  We use the rotation curves of more than 100 disk galaxies to examine whether
the structure of their surrounding dark halos is consistent with the universal
density profile proposed by Navarro, Frenk & White (NFW profiles). Rotation
curve shape is a strong function of galaxy surface brightness: low-surface
brightness galaxies (LSBs) have slowly rising rotation curves while in
high-surface brightness systems the rotation speed rises sharply and stays flat
or even declines beyond the optical radius. These observations are consistent
with NFW halo profiles, with the possible exception of a few LSBs where the
rotation curves are better described by shallower central density profiles.
Consistency with observational trends requires that halos have lower
characteristic densities than expected in the standard biased cold dark matter
(CDM) scenario, but roughly compatible with COBE-normalized, low-density, flat
CDM universes. The data also imply that disk mass-to-light ratios increase
gently with luminosity, and that the halo circular velocity, V_200, is not
directly proportional to the disk rotation speed, V_rot. Slowly rotating disks
are surrounded by halos of higher circular velocities, whereas faster rotators
(V_rot > 150 km/s) are all surrounded by halos of similar mass, corresponding
to V_200 ~ 200 km/s. We speculate that this is because the efficiency of
assembly of baryons into galaxies is high in massive halos, leading to disks
too massive to be stable in systems that exceed the ``critical'' V_200 ~ 200
km/s. This modeling also provides a natural explanation for the distribution of
sizes and rotation speeds of disk galaxies. (Abridged.)
