[AndrewC]

It's a huge problem, as according to Kepler's laws, the speed should be inversely proportional to the square root of the radius. It's not.

I concur. Thanks, Herr Physiker, it is wise to get out of the way of the pros so I can add nothing. I also found another paper on the calculation of galactic angular velocity. The Disk Rotation of the Milky Way Galaxy

[VadeRetro]

Haven't read the whole paper, but the first paragraph is worth quoting:

The rotation of the disk of the Milky Way Galaxy is analyzed. It rotates neither as a solid disk, nor in accordance with individual keplerian orbits. Rather the disk executes a form of differential rotation which suggests that a considerable fraction of its mass resides in the outer portions of the Galaxy.