New Candidate Supernova Remnants in M31

Astronomy and Astrophysics – Astronomy

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Scientific paper

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(

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