Revisiting the Equipartition Assumption in Star-Forming Galaxies
School of Mathematics, Statistics and Physics, Newcastle University, Newcastle Upon Tyne NE1 7RU, UK
Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Author to whom correspondence should be addressed.
Received: 7 March 2019 / Revised: 27 March 2019 / Accepted: 28 March 2019 / Published: 8 April 2019
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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 ≈
, 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 (≈
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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MDPI and ACS Style
Seta, A.; Beck, R. Revisiting the Equipartition Assumption in Star-Forming Galaxies. Galaxies 2019, 7, 45.
Seta A, Beck R. Revisiting the Equipartition Assumption in Star-Forming Galaxies. Galaxies. 2019; 7(2):45.
Seta, Amit; Beck, Rainer. 2019. "Revisiting the Equipartition Assumption in Star-Forming Galaxies." Galaxies 7, no. 2: 45.
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