H i Kinematics along the Minor Axis of M82

Abstract 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 H i observations that reveal the cold gas kinematics along the minor axis in unprecedented detail. This includes the detection of H i 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 H i velocity decreases substantially from about 120 to from 1.5 to 10 kpc off the midplane. The velocity profile is not consistent with the H i 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 H i 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 H i kinematics are inconsistent with a simple conical outflow centered on the nucleus, but instead require the more widespread launch of the H i over the ∼1 kpc extent of the starburst region. Regardless of the launch mechanism for the H i gas, the observed velocity decrease along the minor axis is sufficiently great that the H i may not escape the halo of M82. The inferred H i outflow rate at 10 kpc off the midplane is much less than 1 yr −1 .