The Integrated Polarization of Spiral Galaxy Disks

TL;DR

This paper studies the polarization properties of nearby spiral galaxies at 4.8 GHz, models their polarization as a function of inclination, and discusses implications for magnetic field studies and intergalactic medium detection.

Contribution

It provides new models for integrated polarization of spiral galaxy disks considering inclination and magnetic field components, and analyzes polarization dependence on galaxy luminosity and orientation.

Findings

Polarization ranges from <1% to 17.6% in observed galaxies.

Polarization depends on magnetic field ratio at low inclinations.

Higher luminosity galaxies show lower polarization (<4%).

Abstract

We present integrated polarization properties of nearby spiral galaxies at 4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a function of inclination. Spiral galaxies in our sample have observed integrated fractional polarization in the range < 1% to 17.6%. At inclinations less than 50 degrees, the fractional polarization depends mostly on the ratio of random to regular magnetic field strength. At higher inclinations, Faraday depolarization associated with the regular magnetic field becomes more important. The observed degree of polarization is lower (<4%) for more luminous galaxies, in particular those with L_{4.8} > 2 x 10^{21} W/Hz. The polarization angle of the integrated emission is aligned with the apparent minor axis of the disk for galaxies without a bar. In our axially symmetric models, the polarization angle of the integrated emission is independent of wavelength. Simulated distributions of fractional polarization for randomly oriented spiral galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization measurements, e.g. with the SKA, of unresolved spiral galaxies allow statistical studies of the magnetic field in disk galaxies using large samples in the local universe and at high redshift. As these galaxies behave as idealized background sources without internal Faraday rotation, they can be used to detect large-scale magnetic fields in the intergalactic medium.