The Structure and Kinematics of Galactic Disks
Robin Ciardullo (Penn State University)
Much of our understanding of dark matter halos comes from the
largely-untested assumption that the mass-to-light ratio of spiral disks
is constant is radius. To test this hypothesis, we have used planetary
nebula kinematics to measure the disk mass surface density of five
nearby,
low-inclination spiral galaxies. We show that, in general, disks do
have
a constant M/L out to ~ 3 optical scale lengths, but once outside this
radius, the vertical velocity dispersion of old disk stars is
independent
of distance. We suggest that this is due to the increased importance
of a thick disk, and possible disk heating of the thin disk by dark
halo substructure. We show that the disks of early type spirals have
higher M/L
values and are closer to maximal than the disks of later-type
spirals, and
that the residual unseen dark halos are better fit by pseudo-
isothermal laws
than by NFW profiles.
