Equipartition and Cosmic Ray Energy Densities in Central Molecular Zones
  of Starbursts
  The energy densities in magnetic fields and cosmic rays (CRs) in galaxies are
often assumed to be in equipartition, allowing for an indirect estimate of the
magnetic field strength from the observed radio synchrotron spectrum. However,
both primary and secondary CRs contribute to the synchrotron spectrum, and the
CR electrons also loose energy via bremsstrahlung and inverse Compton. While
classical equipartition formulae avoid these intricacies, there have been
recent revisions that account for the extreme conditions in starbursts. Yet,
the application of the equipartition formula to starburst environments also
presupposes that timescales are long enough to reach equilibrium. Here, we test
equipartition in the central molecular zones (CMZs) of nearby starburst
galaxies by modeling the observed gamma-ray spectra, which provide a direct
measure of the CR energy density, and the radio spectra, which provide a probe
of the magnetic field strength. We find that in starbursts, the magnetic field
energy density is significantly larger than the CR energy density,
demonstrating that the equipartition argument is frequently invalid for CMZs.
