A multiwavelength study of supernova remnants in six nearby galaxies – II. New optically selected supernova remnants

We present results from a study of optically emitting Supernova Remnants (SNRs) in six nearby galaxies (NGC 2403, NGC 3077, NGC 4214, NGC 4395, NGC 4449 and NGC 5204) based on deep narrow band H and SII images as well as spectroscopic observations. The SNR classification was based on the detected sources that fulfill the well-established emission line flux criterion of SII/H > 0. 4. This study revealed ~400 photometric SNRs down to a limiting H flux of 10^ (-15) erg sec^ (-1) cm^ (-2). Spectroscopic observations confirmed the shock-excited nature of 56 out of the 96 sources with (SII/H) ₏₇₎ₓ> 0. 3 (our limit for an SNR classification) for which we obtained spectra. 11 more sources were spectroscopically identified as SNRs although their photometric SII/H ratio was below 0. 3. We discuss the properties of the optically-detected SNRs in our sample for different types of galaxies and hence different environments, in order to address their connection with the surrounding interstellar medium. We find that there is a difference in NII/H line ratios of the SNR populations between different types of galaxies which indicates that this happens due to metallicity. We cross-correlate parameters of the optically detected SNRs (SII/H ratio, luminosity) with parameters of coincident X- ray emitting SNRs, resulted from our previous studies in the same sample of galaxies, in order to understand their evolution and investigate possible selection effects. We do not find a correlation between their H and X-ray luminosities, which we attribute to the presence of material in a wide range of temperatures. We also find evidence for a linear relation between the number of luminous optical SNRs (10^ (37) erg sec^ (-1) ) and SFR in our sample of galaxies.