Radio Continuum Imaging of the Spiral Galaxy NGC 4258

We analyze 3.″5 resolution, high-sensitivity radio continuum images of the nearby spiral galaxy NGC 4258 at 6 and 20 cm derived from multiple observations used to monitor the radio supernova SN 1981K by Van Dyk et al. Seven bright H II regions and five supernova remnant candidates are identified. Extinctions to the H II regions are estimated for the first time from a comparison of radio flux densities to new optical fluxes derived from Hα observations by Dutil & Roy. The bright end of the H II region luminosity function is established at each wavelength. The luminosity functions are best fit by power laws consistent with the shape of previously published radio and optical luminosity functions for a number of galaxies. The supernova remnants are all about 2-3 times the radio luminosity of Cas A. In addition, the galaxy's nucleus is shown to have been variable over the SN 1981K monitoring period. The spectral index (α) distribution of the anomalous radio arms is investigated and found to be relatively uniform at α = -0.65 ± 0.10.