Radio polarization and magnetic field structure in M 101

TL;DR

This study uses radio observations to analyze the magnetic field structure and emission components of galaxy M 101, revealing a major structural change near 16 kpc and the dominance of random magnetic fields influenced by star formation.

Contribution

It provides detailed measurements of magnetic field strength, structure, and emission distribution in M 101, highlighting a significant transition at 16 kpc and the role of energetic events in magnetic field properties.

Findings

Radial distribution of emissions shows a break at 16 kpc.

Magnetic field strength decreases with radius, reaching 0.3 μG at 70 kpc.

Random magnetic field correlates with star formation rate.

Abstract

We observed total and polarized radio continuum emission from the spiral galaxy M 101 at 6.2 cm and 11.1 cm wavelengths with the Effelsberg telescope. We use these data to study various emission components in M 101 and properties of the magnetic field. Separation of thermal and non-thermal emission shows that the thermal emission is closely correlated with the spiral arms, while the non-thermal emission is more smoothly distributed indicating diffusion of cosmic ray electrons away from their places of origin. The radial distribution of both emissions has a break near R=16 kpc, where it steepens to an exponential scale length of about 5 kpc, which is about 2.5 times smaller than at R<16 kpc. The distribution of the polarized emission has a broad maximum near R=12 kpc and beyond R=16 kpc also decreases with about 5 kpc scalelength. It seems that near R=16 kpc a major change in the structure of M 101 takes place, which also affects the distributions of the strength of the random and ordered magnetic field. Beyond R=16 kpc the radial scale length of both fields is about 20 kpc, which implies that they decrease to about 0.3 \mu G at R=70 kpc, which is the largest optical extent. The equipartition strength of the total field ranges from nearly 10 \mu G at R<2 kpc to 4 \mu G at R=22-24 kpc. As the random field dominates in M 101, wavelength-independent polarization is the main polarization mechanism. We show that energetic events causing HI shells of mean diameter <625 pc could partly be responsible for this. At radii <24 kpc, the random magnetic field depends on the star formation rate per area with a power-law exponent of 0.28+-0.02. The ordered magnetic field is generally aligned with the spiral arms with pitch angles that are about 8{\deg} larger than those of HI filaments.