A VLBI Study of Free-Free Absorbed Compact Radio Sources in the NGC 253 Starburst

The first 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 1.4 GHz image with angular resolution matched to the Very Large Array (VLA) at 22 GHz. Two compact radio sources have been detected with these LBA observations, neither of which corresponds to the assumed nucleus of the galaxy, which is prominent in VLA 22 GHz images. A multifrequency comparison of the high-resolution radio structure in NGC 253 has been made. In particular, the radio spectra of the two detected sources and the assumed nucleus are discussed. A case is made that the radio emission from the compact sources in NGC 253 is absorbed by partially ionized gas with a free-free optical depth ranging at least between 1 and 4 at 1.4 GHz, in broad agreement with previous low-resolution optical and radio studies of NGC 253 and high-resolution studies of a similar starburst galaxy, M82.