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  1. Why doesn't the dark matter halo co-rotate with the luminous disk?

  2. What keeps it from falling into the center if not angular momentum?

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Because it does not interact much the individual dark matter components (whatever they may be) have no efficient way to shed their energy and end in lower orbits.

They do fall into the center and then fall right back out again, never losing significant amounts of energy.

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In that case wouldn't a lot of them be absorbed by the black hole in the center? And wouldn't we expect most of them to be in a roughly periodic motion that passes through the galactic center and goes to its edges, rather than a mostly "static" halo that the luminous disk is flying through? – Disputationist Dec 11 '12 at 23:05
Two things (1) the black hole is tiny (yes, it a monster of a black hole but it is still very small on galactic scales) and (2) the orbits of the dark matter components are orbits which means they are elliptical. Any "piece" of dark matter with even a very small amount of angular momentum relative the hole will simply miss it over and over again. That said, I would be interested in seeing an estimate of how fast it was eating the dark matter. – dmckee Dec 11 '12 at 23:11
@dmckee: or what fraction of the initial orbital angular momentum/energy of DM had been radiated away as gravitational radiation over the lifetime of the galaxy. – Jerry Schirmer Dec 11 '12 at 23:12
Is it the case that 1) gas interacts readily and so clusters at the center, 2) stars form from the gas, and hence also cluster there, 3) dark matter doesn't interact and stays far out? I ask because of this 2006 report which showed both stars and dark matter separating from gas in a galactic collision, indicating stars interact similarly to dark matter (?):… – Art Brown Dec 12 '12 at 0:10
@AntBrown The stars form from gas - it is the gas that behaves differently to dark matter. Stars once formed do behave a bit like dark matter on similar orbits because they are essentially a collisionless. Stars formed at the beginning of the galaxy from gas before the disk formed are still in a spherical halo - they are called... halo stars. – Rob Jeffries Mar 8 '15 at 18:12

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