RADIO OBSERVATIONS OF M83 AND ITS SUPERNOVA-REMNANTS

We report the rapid decline in radio emission from SN 1957D in M83, related to the recently reported drop in optical emission. Comparison between observations during 1983-1984 and 1990-1992 shows a drop relative to the mean flux density of 38% +/- 4% and 19% +/- 3% at 20 and 6 cm, respectively. The radio spectral index (S-upsilon proportional to upsilon(alpha)) of the source at the position of the supernova is now alpha = 0.11 +/- 0.06, suggesting that the radio emission from the supernova has faded below the level of radio emission coming from a nearby (and possibly associated) H II region. The change in the observed value of the flux density at 20 cm at the site of SN 1957D is consistent with a power-law index of -2.9 +/- 0.3. While this may be only a lower limit on the decay, it is already steeper than the measured decline in the intermediate-age supernova SN 1970G in M101 and much steeper than in the early evolution of other Type II radio supernovae. The rapid decline in emission from SN 1957D suggests that the supernova shock has overrun the circumstellar material lost by the progenitor star. The new observations strengthen our previous tentative detection of SN 1950B in M83, which has changed little with time and has a spectral index of alpha = -0.57 +/- 0.08. The new observations further suggest that several additional point sources in M83 that were not optically identified as supernovae may be SNRs. Radio emission was not detected from SN 1923A, SN 1945B or SN 1968L, while the Type Ib supernova SN 1983N, which had been briefly observed in 1983-1984, continues to be below detectable limits. We also report on a group of three sources near the nucleus of M83, which may form a single structure. Possible explanations for the origin of this structure include a background radio galaxy, a nuclear radio jet, or a bow shock. The observations also show diffuse emission along the spiral arms, with the emission along the western arm bounded sharply along a straight line, and most likely resulting from a shock associated with the bar of M83.