BRIGHT RADIO-CONTINUUM EMISSION FROM STAR-FORMATION IN THE CORES OF NEARBY SPIRAL GALAXIES

We have mapped 6 and 2 cm continuum emission from the nuclear regions of ten normal spiral galaxies (Maffei 2, NGC 2403, M81, NGC 4236, NGC 4258, NGC 4736, NGC 4826, M51, M83, and M101) using the VLA. The galaxies were chosen because they are nearby(less than or similar to 5 Mpc) and likely to have bright continuum emission. Matching beamsizes of similar to 1''.5 allow the construction of spectral index maps from which the relative amounts of thermal bremsstrahlung and nonthermal synchrotron emission are estimated. Bright radio continuum is detected within the inner arcminute of eight of the 10 galaxies. Even at arcsecond resolution, corresponding to sizescales of 10-30 pc, nonthermal synchrotron emission from supernova remnants (SNRs) dominates the 6 cm continuum fluxes. High intensities suggest that this nonthermal component originates in a large population of young SNRs associated with recent star formation. Candidates for radio supernovae appear in several galaxies, including a candidate for SN 1986L in M83. Weak nonthermal core sources, similar to the Galactic center source Sgr A(*), may be present in four of the galaxies. The estimated thermal fluxes indicate that many of the galaxies have significant Lyman continuum fluxes in their cores, N(Lyc)similar to 10(51)-10(53) S-1, corresponding to OB luminosities of 2 x 10(8) L(.) to 5 x 10(9) L(.), generally consistent with observed far-infrared luminosities. Recent star formation appears to dominate both infrared and radio continuum fluxes in these galaxies. We present maps of the separated thermal and nonthermal emission for the starburst sources in Maffei 2 and M83.