Multi-Epoch Very Long Baseline Interferometric Observations of the
  Nuclear Starburst Region of NGC 253: Improved modelling of the supernova and
  star-formation rates
  The results of multi-epoch observations of the southern starburst galaxy, NGC
253, with the Australian Long Baseline Array (LBA) at 2.3 GHz are presented. As
with previous radio interferometric observations of this galaxy, no new sources
were discovered. By combining the results of this survey with Very Large Array
observations at higher frequencies from the literature, spectra were derived
and a free-free absorption model was fitted of 20 known sources in NGC 253. The
results were found to be consistent with previous studies. The supernova
remnant, 5.48-43.3, was imaged with the highest sensitivity and resolution to
date, revealing a two-lobed morphology. Comparisons with previous observations
of similar resolution give an upper limit of 10,000 km/s for the expansion
speed of this remnant. We derive a supernova rate of <0.2 per yr for the inner
300 pc using a model that improves on previous methods by incorporating an
improved radio supernova peak luminosity distribution and by making use of
multi-wavelength radio data spanning 21 years. A star formation rate of SFR(M>5
M_solar)<4.9 M_solar per yr was also estimated using the standard relation
between supernova and star-formation rates. Our improved estimates of supernova
and star-formation rates are consistent with studies at other wavelengths. The
results of our study point to the possible existence of a small population of
undetected supernova remnants, suggesting a low rate of radio supernovae
production in NGC 253.
