# Revisiting the Equipartition Assumption in Star-forming Galaxies

## TL;DR

This paper critically examines the common assumption of energy equipartition between cosmic rays and magnetic fields in star-forming galaxies, revealing it is invalid at scales below approximately 100 parsecs based on simulations and observations.

## Contribution

It provides a detailed analysis of the validity of the equipartition assumption, combining observational data and simulations to identify the relevant physical scales.

## Key findings

- Equipartition is invalid below ~100 pc in spiral galaxies.
- Observations support equipartition at large scales but show deviations at smaller scales.
- Simulations indicate the lower limit for valid equipartition is around 100 pc.

## Abstract

Energy equipartition between cosmic rays and magnetic fields is often assumed to infer magnetic field properties from the synchrotron observations of star-forming galaxies. However, there is no compelling physical reason to expect the same. We aim to explore the validity of the energy equipartition assumption. After describing popular arguments in favour of the assumption, we first discuss observational results which support it at large scales and how certain observations show significant deviations from equipartition at scales smaller than $\approx 1 \, {\rm kpc}$, probably related to the propagation length of the cosmic rays. Then we test the energy equipartition assumption using test-particle and MHD simulations. From the results of the simulations, we find that the energy equipartition assumption is not valid at scales smaller than the driving scale of the ISM turbulence ($\approx 100 \, {\rm pc}$ in spiral galaxies), which can be regarded as the lower limit for the scale beyond which equipartition is valid. We suggest that one must be aware of the dynamical scales in the system before assuming energy equipartition to extract magnetic field information from synchrotron observations. Finally, we present ideas for future observations and simulations to investigate in more detail under which conditions the equipartition assumption is valid or not.