Program: GS-2011A-C-1

Using a spectacular set of emission-line images of M83 we obtained from Magellan/IMACS in April 2009, we have identified 118 new emission nebulae that are likely to be supernova remnants (SNRs), for a total of almost 200 when combined with ones identified in our previous study with the Blanco. Of these, 33 have diameters $\lesssim 1\arcsec$ (= 22 pc at M83) and hence probably are relatively young. The great majority (27) of these are coincident with soft X-ray sources (a further indicator of youthful vigor), and at least four are dominated by \oiii\ emission. These four, and possibly others, are probably ejecta-dominated core-collapse SNRs akin to Cas A---rare birds indeed, with only 8 known in the Universe. The primary goal of this proposal is to obtain optical spectra of all the young SNR candidates in order to search for high-velocity ejecta and characterize the elemental abundances, in order to positively identify any that are indeed ejecta-dominated. In addition, we hope to obtain spectra from most of the entire SNR sample in M83 to study the physical conditions, kinematics, and abundances of the SNR population. The result may well be the largest optically well-studied sample of SNRs in any single galaxy.