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

TL;DR

This paper proposes using velocity structure from spectroscopic data as a reliable method to distinguish supernova remnants from HII regions in nearby galaxies, improving sample purity.

Contribution

It introduces velocity broadening as a diagnostic tool for separating SNRs from HII regions, enhancing identification accuracy over traditional ratio-based methods.

Findings

SNRs show significant velocity broadening distinguishable from HII regions.

Spectroscopic observations with ~50 km/s resolution can effectively separate SNRs from HII regions.

Velocity structure is a robust criterion even at large SNR diameters.

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$.