Properties of the Magneto-ionic Medium in the Halo of M51 revealed by Wide-band Polarimetry

Abstract

We present a study of the magneto-ionic medium in the Whirlpool galaxy (M51) using new wide-band multi-configuration polarization data at L band (1-2 GHz) obtained at the Karl G. Jansky Very Large Array. By fitting the observed diffuse complex polarization $Q$+$iU$ as a function of wavelength directly to various depolarization models, we find that polarized emission from M51 at 1-2 GHz originates from the top of the synchrotron disk and then experiences Faraday rotation in the near-side thermal halo of the galaxy. Thus, the scale height of the thermal gas must exceed that of the synchrotron emitting gas at L band. The observed Faraday depth distribution at L band is consistent with a halo field that comprises of a plane-parallel bisymmetric component and a vertical component which produces a Faraday rotation of $\sim$ $-$9 rad m${}^{-2}$. The derived rotation measure structure functions indicate a characteristic scale of rotation measure fluctuations of less than 560 pc in the disk and approximately 1 kpc in the halo. The outer scale of turbulence of 1 kpc found in the halo of M51 is consistent with superbubbles and the Parker instability being the main energy injection mechanisms in galactic halos.

arXiv comment36 pages, 19 figures, 3 tables. Accepted for publication in ApJ

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• Modelling the cosmic ray electron propagation in M 51
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• The magnetized disk-halo transition region of M51
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• The origin of the structure of large-scale magnetic fields in disc galaxies
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• A large-scale, regular intergalactic magnetic field associated with Stephan's Quintet?
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• Probing the Morphology of Polarized Emission Induced by Fluctuation Dynamo using Minkowski Functionals