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The energy density of energetic protons, U_p, in several nearby starburstnuclei (SBNs) has been directly deduced from gamma-ray measurements of theradiative decay of neutral pions produced in interactions with ambient protons.Lack of sufficient sensitivity and spatial resolution makes this directdeduction unrealistic in the foreseeable future for even moderately distantSBNs. A more viable indirect method for determining U_p in star-forminggalaxies is to use its theoretically based scaling to the energy density ofenergetic electrons, U_e, which can be directly deduced from radio synchrotronand possibly also nonthermal hard X-ray emission. In order to improve thequantitative basis and diagnostic power of this leptonic method we reformulateand clarify its main aspects. Doing so we obtain a basic expression for theratio U_p/U_e in terms of the proton and electron masses and the power-lawindices that characterize the particle spectral distributions in regions wherethe total particle energy density is at equipartition with that of the meanmagnetic field. We also express the field strength and the particle energydensity in the equipartition region in terms of the region's size, mean gasdensity, IR and radio fluxes, and distance from the observer, and determinevalues of U_p in a sample of nine nearby and local SBNs.

Estimates of relativistic electron and proton energy densities in starburst galactic nuclei from radio measurements