Supernova Remnants in the Irregular Galaxy NGC 4449

The nearby irregular galaxy NGC 4449 has a star formation rate of ∼0.4 M _☉ yr ^−1 and should host of order 70 supernova remnants (SNRs) younger than 20,000 yr, a typical age for SNRs expanding into an interstellar medium (ISM) with a density of 1 cm ^−3 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 [S ii ]:H α is omnipresent in NGC 4449, causing confusion when using this common diagnostic for SNRs. Using narrowband interference-filter images, we first identified 49 objects that have elevated [S ii ]:H α ratios compared to nearby H ii regions. Using Gemini-N and GMOS, we then obtained high-resolution spectra of 30 of these SNR candidates, 25 of which have [S ii ]:H α ratios greater than 0.5. Of these, 15 nebulae are almost certainly SNRs, based on a combination of characteristics: higher [O i ]:H α ratios and broader line widths than observed from H ii 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 et al. While NGC 4449 is likely an extreme case because of the high surface brightness and elevated [S ii ]:H α 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.