A Spectroscopic Study of the Rich Supernova Remnant Population in M83
  We report the results from a spectrophotometric study sampling the roughly
300 candidate supernova remnants (SNRs) in M83 identified through optical
imaging with Magellan/IMACS and HST/WFC3. Of the 118 candidates identified
based on a high [S II] $\lambda\lambda$ 6716,6731 to H$\alpha$ 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] 5007 $\lambda$ to H$\alpha$ emission ratio,
selected in an attempt to identify young ejecta-dominated SNRs like Cas A,
reveal only one (previously reported) object with the broad (over 1000 km/s)
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 shows broad emission lines; instead their spectra stem from relatively
slow (< 200 km/s) 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.
