Interstellar Gas and a Dark Disk
  We introduce a potentially powerful method for constraining or discovering a
thin dark matter disk in the Milky Way. The method relies on the relationship
between the midplane densities and scale heights of interstellar gas being
determined by the gravitational potential, which is sensitive to the presence
of a dark disk. We show how to use the interstellar gas parameters to set a
bound on a dark disk and discuss the constraints suggested by the current data.
However, current measurements for these parameters are discordant, with the
uncertainty in the constraint being dominated by the molecular hydrogen
midplane density measurement, as well as by the atomic hydrogen velocity
dispersion measurement. Magnetic fields and cosmic ray pressure, which are
expected to play a role, are uncertain as well. The current models and data are
inadequate to determine the disk's existence, but, taken at face value, may
favor its existence depending on the gas parameters used.
