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

TL;DR

This study uses wide-band polarimetry data to analyze the magneto-ionic medium in M51, revealing the structure and scale of magnetic fields and turbulence in the galaxy's halo and disk.

Contribution

It provides new insights into the magnetic field configuration and turbulence scales in M51's halo using advanced polarization modeling techniques.

Findings

Polarized emission originates from the top of the synchrotron disk.

Halo magnetic field includes a bisymmetric and vertical component.

Turbulence scale in the halo is approximately 1 kpc.

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.