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

Abstract

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$.

19 pages, 9 figures. To be published in ApJ

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• Supernova Remnants in M83 as Observed with MUSE
• Internal kinematics of giant H II regions in M101 with the Keck Cosmic Web Imager
• New ASKAP Radio Supernova Remnants and Candidates in the Large Magellanic Cloud
• MaNGA galaxies with off-centered spots of enhanced gas velocity dispersion
• A Kinematical Study of the Dwarf Irregular Galaxy NGC 1569 and its Supernova Remnants