Spectroscopy of Supernova Remnants and Candidates in M31

Abstract

With a star formation rate of order 0.4 M$_\odot $ yr$^{-1}$, M31 should have significant population of supernova remnants (SNRs), and, in fact, 156 SNR and SNR candidates have been suggested by Lee et al. (2014) by searching for nebulae with elevated [SII]/H${\alpha}$ ratios in narrow band images. Here we use a combination of low and high resolution optical spectroscopy obtained with Hectospec on the MMT to characterize 152 of these nebulae. Of these candidates, we find 93 nebulae that have [SII]/H${\alpha}$ ratios that exceed 0.4, the traditional ratio used to separate SNRs from HII regions, strongly suggesting that at least these objects are SNRs. Our high resolution spectroscopy reveals 108 nebulae that have velocity widths in H${\alpha} $ (full-width at 20% peak flux) that exceed 50 km s$^{-1}$, significantly larger than found in HII regions. There are 72 objects that satisfy both tests. Here we discuss the spectroscopic characteristics of all of the objects in our sample, and the likelihood that other objects in the sample of Lee et al. are also SNRs, and we briefly consider confirmation by X-ray, radio and UV observations. We also discuss several new candidates that have been identified serendipitously in the course of examining a large amount of archival Hectospec data.