The central regions of M82.

This paper discusses observations of the structure, kinematics, and emission-line spectra of the bright central regions of M82 and the base of the H-alpha-emitting filament system that emerges from those regions. Evidence is found that intense bursts of star formation have continued to the present time in the central parts of the galaxy and also occurred in the disk about 100 million years ago. The results also indicate that the inner regions of the filaments are ionized and that their line radiation is intrinsic rather than scattered by dust from other H II regions, as expected from the most popular current models. An interpretation that is qualitatively consistent with the known properties of M82 is shown to be a modified version of a tidal model, wherein the large amount of cool matter (H I, H2, and dust) now dispersed across the face of the galaxy and into its halo was originally in the plane of M82 (then an Sc-Irr galaxy), was expelled during a tidal encounter with M81, and is now being slowly reacquired, producing a wave of star-formation bursts in the main body.