2ND-EPOCH VLA OBSERVATIONS OF COMPACT RADIO-SOURCES IN NGC-253

The nearby starburst galaxy NGC 253 has been reobserved with the highest resolution configuration of the VLA at 6 cm, and observed for the first time at 3.6 cm. No new compact radio sources stronger than 3 mJy at 6 cm appeared in the 18 months since the first-epoch observations at 6 cm. This flux-density limit corresponds to an intrinsic luminosity three times that of the galactic supernova remnant Cas A. The 95% confidence upper limit to the type II supernova rate is 3.0 yr-1, assuming that 2/3 of such supernovae reaching their 6 cm maxima between epochs would have been strong enough radio sources to be detected at the second epoch. None of the 6 cm sources stronger than 1 mJy was found to undergo significant flux decreases over 18 months, a result differing from that found for M82 by Kronberg & Sramek [Sci, 227, 88 (1985)]. Errors in the flux-density scales should not have caused this difference between NGC 253 and M82, implying that the sources in NGC 253 are intrinsically different from those in M82. In principle, the bulk of the sources in NGC 253 could be supernova remnants or H II regions, or they may be young supernovae of a different type from those in M82. The 3.6 cm data show a substantial reduction in confusion and support the estimate that there are approximately 100 compact radio sources in NGC 253; some of the sources seen at both 3.6 and 6 cm probably have flat spectra and may be H II regions. A shell supernova remnant whose intrinsic radio size and luminosity are similar to Cas A has been discovered about 330 pc from the galaxy nucleus.