Both the spectral and spatial information in high-resolution radio continuum maps is used to decouple thermal emission due to H II regions from the extended, strong synchrotron emission found in galactic cores. A first-order analysis, assuming constant spectral indices across the maps, yields thermal fluxes to within a factor of 2. The presence of large numbers of H II regions and young stars in the nuclear regions of three nearby spiral galaxies is thus confirmed.
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Turner et al. (1983) studied this question.