Atomic hydrogen shells in the Messier 82 starburst

We report on a search for atomic hydrogen holes and shells in the nearby starburst galaxy M82, using high angular resolution (~1.3arcsec) VLA HI absorption observations. From this study, we have detected four HI shells in the central kiloparsec of M82. The sizes of these shells (~30-50pc) are smaller than those of the majority of shells observed in the Large Magellanic Cloud, although the M82 shells have higher expansion velocities (~30kms^-1) and typical kinetic energies of 10⁵¹-10⁵²erg. Because our observations were made in absorption, strong selection effects are present which hinder the detection of shells that could be present outside, or behind, the extended radio continuum associated with the starburst. Nevertheless, our detection of four shells in M82 actually represents a higher density of shells per unit area compared with the Large Magellanic Cloud. We also discuss the gas dynamics in the central kiloparsec of M82, and discuss the velocity structure of gas in a barred potential and in wind-driven shells. We conclude that in M82 the observed gas dynamics are most likely a superposition of both effects.