Subarcsecond?Resolution Radio Maps of Nearby Spiral Galaxies

Abstract

We report subarcsecond-resolution Very Large Array imaging of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, compact radio continuum sources are identified in the central ~15'' × 15'' region. These compact sources are responsible for 20%-30% of the total emission from the central kiloparsec of the host galaxies at 2 cm but only ~5%-10% at 6 cm. More than half of the compact sources appear to be H II regions. The H II regions with rising spectra must be fairly dense (ni ~ 104 cm-3) and are presumably very young. The largest of these H II regions require the excitation of 500-800 O stars within regions only a few parsecs in extent. These clusters approach the sizes expected for globular clusters. Thermal free-free emission from compact sources contributes more significantly at 2 cm, while diffuse synchrotron emission dominates at 6 cm. The radio H II regions are found near the centers of giant molecular clouds in projection and do not have obvious visual counterparts.