The subparsec-scale radio properties of southern starburst galaxies. I. Supernova remnants, the supernova rate, and the ionized medium in the NGC 253 starburst

Wide-field, very long baseline interferometry observations of the nearby starburst galaxy NGC 253, obtained with the Australian Long Baseline Array, 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 modeled 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) similar to 1 to > 20), implying that the overall structure of the ionized medium is clumpy. Of the 20 sources, 9 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 yr 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 kilometers per second. A star formation rate of 3.4(v/10(4)) M-circle dot yr(-1) < SFR(M >= 5 M-circle dot) < 59 M-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 and radio luminosities.