Revisiting the Equipartition Assumption in Star-Forming Galaxies
Date
2019
Authors
Seta, Amit
Beck, R
Journal Title
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Publisher
MDPI
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 that support it at large scales and how certain observations
show significant deviations from equipartition at scales smaller than ≈1 kpc, probably related to
the propagation length of the cosmic rays. Then, we test the energy equipartition assumption using
test-particle and magnetohydrodynamic (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 (≈100 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.
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Galaxies
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Journal article
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Open Access
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Creative Commons Attribution (CC BY) license
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