Using [FeII] to Search for Supernova Remnants in NGC 6946

Supernovae and supernova remnants (SNRs) play an important role in the evolution of the interstellar medium of their host galaxies. A crucial part in gaining a better understanding of this role is in obtaining a large sample of such objects and studying how their properties relate to their physical environment. Usually, SNRs are identified at optical wavelengths as emission nebulae with high [SII]:Hα flux ratios compared to HII regions. However, in the IR, shock models indicate and observations show that SNRs emit strongly in [Fe II] at 1.64 μm. Here we report on an attempt to use the [Fe II] 1.64 μm line to search for SNRs in the nearby starburst galaxy NGC 6946. For this study we have used the WIYN High Resolution Infrared Camera (WHIRC) on the WIYN 3.5m telescope to image NGC 6946 in broad bands J and H and narrow bands [FeII], [FeII]-4500, Paβ and Paβ-4500 (where -4500 indicates an offset ~4500 km s-1 from the named filter). We processed the images to create a mosaic image of NGC 6946 in each of the six filters and visually searched these mosaics to find potential supernova remnants using the criteria that candidates should show significant [FeII] emission, but little of no emission in the other filters. We have identified 47 supernova remnant candidates (SNRcs). Only one of these objects is coincident with an optically selected SNR. The [FeII] fluxes range from 1.5e-16 to 4.2e-15 erg s-1 cm-2. These fluxes are at the highest end of previously published extragalactic SNR [FeII] fluxes, which suggests there is a much larger population of SNRs in NGC 6946 below the detection threshold of our data. All of the candidates now need to be confirmed spectroscopically. However, the fact that we detect as many objects as we did, suggests that [FeII] can be used as an effective search tool to find extragalactic SNRs.