IS M82 A DISK-TRUNCATED BULGE BY A CLOSE ENCOUNTER WITH M81

The rotation curve of the small-mass starburst galaxy M82 has a steep nuclear rise, peaking at 200 pc radius, which then declines in a Keple rian fashion. This rotation curve mimics that for a central bulge of s piral galaxies with a high concentration of stellar mass. The declinin g rotation indicates that its extended disk mass is missing. In order to explain this peculiar rotation characteristic, we propose a hypothe sis 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 simulated a tidal truncation of the disk of a companion galaxy by a t idal penetration through its more massive parent galaxy. The model can well reproduce the observed peculiar feature of M82.