MMT Spectroscopy of Supernova Remnant Candidates in M33

Abstract

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 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.

The impact of pre-supernova feedback and its dependence on environment

“…instrumental), while the Doppler broadened lines of >100 km s −1 , characteristic of radiative shocks in younger SNRs (e.g. Long et al 2018), are expected to be resolved in our observations. Older SNRs with line widths <100 km s −1 might therefore be misclassified, however, these also are typically very faint and would likely fall below our detection threshold.…”

“…The most commonly used line ratio diagnostic to identify SNRs in external galaxies is [S II]/H𝛼, as it gives a relative measure of the ionisation stages of sulphur within a given region. In H II regions, where the gas is mostly photoionised, a larger fraction of sulphur is expected to be in the higher excitation state (S ++ ), and [S II]/H𝛼 ratios are typically around 0.1 (Long et al 2018). In SNRs, where shocks contribute to populating S + , [S II]/H𝛼 values are expected to be 0.4 or higher (Allen et al 2008).…”

Kinematics: A Clean Diagnostic for Separating Supernova Remnants from H ii Regions in Nearby Galaxies

“…A potential diagnostic for SNRs is to take advantage of the fact that the material behind a SNR shock will have bulk motions comparable to the local shock speed, several hundred km s −1 , whereas material in H II regions will for the most part have velocities of order 10 km s −1 . Unfortunately, although spectroscopic follow-up of a large number of the nebulae identified as SNRs in extragalactic surveys has taken place (e.g., Matonick & Fesen 1997;Blair & Long 1997;Gordon et al 1998;Lee & Lee 2014a,b;Lee et al 2015;Winkler et al 2017;Long et al 2018Long et al , 2019, most of these spectroscopic surveys have not had sufficient spectral resolution to measure the expected line broadening in SNRs and use it as a meaningful diagnostic.…”

The Supernova Remnant Populations of the galaxies NGC 45, NGC 55, NGC 1313, NGC 7793: Luminosity and Excitation Functions

“…An additional correction needed to obtain the net H𝛼 flux is to subtract the contribution of the [N ]𝜆𝜆6548,6584Å lines . We adopted a ([N ]𝜆𝜆6548, 6584)/H𝛼 flux ratio of 0.27, based on emissionline measurements of spectroscopically identified SNRs of galaxies with similar metallicities of our sample (Long et al 2018, Leonidaki, Boumis, & Zezas 2013, Blair & Long 1997.…”