Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apjs..113..333m&link_type=abstract
Astrophysical Journal Supplement v.113, p.333
Astronomy and Astrophysics
Astronomy
22
Galaxies: Individual Ngc Number: Ngc 2403, Galaxies: Ism, Ism: Supernova Remnants
Scientific paper
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' 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 >= 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, 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]/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 ~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.
Blair William Patrick
Fesen Robert A.
Long Knox S.
Matonick David M.
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