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We have used pure emission-line images (deep CCD images from which a continuum image has been subtracted; see Winkler \& Williams, companion paper) to search for supernova remnant (SNR) candidates in M31. The ratio of \sii\ $\lambda\ 6724$\ to \ha\ is used to distinguish SNRs from \hii\ regions, a technique that has been used effectively in other galaxies as well as in earlier photographic surveys of M31 (D'Odorico et al 1980, A\&A Supp 40, 67; Blair et al 1981, Ap J 247, 879). The extensive cooling zone typically found behind SNR shocks emits strongly in \sii, while in photoionized material such as \hii\ regions most of the S is twice ionized and hence \sii\ is weak. SNRs almost always have \sii/\ha\ ratios $> 0.4$\ vs values $< 0.2$\ for \hii\ regions. Using this criterion, we have identified 105 nebulae in M31 which are strong SNR candidates, of which 13 were previously identified by D'Odorico et al and/or Blair et al. While this number probably represents a small minorit y of the SNRs which are actually present in M31, confirmation of a substantial fraction of these would increase the population of known remnants in M31 several-fold and would make it the galaxy with the largest sample of known optical SNRs, including our own.
For the cumulative number of SNRs as a function of diameter we find
Observations were made with the Burrell Schmidt telescope of the
Warner and Swasey Observatory, Case Western Reserve University.
This research is sponsored by NSF grant AST-9315967
and by the Keck Northeast Astronomy Consortium.
Observations were made with the Burrell Schmidt telescope of the Warner and Swasey Observatory, Case Western Reserve University. This research is sponsored by NSF grant AST-9315967 and by the Keck Northeast Astronomy Consortium.
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