2018ApJ...855..140L

To date, over 220 emission nebulae in M33 have been identified as supernova remnants (SNRs) or SNR candidates, principally through [S II]:Hα line ratios that are elevated compared to those in H II regions. In many cases, the determination of a high [S II]:Hα line ratio was made using narrow-band interference filter images and has not been confirmed spectroscopically. Here, we present MMT 6.5 m optical spectra that we use to measure [S II]:Hα and other line ratios in an attempt to determine the nature of these suggested candidates. Of the 197 objects in our sample, 120 have no previously published spectroscopic observations. We confirm that the majority of candidate SNRs have emission line ratios characteristic of SNRs. While no candidates show Doppler-broadened lines expected from young, ejecta-dominated SNRs (>=1000 km s^–1), a substantial number do exhibit lines that are broader than H II regions. We argue that the majority of the objects with high [S II]:Hα line ratios (>0.4) are indeed SNRs, but the distinction between H II regions and SNRs becomes less obvious at low surface brightness, and additional criteria, such as X-ray detection, are needed. We discuss the properties of the sample as a whole and compare it with similar samples in other nearby galaxies.