View Full Version : Does dark matter rotate with the galaxy

2013-Dec-16, 10:54 PM
Does dark matter rotate with the galaxy?
If the dark matter halo rotated with the galaxy, wouldn't it flatten and not be spherical?
If it doesn't rotate, wouldn't there be a gravitational drag from stars moving through it, and stirring it up?

2013-Dec-17, 02:22 AM
Dark Matter probably does not rotate with the stars in the galaxy. It is possible that some small fraction of the dark matter has picked up some rotational inertia from stars as the particles pass them.

2013-Dec-17, 02:33 AM
1. Unknown. On very general grounds, lacking the energy-loss processes of baryons, they might at most be oblate spheroids, not forming thin disks like the gas and young stars.

2. Yes, but the limits on oblateness of dark matter distributions in galaxies are not very helpful on this point - they come mainly from rotation curves of polar rings left over from disruption of companion galaxies or X-ray detections of hot gas in galaxies where the disks are minor constituents (one of those, for NGC 720, gives an axial ratio of about 0.6 for the gravitational potential shape).

3. Yes, but this process of (a form of dynamical friction) is slow compared to the age of the Universe, since a typical dark-matter particle would complete many galactic orbits before a strong gravitational scattering event with a star(massive interstellar clouds may be more effective, come to think of it).

This uncertainty factors in to interpretation of direct-detection dark-matter experiments, since the interaction rate is would go down if the dark matter particles are systematically moving more or less with the Sun around the galactic center. It's not much help that some simulations take it as a given that the dark-matter halos have no net rotation.

2013-Dec-17, 10:54 AM
Dark matter seems to be a bigger mystery than ever, well to me anyway.

Some older models of galaxies merging, seem to assume that dark matter picks up angular momentum from off-centre collisions, but as ngc3314 says, others do not.
At the moment, I can't find a gravitation lensing article or paper that shows non-spherical dark matter halos around galaxies.