MMT Spectroscopy of Supernova Remnant Candidates in M33

TL;DR

This study uses optical spectroscopy to confirm supernova remnant candidates in M33, showing most with characteristic emission line ratios are indeed SNRs, though some require additional criteria for confirmation.

Contribution

First spectroscopic confirmation of many SNR candidates in M33, providing a more reliable classification based on emission line ratios and broadening.

Findings

Majority of candidates confirmed as SNRs based on line ratios

No young ejecta-dominated SNRs detected in the sample

Broader lines observed in some SNR candidates

Abstract

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.