Identification of Supernova Remnants in the Sculptor Group Galaxies NGC 300 and NGC 7793

Abstract

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 galaxy NGC 7793, using interference filters to isolate emission from Hα + [N II], [S II], and a continuum band near 6100 A. Using these images, we have identified isolated nebulae that are bright in [S II] relative to Hα, indicating probable shock heating, and that have morphologies consistent with identification as supernova remnants (SNRs). Our lists of candidates contain 28 objects each in NGC 300 and NGC 7793. We then obtained optical (4800-6900 A) long-slit CCD spectra of many of these nebulae, as well as many adjacent photoionized regions along the slits for comparison. The spectra confirm that the SNR candidates have [S II]:Hα ratios in excess of 0.4, thus satisfying the usual optical criterion for being supernova remnants. Approximately 80% of the objects observed spectroscopically show additional emission lines that support their identification as supernova remnants. These are the first large samples of supernova remnants identified in galaxies outside the Local Group. In general, many properties of these SNR samples parallel the more extensive sample identified in M33. However, the dividing line between the [S II]:Hα ratios appropriate for photoionized and shock-heated nebulae appears to be much less distinct in these galaxies than for Local Group galaxies, causing confusion and incompleteness in these samples. This confusion effect is most prevalent in NGC 7793, where the observed [S II]:Hα ratio in the most diffuse photoionized gas rises well above the value of 0.4 normally used to discriminate SNRs from H II regions.