Discovery of a population of compact radio sources in the spiral galaxy NGC 4736

The discovery of 19 compact radio sources in the near-nuclear region of the spiral galaxy NGC 4736 is reported. Nine are found to have thermal bremsstrahlung spectra. Their small sizes of 1-2 arcsec (20-40 pc at a distance of 4 Mpc), combined with typical flux densities of 0.3-1.0 mJy at 20 cm, suggest high gas densities of 60-270/cu cm. It is proposed that these are H II regions associated with recently formed stars. The remaining 10 sources are all unresolved to 1 arcsec (20 pc) and have synchrotron spectra. Equipartition calculations show that the limits on the relativistic particle energies and magnetic field strengths are consistent with values found for Galactic SNRs. Furthermore, the radio luminosities and small sizes of these sources suggest that these are young SNRs. 20 references