MMT Spectroscopy of Supernova Remnant Candidates in M33
  To date, over 220 emission nebulae in M33 have been identified as supernova
remnants (SNRs) or SNR candidates, principally through [SII]:Halpha line ratios
that are elevated compared to those in H II regions. In many cases, the
determination of a high [SII]:Halpha 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 [SII]:Halpha 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), a substantial number do exhibit lines that are broader than H II
regions. We argue that the majority of the objects with high [SII]:Halpha line
ratios (>0.4) are indeed SNRs, but at low surface brightness the distinction
between H II regions and SNRs becomes less obvious, 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.
