The Rotation Velocity Attributable to Dark Matter at Intermediate Radii in Disk Galaxies

We examine the amplitude of the rotation velocity that can be attributed to the dark matter halos of disk galaxies, focusing on well-measured intermediate radii. The data for 60 galaxies spanning a large range of mass and Hubble types, taken together, are consistent with a dark halo velocity logV_h=C+B logR with C=1.47^+0.15_-0.19 and B~1/2 over the range of 1<R<74 kpc. The range in C stems from different choices of the stellar mass estimator, from minimum to maximum disk. For all plausible choices of stellar mass, the implied densities of the dark halos are lower than expected from structure formation simulations in ΛCDM, which anticipate C>1.6. This problem is not specific to a particular type of galaxy or to the innermost region of the halo (cusp or core); the velocity attributable to dark matter is too low at all radii.