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Revisiting the Equipartition Assumption in Star-Forming Galaxies

1,2,* and 3
1
School of Mathematics, Statistics and Physics, Newcastle University, Newcastle Upon Tyne NE1 7RU, UK
2
Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
3
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
*
Author to whom correspondence should be addressed.
Galaxies 2019, 7(2), 45; https://doi.org/10.3390/galaxies7020045
Received: 7 March 2019 / Revised: 27 March 2019 / Accepted: 28 March 2019 / Published: 8 April 2019
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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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. View Full-Text
Keywords: galaxies; spirals; ISM; magnetic fields; cosmic rays; radio continuum galaxies; spirals; ISM; magnetic fields; cosmic rays; radio continuum
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Seta, A.; Beck, R. Revisiting the Equipartition Assumption in Star-Forming Galaxies. Galaxies 2019, 7, 45.

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