Revisiting the Equipartition Assumption in Star-forming Galaxies
  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.
