Kavli Affiliate: Susan E. Clark
| First 5 Authors: Tara Dacunha, Sergio Martin-Alvarez, Susan E. Clark, Enrique Lopez-Rodriguez,
| Summary:
Understanding the role that magnetic fields play on the stage of galaxy
formation requires accurate methods for inferring the properties of
extragalactic magnetic fields. Radio synchrotron emission has been the most
promising avenue to infer magnetic field strengths across galaxies, with the
application of a central assumption: that galactic cosmic rays are in energy
equipartition with the magnetic field. In this work, we leverage flexible
synthetic observations of a high-resolution magnetohydrodynamic simulation of a
Milky Way-like galaxy to review whether true equipartition is capable of
reproducing radio observations of galaxies, and investigate its impact on the
inference of magnetic field strengths when varying the properties and density
distribution of the cosmic rays. We find that imposing equipartition
(regardless of scale length) results in cosmic ray electron densities that are
unable to generate either the amplitude or the shape of the radio intensity
profiles typically observed in spiral galaxies. Instead, observationally
motivated smooth distributions of cosmic ray electrons across the galaxy
provide a remarkable match to observations. We further demonstrate that
assuming equipartition with those mock observations can lead to significant
overestimation of the magnetic field strength ($sim10-50times$). This
overestimation varies with cosmic ray electron densities, cosmic ray spectrum
power-law index, and galactic environment, aggravated in inter-arm regions and
attenuated in star-forming regions. Our results promote caution when assuming
equipartition in observations, and suggest that additional theoretical and
numerical work is required to leverage the upcoming generation of radio
observations poised to revolutionize our understanding of astrophysical
magnetic fields.
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