A New, Larger Sample of Supernova Remnants in NGC 6946
  The relatively nearby spiral galaxy NGC~6946 is one of the most actively star
forming galaxies in the local Universe. Ten supernovae (SNe) have been observed
since 1917, and hence NGC6946 surely contains a large number of supernova
remnants (SNRs). Here we report a new optical search for these SNRs using
narrow-band images obtained with the WIYN telescope. We identify 147 emission
nebulae as likely SNRs, based on elevated [SII]:Halpha ratios compared to HII
regions. We have obtained spectra of 102 of these nebulae with Gemini
North-GMOS; of these, 89 have [SII]:Halpha ratios greater than 0.4, the
canonical optical criterion for identifying SNRs. There is very little overlap
between our sample and the SNR candidates identified by Lacey et al. (2001)
from radio data. Also, very few of our SNR candidates are known X-ray sources,
unlike the situation in some other galaxies such as M33 and M83. The emission
line ratios, e.g., [NII]:Halpha, of the candidates in NGC6946 are typical of
those observed in SNR samples from other galaxies with comparable metallicity.
None of the candidates observed in our low-resolution spectra show evidence of
anomalous abundances or significant velocity broadening. A search for emission
at the sites of all the historical SNe in NGC6946 resulted in detections for
only two: SN1980K and SN2004et. Spectra of both show very broad, asymmetric
line profiles, consistent with the interaction between SN ejecta and the
progenitor star's circumstellar material, as seen in late spectra from other
core-collapse SNe of similar age.
