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.

Wide-field, very long baseline interferometry observations of the nearby starburst galaxy NGC 4945, obtained with the Australian Long Baseline Array, have produced 2.3 GHz images over two epochs with a maximum angular resolution of 15 mas (0.3 pc). Fifteen sources were detected, 13 of which correspond to sources identified in higher frequency (3 cm and 12 mm) Australian Telescope Compact Array images. Four of the sources are resolved into shell-like structures ranging between 60 and 110 mas (1.1 to 2.1 pc) in diameter. From these data the spectra of 13 compact radio sources in NGC 4945 were modelled; nine were found to be consistent with free-free absorbed power laws and four with a simple power law spectrum. The free-free opacity is highest toward the nucleus but significantly varies throughout the nuclear region (tau(0) similar to 6-23), implying that the overall structure of the ionized medium is clumpy. Of the 13 sources, 10 have steep intrinsic spectra associated with synchrotron emission from supernova remnants, the remaining sources have flat intrinsic spectra which may be associated with thermal radio emission. A non-thermal source with a jet-like morphology is detected similar to 1 '' from the assumed location of the active galactic nucleus. A Type II supernova rate upper limit of 15.3 yr(-1) is determined for the inner 250 pc region of the galaxy at the 95% confidence level, based on the lack of detection of new sources in observations spanning 1.9 years and a simple model for the evolution of supernova remnants. A Type II supernova rate of > 0.1(nu/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 (SFR) of 2.4 (nu/10(4)) < SFR(M >= 5M(circle dot)) < 370M(circle dot) yr(-1) has been estimated directly from the supernova rate limits and is of the same order of magnitude as rates determined from integrated far-IR (1.5 M-circle dot yr(-1)) and radio luminosities (14.4 +/- 1.4 (Q/8.8) M-circle dot yr(-1)). The supernova rates and star formation rates determined for NGC 4945 are comparable to those of NGC 253 and M82