Kinematics: A Clean Diagnostic for Separating Supernova Remnants from
  HII 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 narrow-band
imaging, separating SNRs from HII regions on the basis of [SII]:H-alpha ratios
that are elevated compared to HII regions. However, the boundary between SNRs
and HII regions is not always distinct, especially at low surface brightness.
Here we explore velocity structure as a possible criterion for separating SNRs
from HII regions, using a sample of well-studied SNRs in the Large Magellanic
Cloud (LMC) 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 HII 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$.
