Properties of Optically Selected Supernova Remnant Candidates in M33

Abstract: Narrow band images covering strong emission lines are efficient to survey supernova remnants (SNRs) in nearby galaxies. Using the narrow band images provided by the Local Group Galaxy Survey we searched for SNRs in M33. Culling the objects with enhanced [SII]/Ha and round morphology in the continuum-subtracted Ha and [SII] images, we produce a list of 199 sources. Among them 79 are previously unknown. Their progenitor and morphology types were classified. A majority of the sample (170 objects) are likely remnants of core-collapse SNe, and 29 are remnants of Type Ia SNe. The cumulative size distribution of these objects is found to be similar to that of the M31 remnants derived in the similar way. We obtain a power law slope, a = 2.38 + 0:05. Thus a majority of the sources are considered to be in the Sedov-Taylor phase, consistent with previous findings. The histogram of the emission line ratio ([SII]/Ha) of the remnants has two concentrations at [SII]/Ha ~0.55 and ~0.8, as in M31. Interestingly Lx (and L20cm) of the compact center-bright objects are correlated with their optical luminosity. The remnants with X-ray emission have brighter optical surface brightness and smaller diameters than those without X-ray emission.