NGC 3310, a galaxy merger?

The HI structure and kinematics of the peculiar starburst galaxy NGC 3310 (Arp 217, UGC 5786) are discussed. New evidence bearing on the origin of the starburst is presented. The bulk of HI coincides with the bright optical disk and shows differential rotation. Its velocity dispersion is, however, unusually large for a spiral galaxy (up to 40 km/s), suggesting that the disk is highly perturbed as already indicated by optical emission line spectroscopy. There are, in addition, two prominent HI tails, one extending to the north-west and the other, somewhat patchy, to the south. These HI tails, the perturbed kinematics and the peculiar optical morphology strongly suggest a recent merger between two gas-rich galaxies. This seems to have been a major merger in which most of the gas in the inner parts has been preserved in neutral atomic form and either one of the progenitor disks has survived or a new disk has formed.