Is M82 a Disk-Truncated Bulge by a Close Encounter with M81?

Abstract

The rotation curve of the small-mass starburst galaxy M82 has a steep nuclear rise, peaking at 200 pc radius, and then, declines in a Keplerian fashion. This rotation curve mimics that for a central bulge of spiral galaxies with high concentration of stellar mass. The declining rotation indicates that its extended disk mass is missing. In order to explain this peculiar rotation characteristics, we propose a hypothesis that M82 is a surviving central bulge of a much larger disk galaxy, whose outer disk was truncated during a close encounter with M81. We simulate a tidal truncation of the disk of a companion galaxy by a tidal penetration through its more massive parent galaxy. The model can well reproduce the observed peculiar feature of M82.