Kinematics of Planetary Nebulae in M51's Tidal Debris

We report the results of a radial velocity survey of planetary nebulae (PNe) located in the tidal features of the well-known interacting system NGC 5194/95 (M51). We find clear kinematic evidence that M51's northwestern tidal debris consists of two discrete structures that overlap in projection—NGC 5195's own tidal tail and diffuse material stripped from NGC 5194. We compare these kinematic data to a new numerical simulation of the M51 system and show that the data are consistent with the classic "single-passage" model for the encounter, with a parabolic satellite trajectory and a 2 : 1 mass ratio. We also comment on the spectra of two unusual objects: a high-velocity PN that may be associated with NGC 5194's halo and a possible interloping high-redshift galaxy.