21-cm observations of the interacting galaxies M81 and M82

Abstract

The Cambridge Half-Mile Telescope has been used to map the H I distribution and velocity field in the region surrounding the peculiar Irr II galaxy M82 with an angular resolution 2 × 2.1 arcmin. The maps reveal a clumpy H I bridge stretching between M81 and M82 which, at either end, merges into the galaxies with radial velocities appropriate to their optical recession velocities. Neutral hydrogen above the plane of M82 exhibits a radial velocity gradient in the sense that higher H I radial velocities are found in the north of the galaxy than in the south. The velocity gradient is in the same sense as that found optically in the filaments and may be gas which has been captured from M81 in a single hyperbolic tidal encounter $$\sim\,1.8\,\times\,{10}^{8}$$ yr ago. Evidence for a diminution in the H I surface density along the north axis of M82 coincident with part of the Hα filamentary system suggests that there is dynamical coupling between filaments and gas. A possible model of the filaments is one in which they are composed of dusty plasma accelerated away from the galactic nucleus under radiation pressure forces transmitted via the dust grains, and have swept out cavities in the surrounding H I cloud. In order to account for the observed blueshifts of the filaments on the south of M82, these must have interacted with the gas in the outer parts of the galaxy.