  We present the results of a kinematic study of planetary nebulae in the
extreme outskirts of two spiral galaxies, M83 (NGC 5236) and M94 (NGC 4736). We
find that in the inner regions of the galaxies, the vertical velocity
dispersion (sigma_z) falls off exponentially with the light, as expected for a
constant mass-to-light ratio, constant thickness disk. However, starting at
four optical scale lengths, sigma_z asymptotes out at roughly 20 km/s. Our
analysis finds evidence for significant flaring in the outer regions as well,
especially in M94. These observations are in excellent agreement with
predictions derived from models of disk heating by halo substructure, and
demonstrate how kinematic surveys in the outer disks of spirals can be used to
test hierarchical models of galaxy formation.
