Kinematics: A Clean Diagnostic for Separating Supernova Remnants from H II Regions in Nearby Galaxies

Many more supernova remnants (SNRs) are now known in external galaxies than in the Milky Way. Most of these SNRs have been identified using narrowband imaging, separating SNRs from H II regions on the basis of [S II]:H alpha ratios that are elevated compared to H II regions. However, the boundary between SNRs and H II regions is not always distinct, especially at low surface brightness. Here we explore velocity structure as a possible criterion for separating SNRs from H II regions, using a sample of well-studied SNRs in the Large Magellanic Cloud as well as a small number of SNRs in the galaxy M83. We find, perhaps not surprisingly, that even at large diameters, SNRs exhibit velocity broadening sufficient to readily distinguish them from H II regions. We thus suggest that the purity of most extragalactic samples would be greatly improved through spectroscopic observations with a velocity resolution of order 50 km s(-1).