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Wide-field VLBI observations of the nearby starburst galaxy NGC 4945,obtained with the Australian LBA, have produced 2.3 GHz images over two epochswith a maximum spatial resolution of 0.3 pc. 15 sources were detected, 13 ofwhich correspond to sources identified in higher frequency (3 cm and 12 mm)ATCA images. Four of the sources are resolved into shell-like structuresranging between 1.1 to 2.1 pc in diameter. From these data the spectra of 13compact radio sources in NGC 4945 were modelled; nine were found to beconsistent with free-free absorbed power laws and four with a simple power lawspectrum. The free-free opacity is highest toward the nucleus but variessignificantly throughout the nuclear region, implying that the overallstructure of the ionised medium is clumpy. Of the 13 sources, 10 have steepintrinsic spectra associated with synchrotron emission from supernova remnants,the remaining sources have flat intrinsic spectra which may be associated withthermal radio emission. A non-thermal source with a jet-like morphology isdetected ~1" from the assumed location of the AGN. A type II supernova (SN)rate upper limit of 15.3/yr is determined for the inner 250 pc region of thegalaxy at the 95% confidence level, based on the lack of detection of newsources in observations spanning 1.9 years and a simple model for the evolutionof supernova remnants. A type II SN rate of >0.1(v/1e4)/yr is implied fromestimates of supernova remnant source counts, sizes and expansion rates, wherev is the radial expansion velocity of the supernova remnant in km/s. A SFR of2.4(v/1e4)<SFR(M>=5Msun)<370 Msun/yr has been estimated directly from the SNrate limits and is of the same order of magnitude as rates determined fromintegrated FIR and radio luminosities.

the astronomical journal

The Sub-parsec Scale Radio Properties of Southern Starburst Galaxies.  II. Supernova Remnants, the Supernova Rate, and the Ionised Medium in the NGC  4945 Starburst