An Optical Search For Supernova Remnants in NGC 2403

Abstract

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 B8@ ] 8@ eld covering nearly all of this galaxy. Our SNR identication technique consisted of constructing continuum-subtracted Ha and [S II] jj6716, 6731 images, obtaining spectra of a few [S II]bright emission nebulae found in these images, and then using the [S II]/Ha ratios obtained from these spectra to correct the image [S II]/Ha ratios. Thirty-ve emission nebulae with [S II]/Ha 0.45 were identied as SNRs and have diameters of 20200 pc, reddeningcorrected Ha intensities of 2.2 ] 10~15 to 1.4 ] 10~13 ergs cm~2 s~1, [S II]/Ha ratios of 0.451.11, and a variety of optical morphologies (shell, arc, and lled). Spectra of 15 of these remnants were obtained and are presented. The two NGC 2403 SNRs previously identied by DOdorico, Dopita, & Benvenuti were among the 35 SNRs detected. Our nal 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]/Ha 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 ;