Radio Continuum Emission as a Star Formation Tracer: Bridging the Spatial Scales
Radio continuum emission has been demonstrated to be an excellent tracer of recent star formation, yet current understanding remains empirical, and is primarily based upon integrated multi–frequency or resolved mono–chromatic radio continuum studies. In this thesis, we gain further insight into the relationship between the observed radio continuum emission and recent star formation through the use of spatially resolved, multi–frequency radio continuum observations, focusing our analysis on the relatively simple star–forming dwarf irregular galaxies, IC10 and NGC1569. High—resolution (∼1 pc) 1.5GHz and 5.0GHz e–MERLIN observations of IC10, combined with a multi–wavelength classification scheme, reveal that the majority of the compact star–formation products found within low mass, star forming galaxies are HII regions, with radio supernova and supernova remnants being observedmore rarely. By taking a census of these star–formation products, we derive useful lower limits on the instantaneous star–formation rate in this simple stellar system. We further demonstrate that high–resolution observations can be used to remove the contribution of contaminating background galaxies to integrated galaxy radio spectra, which can be a significant correction in very low mass systems. On larger spatial scales (∼100 pc), we developed a Bayesian Markov–Chain Monte– Carlo approach to separate the thermal and non–thermal radio continuum emission components on a resolved basis. We applied this procedure to VLA observations of NGC1569 that span a wide frequency range (1–34GHz). During our fitting procedure, we use Ha maps found in the literature to constrain the thermal radio continuumemission component. We find that NGC1569 exhibits a high average thermal fraction at 1GHz (∼25%) which is in line with recent integrated studies, and that the average non—thermal spectral index for the main disk is a = −0.53, which is consistent with the injection spectrum found in galactic supernova remnants. Taken together, these results indicate that NGC1569 has recently finished a star formation burst, which is in close agreement with the literature where this galaxy is classified as being in a post– starburst phase, and that supernova remnants are the predominant source of cosmic ray electrons in this system. We further demonstrate that the resulting maps from our separation procedure can be used to further constrain additional quantities, such as the line of sight reddening at arcsecond resolution, providing for example a measure of internal extinction by dust in NGC1569’s HII regions as well as spatially resolved equipartition magnetic field strengths. With our low–resolution VLA observations, we found that the supernova remnant, NGC1569–38, exhibits a break in its radio spectral energy distribution at ∼ 7GHz. We followed this observation up with e–MERLIN observations of NGC1569 and found that the surrounding InterstellarMediumsignificantly contributes to the observed spectral energy distribution in the lower resolution maps. Based on NGC1569–38’s observed surface brightness and minimum energy magnetic field strength compared to a sample of galactic and extra—galactic supernova remnants,we determine thatNGC1569– 38 is young and is just entering the Sedov–Taylor phase of expansion. We demonstrate that the observed break in a spatially resolved radio continuum spectral energy distribution of a young supernova Remnant, even at resolutions of about 100 pc, is due to contamination by emission from the surrounding Interstellar medium. To reveal effects such as cosmic ray electron ageing, higher resolution, matched to the size of the supernova remnant, multi–frequency radio spectra are required.
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