A multi-wavelength 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{\alpha} 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{\alpha} > 0.4. This study revealed ~400 photometric SNRs down to a
limiting H{\alpha} 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{\alpha})$_{phot}$> 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{\alpha} 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{\alpha} 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{\alpha} 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{\alpha} 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.
