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

TL;DR

This study tests the validity of equipartition between magnetic fields and cosmic rays in starburst galaxy centers by comparing gamma-ray and radio observations, finding that magnetic energy dominates over cosmic rays, challenging the equipartition assumption.

Contribution

The paper provides a direct test of equipartition in starburst galaxy centers using gamma-ray and radio data, revealing that magnetic energy density exceeds cosmic ray energy density.

Findings

Magnetic field energy density is significantly larger than cosmic ray energy density in starbursts.

Equipartition assumption often does not hold in the central regions of starburst galaxies.

Gamma-ray spectra effectively measure cosmic ray energy densities in these environments.

Abstract

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.