A multi-frequency analysis of radio supernova remnants and their environments in Sculptor Group SD Galaxy NGC 300 and the Small Magellanic Cloud

While observations and analysis of supernova remnants (SNRs) located within the Milky Way Galaxy have been extensive, severe limitations have been imposed by massive photoelectric absorption along the Galactic plane and significant uncertainties in distance. Alternatively, the study of extragalactic SNRs offers a unique opportunity to examine them at a relatively known distance without Galactic plane interference. Here, I present multi-frequency identification and analysis of radio SNRs and their local environments in the direction of NGC 300 and the Small Magellanic Cloud (SMC). Using both Australia Telescope Compact Array (ATCA) and Very Long Array observations, 18 radio SNRs (with 3 radio candidate SNRs and one microquasar candidate) were identified in NGC 300 based on selection criteria including spectral index and cross identification with optical and X-ray sources. X-ray co-identifications include those found in newly reduced data from the XMM-Newton archive and use of X-ray spectral models and hardness ratios. Five of these 18 radio remnants are associated with reported optical remnants, 3 have X-ray counterparts and 12 are found in archived [S ii] images. The 1.374 GHz luminosity function suggests that only the brightest SNRs are seen and correlations with OB associations or H ii regions suggest they originate within star forming regions. In the SMC, multi-wavelength identification criteria identified a total of 21 radio SNRs (16 known and 5 new candidates). Out of a total of 717 SMC radio sources found using the ATCA, 71 are classified as H ii regions, 616 as background objects, 2 as microquasar candidates and 2 as planetary nebulae. Follow-up optical spectra also confirmed one of the radio candidates as an SNR. Statistics for SNRs, H ii regions and background objects are presented for both galaxies; extensively for the SMC. These statistics suggest that for this mature population of remnants, radio brightness and density/abundance-sensitive optical spectral line intensities are not directly related to diameter, but rather reflect the local interstellar environment. An average of their 'metal’abundances (10−4.1), based on these optical line intensities, agree with those previously reported for the SMC, obtained by other methods. Observations of extragalactic SNRs reflect both their nature (progenitor) and their nurture (environment) since in the evolution of their life cycle, the two cannot be separated