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The results of multi-epoch observations of the southern starburst galaxy, NGC253, with the Australian Long Baseline Array (LBA) at 2.3 GHz are presented. Aswith previous radio interferometric observations of this galaxy, no new sourceswere discovered. By combining the results of this survey with Very Large Arrayobservations at higher frequencies from the literature, spectra were derivedand a free-free absorption model was fitted of 20 known sources in NGC 253. Theresults were found to be consistent with previous studies. The supernovaremnant, 5.48-43.3, was imaged with the highest sensitivity and resolution todate, revealing a two-lobed morphology. Comparisons with previous observationsof similar resolution give an upper limit of 10,000 km/s for the expansionspeed of this remnant. We derive a supernova rate of <0.2 per yr for the inner300 pc using a model that improves on previous methods by incorporating animproved radio supernova peak luminosity distribution and by making use ofmulti-wavelength radio data spanning 21 years. A star formation rate of SFR(M>5M_solar)<4.9 M_solar per yr was also estimated using the standard relationbetween supernova and star-formation rates. Our improved estimates of supernovaand star-formation rates are consistent with studies at other wavelengths. Theresults of our study point to the possible existence of a small population ofundetected supernova remnants, suggesting a low rate of radio supernovaeproduction in NGC 253.

the astronomical journal

Multi-Epoch Very Long Baseline Interferometric Observations of the  Nuclear Starburst Region of NGC 253: Improved modelling of the supernova and  star-formation rates