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The energy densities in magnetic fields and cosmic rays (CRs) in galaxies areoften assumed to be in equipartition, allowing for an indirect estimate of themagnetic field strength from the observed radio synchrotron spectrum. However,both primary and secondary CRs contribute to the synchrotron spectrum, and theCR electrons also loose energy via bremsstrahlung and inverse Compton. Whileclassical equipartition formulae avoid these intricacies, there have beenrecent revisions that account for the extreme conditions in starbursts. Yet,the application of the equipartition formula to starburst environments alsopresupposes that timescales are long enough to reach equilibrium. Here, we testequipartition in the central molecular zones (CMZs) of nearby starburstgalaxies by modeling the observed gamma-ray spectra, which provide a directmeasure of the CR energy density, and the radio spectra, which provide a probeof the magnetic field strength. We find that in starbursts, the magnetic fieldenergy density is significantly larger than the CR energy density,demonstrating that the equipartition argument is frequently invalid for CMZs.

Equipartition and Cosmic Ray Energy Densities in Central Molecular Zones of Starbursts