Supernova Remnants in the Irregular Galaxy NGC4449

The nearby irregular galaxy NGC4449 has a star formation rate of about 0.4 solar masses/yr and should host of order 70 SNRs younger than 20,000 years, a typical age for SNRs expanding into to an ISM with unit density to reach the radiative phase. We have carried out an optical imaging and spectroscopic survey in an attempt to identify these SNRs. This task is challenging because diffuse gas with elevated ratios of [SII]:H-alpha is omnipresent in NGC4449, {causing confusion when using this common diagnostic for SNRs. Using narrow-band interference-filter images, we first identified 49 objects that have elevated [SII]:H-alpha ratios compared to nearby HII regions. Using Gemini-N and GMOS, we then obtained high-resolution spectra of 30 of these SNR candidates, 25 of which have [SII]:H-alpha ratios greater than 0.5. Of these, 15 nebulae are almost certainly SNRs, based on a combination of characteristics: higher [OI]:H-alpha ratios and broader line widths than observed from HII regions. The remainder are good candidates as well, but need additional confirmation. Surprisingly, despite having superior imaging and spectroscopic data sets to examine, we are unable to confirm most of the candidates suggested by Leonidaki (2013). While NGC4449 is likely an extreme case because of the high surface brightness and elevated [SII]:H-alpha ratio of diffuse gas, it highlights the need for sensitive high-resolution optical spectroscopy, or high spatial resolution radio or X-ray observations that can ensure accurate SNR identifications in external galaxies.