HI Kinematics Along The Minor Axis of M82
  M82 is one of the best studied starburst galaxies in the local universe, and
is consequently a benchmark for studying star formation feedback at both low
and high redshift. We present new VLA HI observations that reveal the cold gas
kinematics along the minor axis in unprecedented detail. This includes the
detection of HI up to 10 kpc along the minor axis toward the South and beyond 5
kpc to the North. A surprising aspect of these observations is that the
line-of-sight HI velocity decreases substantially from about 120 km/s to 50
km/s from 1.5 to 10 kpc off the midplane. The velocity profile is not
consistent with the HI gas cooling from the hot wind. We demonstrate that the
velocity decrease is substantially greater than the deceleration expected from
gravitational forces alone. If the HI consists of a continuous population of
cold clouds, some additional drag force must be present, and the magnitude of
the drag force places a joint constraint on the ratio of the ambient medium to
the typical cloud size and density. We also show that the HI kinematics are
inconsistent with a simple conical outflow centered on the nucleus, but instead
require the more widespread launch of the HI over the ~1 kpc extent of the
starburst region. Regardless of the launch mechanism for the HI gas, the
observed velocity decrease along the minor axis is sufficiently great that the
HI may not escape the halo of M82. We estimate the HI outflow rate is much less
than 1 M$_{\odot}$ per year at 10 kpc off the midplane.
