Constraining regular and turbulent magnetic field strengths in M51 via Faraday depolarization

Abstract

We employ an analytical model that incorporates both wavelength-dependent and wavelength-independent depolarization to describe radio polarimetric observations of polarization at $\lambda \lambda \lambda 3.5,6.2,20.5$ cm in M51 (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 $18\mu$G and a total magnetic field strength in the halo of $\sim 4-6\mu$G. 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.

Accepted for publication in Astronomy & Astrophysics, 10 pages, 5 figures, 5 tables

References in corpus (4)

• Magnetism in the spiral galaxy NGC 6946: magnetic arms, depolarization rings, dynamo modes and helical fields
• The Radio Continuum-Star Formation Rate Relation in WSRT SINGS Galaxies
• High-resolution radio continuum survey of M33: III. Magnetic fields
• Depolarization of synchrotron radiation in a multilayer magneto-ionic medium

Cited by in corpus (6)

• The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR
• CHANG-ES XV: Large-scale magnetic field reversals in the radio halo of NGC 4631
• Properties of the Magneto-ionic Medium in the Halo of M51 revealed by Wide-band Polarimetry
• CMB foreground measurements through broad-band radio spectro-polarimetry: prospects of the SKA-MPG telescope
• Workshop summary 'The Power of Faraday Tomography'
• Pseudo-observation of spiral galaxies in the radio band to verify depolarization models