Radio Continuum Emission as a Star Formation Tracer: Bridging the Spatial Scales
Abstract
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.
Publication date
2019-01-07Published version
https://doi.org/10.18745/th.22123https://doi.org/10.18745/th.22123
Funding
Default funderDefault project
Other links
http://hdl.handle.net/2299/22123Metadata
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