Planetary Nebulae in Face-On Spiral Galaxies. III. Planetary Nebula Kinematics and Disk Mass

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.