IDENTIFICATION OF SUPERNOVA-REMNANTS IN THE SCULPTOR GROUP GALAXIES NGC-300 AND NGC-7793

We have obtained CCD imagery of eight fields in the Sculptor Group Sc spiral galaxy NGC 300 and two fields covering the Sculptor Sd spiral g alaxy NGC 7793, using interference filters to isolate emission from H alpha + [N II], [S II], and a continuum band near 6100 Angstrom. Using these images, we have identified isolated nebulae that are bright in [S II] relative to H alpha, indicating probable shock heating, and tha t have morphologies consistent with identification as supernova remnan ts (SNRs). Our lists of candidates contain 28 objects each in NGC 300 and NGC 7793. We then obtained optical (4800-6900 Angstrom) long-slit CCD spectra of many of these nebulae, as well as many adjacent photoio nized regions along the slits for comparison. The spectra confirm that the SNR candidates have [S II]:H alpha ratios in excess of 0.4, thus satisfying the usual optical criterion for being supernova remnants. A pproximately 80% of the objects observed spectroscopically show additi onal emission lines that support their identification as supernova rem nants. These are the first large samples of supernova remnants identif ied in galaxies outside the Local Group. In general, many properties o f these SNR samples parallel the more extensive sample identified in M 33. However, the dividing line between the [S II]:H alpha ratios appro priate for photoionized and shock-heated nebulae appears to be much le ss distinct in these galaxies than for Local Group galaxies, causing c onfusion and incompleteness in these samples. This confusion effect is most prevalent in NGC 7793, where the observed [S II]:H alpha ratio i n the most diffuse photoionized gas rises well above the value of 0.4 normally used to discriminate SNRs from H II regions.