The Equipartition Magnetic Field Formula in Starburst Galaxies: Accounting for Pionic Secondaries and Strong Energy Losses

Understand

Equipartition arguments provide an easy way to find a characteristic scale for the magnetic field from radio emission, by assuming the energy densities in cosmic rays and magnetic fields are the same.

• Yet most of the cosmic ray content in star-forming galaxies is in protons, which are invisible in radio emission.
• Therefore, the argument needs assumptions about the proton spectrum, typically that of a constant proton/electron ratio.
• In some environments, particularly starburst galaxies, the reasoning behind these assumptions does not necessarily hold: secondary pionic positrons and electrons may be responsible for most of the radio emission, and strong energy losses can alter the proton/electron ratio.