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 ≈ 8' × 8' field covering nearly all of this galaxy. Our SNR identification technique consisted of constructing continuum-subtracted Hα and [S II] λλ6716, 6731 images, obtaining spectra of a few [S II]-bright emission nebulae found in these images, and then using the [S II]/Hα ratios obtained from these spectra to correct the image [S II]/Hα ratios. Thirty-five emission nebulae with [S II]/Hα ≥ 0.45 were identified as SNRs and have diameters of 20-200 pc, reddening-corrected Hα intensities of 2.2 × 10^-15 to 1.4 × 10^-13 ergs cm^-2 s^-1, [S II]/Hα ratios of 0.45-1.11, and a variety of optical morphologies (shell, arc, 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]/Hα 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 ~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.