  Much of our understanding of dark matter halos comes from the assumption that
the mass-to-light ratio (M/L) of spiral disks is constant. The best way to test
this hypothesis is to measure the disk surface mass density directly via the
kinematics of old disk stars. To this end, we have used planetary nebulae (PNe)
as test particles and have measured the vertical velocity dispersion (sigma_z)
throughout the disks of five nearby, low-inclination spiral galaxies: IC 342,
M74 (NGC 628), M83 (NGC 5236), M94 (NGC 4736), and M101 (NGC 5457). By using HI
to map galactic rotation and the epicyclic approximation to extract sigma_z
from the line-of-sight dispersion, we find that, with the lone exception of
M101, our disks do have a constant M/L out to ~3 optical scale lengths.
However, once outside this radius, sigma_z stops declining and becomes flat
with radius. Possible explanations for this behavior include an increase in the
disk mass-to-light ratio, an increase in the importance of the thick disk, and
heating of the thin disk by halo substructure. We also find that the disks of
early type spirals have higher values of M/L and are closer to maximal than the
disks of later-type spirals, and that the unseen inner halos of these systems
are better fit by pseudo-isothermal laws than by NFW models.
