A new optical sample of supernova remnants in M33

We present a new and larger sample of supernova remnants in the nearby spiral galaxy M33. The sample is based upon CCD interference filter observations obtained with the Kitt Peak 4 m telescope and spectroscopic observations obtained with the Multiple Mirror Telescope. Using optical emission-line ratios, supplemented by a radio continuum map of M33 (Duric et al.; Gordon et al.), we have identified 98 supernova remnant (SNR) candidates, of which 53 were previously unknown. We have obtained spectra of 27 SNR candidates, bringing the total number of M33 SNRs for which spectra are available to 72. All the spectra show the characteristic signature of shock-heated gas, which leads us to believe that the rest of the candidates are also supernova remnants. The large sample provides a useful database to investigate the global properties of SNRs, In this paper, we present a new cumulative number-diameter [N(<D)] relation for SNRs, We find that the free expansion model is inconsistent with the observed N(<D) relation and that the Sedov-Taylor (ST) expansion model is a much better fit. The ST expansion model is used to derive a SN rate in M33 of one SN every 360 yr. The spectroscopic sample of SNRs is the largest of its kind in any galaxy. Our investigations show no relation between [N II]:H alpha and [S II] lambda 6717:lambda 6731. We do, however, find weak trends between the SNR diameter and [N II]:H alpha and [S II]:H alpha. Further, we find a strong correlation between [N II]:H alpha and the galactocentric distance (GCD) indicative of an abundance gradient. There is a large dispersion in the [N II]:H alpha ratio at a given GCD that cannot be explained by evolutionary effects and that may be due to a larger abundance dispersion in the inner part of the galaxy.