that black hole was itself causing these motions. Current theory says that's not the case anywhere except very close to the black hole. The two observations you cite are roughly accurate, but they are not related to one another. Even just the luminous matter in a galaxy is several orders of magnitude more massive the central black hole.

In MOND theory "Consequently, the velocity of stars on a circular orbit far from the center is a constant, and doesn't depend on the distance r: the rotation curve is flat."

I didn't realize you were using MOND to make that statement. In MOND, you're right that the rotation curve would be flat a large distance from a central black hole. However, if you assume the mass is entirely in a central black hole and use MOND, then the rotation curve you get will be monotonically decreasing, with a Newtonian dependence at the center and flatness at the outskirts. This is not what we observe. In fact, the rotation curve increases from the center of the galaxy and then becomes flat on the outskirts.

In any case I guess I'm still not convinced a black hole is not or could not be there for the sake of argument. Or even that what they have discovered is a Dark Matter halo which are up to this point as it would seem somewhat theoretical.

The black holes exist, but we're sure they aren't the dominant source of gravity in most of the galaxies we see. I'm not sure if that's what you're trying to say, so if I'm misinterpreting you, my apologies.