A Spectroscopic Study of the Rich Supernova Remnant Population in M83 ∗

Abstract

We report the results from a spectrophotometric study sampling the candidate supernova remnants (SNRs) in M83 identified through optical imaging with Magellan /IMACS and Hubble Space Telescope /WFC3. Of the 118 candidates identified based on a high [S ii ] λλ 6716, 6731 to H α emission ratio, 117 show spectroscopic signatures of shock-heated gas, confirming them as SNRs—the largest uniform set of SNR spectra for any galaxy. Spectra of 22 objects with a high [O iii ] λ 5007 to H α emission ratio, selected in an attempt to identify young ejecta-dominated SNRs like Cas A, reveal only one (previously reported) object with the broad ( ) emission lines characteristic of ejecta-dominated SNRs, beyond the known SN1957D remnant. The other 20 [O iii ]-selected candidates include planetary nebulae, compact H ii regions, and one background QSO. Although our spectroscopic sample includes 22 SNRs smaller than 11 pc, none of the other objects show broad emission lines; instead their spectra stem from relatively slow ( ) radiative shocks propagating into the metal-rich interstellar medium of M83. With six SNe in the past century, one might expect more of M83's small-diameter SNRs to show evidence of ejecta; this appears not to be the case. We attribute their absence to several factors, including that SNRs expanding into a dense medium evolve quickly to the ISM-dominated phase, and that SNRs expanding into regions already evacuated by earlier SNe are probably very faint.