Constraining regular and turbulent magnetic field strengths in M 51 via Faraday depolarization

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden The Netherlands
e-mail: shneider@strw.leidenuniv.nl
² Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ School of Mathematics and Statistics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK

Received: 12 May 2014
Accepted: 18 June 2014

Abstract

We employ an analytical model that incorporates both wavelength-dependent and wavelength-independent depolarization to describe radio polarimetric observations of polarization at λλλ 3.5,6.2,20.5 cm in M 51 (NGC 5194). The aim is to constrain both the regular and turbulent magnetic field strengths in the disk and halo, modeled as a two- or three-layer magneto-ionic medium, via differential Faraday rotation and internal Faraday dispersion, along with wavelength-independent depolarization arising from turbulent magnetic fields. A reduced chi-squared analysis is used for the statistical comparison of predicted to observed polarization maps to determine the best-fit magnetic field configuration at each of four radial rings spanning 2.4 − 7.2 kpc in 1.2 kpc increments. We find that a two-layer modeling approach provides a better fit to the observations than a three-layer model, where the near and far sides of the halo are taken to be identical, although the resulting best-fit magnetic field strengths are comparable. This implies that all of the signal from the far halo is depolarized at these wavelengths. We find a total magnetic field in the disk of approximately 18 μG and a total magnetic field strength in the halo of ~4–6 μG. Both turbulent and regular magnetic field strengths in the disk exceed those in the halo by a factor of a few. About half of the turbulent magnetic field in the disk is anisotropic, but in the halo all turbulence is only isotropic.