An optical search for supernova remnants in NGC 2403

We present the results of an optical search for supernova remnants (SNRs) in the Scd galaxy NGC 2403 using image and spectral observations of a approximate to 8' x 8' field covering nearly all of this galaxy. Our SNR identification technique consisted of constructing continuum-subtracted H alpha and [S II] lambda lambda 6716, 6731 images, obtaining spectra of a few [S II]-bright emission nebulae found in these images, and then using the [S II]/H alpha ratios obtained from these spectra to correct the image [S II]/H alpha ratios. Thirty-five emission nebulae with [S II]/H alpha greater than or equal to 0.45 were identified as SNRs and have diameters of 20-200 pc, reddening-corrected H alpha intensities of 2.2 x 10(-15) to 1.4 x 10(-13) ergs cm(-2) s(-1), [S II]/H alpha ratios of 0.45-1.11, and a variety of optical morphologies (shell, src, and filled). Spectra of 15 of these remnants were obtained and are presented. The two NGC 2403 SNRs previously identified by D'Odorico, Dopita, & Benvenuti were among the 35 SNRs detected. Our final NGC 2403 SNR sample appears biased against detecting large, faint remnants, and no SNRs were detected inside bright H II regions. An examination of the [S II]/H alpha values for all the nebulae measured in NGC 2403 shows no sharp dividing line between SNRs and H II regions, similar to what was found in M31, NGC 300, and NGC 7793 (Walterbos & Braun; Blair & Long). We estimate roughly that there are about 4 times more SNRs in NGC 2403 than we detected, which, assuming the observable lifetime of a SNR is similar to 20,000 yr, yields a crude rate of 1 SN in 150 yr. Analysis of the 15 SNR spectra show a detectable nitrogen gradient, a larger scatter in oxygen abundances than in nitrogen, and a lower mean oxygen abundance than from published H II region data, all of which have been seen in previous SNR studies of other galaxies.