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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
$N(
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.
