VLA observations of NGC 253: Supernova remnants and H II regions at 1 parsec resolution

The edge-on starburst galaxy NGC 253 has been imaged with the VLA at wavelengths ranging from 1.3 to 20 cm, with resolutions between 1 and 15 pc. These images reveal a large number of compact radio sources embedded within the diffuse radio structure in the inner 200 pc of the galaxy. We have identified approximately 64 individual compact radio sources in the galaxy. Of the strongest 17 sources, for which the flux densities are high enough to measure spectral indices with moderately low errors, about half have flat spectra and half have steep spectra; this indicates that perhaps half of the individual radio sources are dominated by thermal radio emission from H II regions. Over an 8 yr timescale, few, if any, of the strongest sources have varied in flux density, with limits of 1%-2% yr(-1) on the rate of decrease. At the highest resolution of 1 pc, a number of radio sources are beginning to be resolved, and at least 15 different sources can be identified within the innermost 20 pc of the galaxy. Individual radio sources have been explored in more detail. The strongest source, 5.79-39.0 (TH2), assumed to be at the nucleus of the galaxy, has a brightness temperature greater than 20,000 K at 22 GHz and greater than 40,000 K at 15 GHz. It is unresolved at the VLA and may be either an active galactic nucleus or a very compact (nonvariable) supernova remnant. A resolved flat-spectrum source, 5.72-40.1 (TH6), is located similar to 20 pc to the southwest. It has an apparent size of 2.4 x 1.2 pc and appears to be an H II region similar to the inner part of 30 Doradus, containing approximately 10(5) M. in stars, as well as similar to 600 M. of ionized gas. Source counts, the lack of variability, and the lack of new sources imply that the radio supernova rate is no more than 0.3 yr(-1), consistent with estimates made in other wave bands.