HI in M81. I. Large Scale Structure and Spiral Density Waves

We present new VLA observations of the HI emission in two fields covering the spiral galaxy M81 and its nearby companions, providing high spatial (12") and spectral (2.5 km s^-1^) resolution and high sensitivity. The northeast arm shows strong signs of a classical density wave: a high concentration of gas on the inside part of the arm, strong streaming motions (up to 50 km s^-1^ in the plane of the galaxy), and velocity dispersions up to 20 km s^-1^ aligned with the intensity Peaks. By analysis of residual velocities along the major and minor axis, we conclude that the radial and tangential streaming motions in most of the northeast arm are consistent with classical density wave theory. The southwest arm shows some signs of classical density wave streaming motions, but they are weaker and not found consistently through the arm. We determine that the disk is stable against fragmentation (Q = 1.5) and there is likely to be a Q barrier in the inner disk keeping the HI spiral pattern from continuing into the nucleus. Qualitatively, the galaxy exhibits traits indicative of modal spiral density waves: the intensity distribution varies in intensity along the spiral arms, and the shape of the intensity profile reverses around corotation.