  Wide-field, very long baseline interferometry (VLBI) observations of the
nearby starburst galaxy NGC 253, obtained with the Australian Long Baseline
Array (LBA), have produced a 2.3 GHz image with a maximum angular resolution of
15 mas (0.3 pc). Six sources were detected, all corresponding to sources
identified in higher frequency (>5 GHz) VLA images. One of the sources,
supernova remnant 5.48-43.3, is resolved into a shell-like structure
approximately 90 mas (1.7 pc) in diameter. From these data and data from the
literature, the spectra of 20 compact radio sources in NGC 253 were modelled
and found to be consistent with free-free absorbed power laws. Broadly, the
free-free opacity is highest toward the nucleus but varies significantly
throughout the nuclear region (tau_0 ~ 1->20), implying that the overall
structure of the ionised medium is clumpy. Of the 20 sources, nine have flat
intrinsic spectra associated with thermal radio emission and the remaining 11
have steep intrinsic spectra, associated with synchrotron emission from
supernova remnants. A supernova rate upper limit of 2.4 yr^-1 is determined for
the inner 320 pc region of the galaxy at the 95% confidence level, based on the
lack of detection of new sources in observations spanning almost 17 years and a
simple model for the evolution of supernova remnants. A supernova rate of >0.14
(v/10^4) yr^-1 is implied from estimates of supernova remnant source counts,
sizes and expansion rates, where v is the radial expansion velocity of the
supernova remnant in km s^-1. A star formation rate of 3.4 (v/10^4) <
SFR(M<=5Msun) < 59 Msun yr^-1 has been estimated directly from the supernova
rate limits and is of the same order of magnitude as rates determined from
integrated FIR and radio luminosities.
