DM Halo Movement: Predicted vs Observed

[jhe1984]

Okay - and this is most likely a gross oversimplification - but (for galaxies thought to have DM halos), the predicted motion/predicted spin of the galaxy would be much tighter, while the observed motion (with the DM accounted for) is actually much wider?

Would this be equivalent to spinning a yo-yo above your head at one power level (representing non-dm galaxy mov't) and then adding a lot of weight to that same yo-yo (representing DM galaxy) but still spinning it at the same power level (so it now goes a lot wider and slower)?

 

[SpaceTiger]

Okay - and this is most likely a gross oversimplification - but (for galaxies thought to have DM halos), the predicted motion/predicted spin of the galaxy would be much tighter, while the observed motion (with the DM accounted for) is actually much wider?

I'm not quite sure what you mean here. At a given distance from the center of a the galaxy, the presence of dark matter leads to a larger rotation velocity. This is because the force of gravity at that radius is stronger (more mass interior) and so a star must have more angular momentum to support a circular orbit.

Would this be equivalent to spinning a yo-yo above your head at one power level (representing non-dm galaxy mov't) and then adding a lot of weight to that same yo-yo (representing DM galaxy) but still spinning it at the same power level (so it now goes a lot wider and slower)?

Well, no, it would be sort of analogous if you left the yoyo the same size and increased the tension of the rope (to simulate the dark matter) by spinning the yoyo faster.

 

[jhe1984]

I'm not quite sure what you mean here. At a given distance from the center of a the galaxy, the presence of dark matter leads to a larger rotation velocity.

Okay, so are you saying that dark matter leads only to a larger rotational velocity, but not necessarily to a different galactic shape (than if DM weren't there)?

 

[SpaceTiger]

Okay, so are you saying that dark matter leads only to a larger rotational velocity, but not necessarily to a different galactic shape (than if DM weren't there)?

It does impact the shape, but not in the way you're thinking. Dark matter is thought to be very weakly interacting, in which case it would form a roughly spherical halo. Luminous matter, however, dissipates energy very easily, so it tends to form a disk. See here for more discussion of spherical vs. disk distributions:

https://www.physicsforums.com/showthread.php?t=93098"