Testing the magnetic field models of galaxies with the SKA

TL;DR

This paper evaluates how the upcoming SKA radio telescope can map and reconstruct magnetic field structures in nearby galaxies using Faraday rotation measures from polarized background sources, considering different galaxy models and turbulence effects.

Contribution

It provides estimates of the polarized source density needed for reliable magnetic field recognition and reconstruction in galaxies with SKA, including the effects of turbulence and different magnetic field configurations.

Findings

A few dozen polarized sources suffice for recognition under ideal conditions.

At least 1200 sources are needed for detailed reconstruction of magnetic fields.

Low-frequency telescopes like LOFAR and ASKAP can assist in field recognition if sources remain polarized.

Abstract

The future new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Hundred and thousands polarized background sources can be measured towards nearby galaxies thus allowing their detailed magnetic field mapping by means of Faraday rotation measures (RM). We aim to estimate the required density of the background polarized sources detected with the SKA for reliable recognition and reconstruction of the magnetic field structure in nearby spiral galaxies. We construct a galaxy model which includes the ionized gas and magnetic field patterns of different azimuthal symmetry (axisymmetric (ASS), bisymmetric (BSS) and quadrisymmetric spiral (QSS), and superpositions) plus a halo magnetic field. RM fluctuations with a Kolmogorov spectrum due to turbulent fields and/or fluctuations in ionized gas density are superimposed. Recognition of magnetic structures is possible from RM towards background sources behind galaxies or a continuous RM map obtained from the diffuse polarized emission from the galaxy itself. Under favourite conditions, about a few dozens of polarized sources are sufficient for a reliable recognition. Reconstruction of the field structure without precognition becomes possible for a large number of background sources. A reliable reconstruction of the field structure needs at least 20 RM values on a cut along the projected minor axis which translates to approximately 1200 sources towards the galaxy. Radio telescopes operating at low frequencies (LOFAR, ASKAP and the low-frequency SKA array) may also be useful instruments for field recognition or reconstruction with the help of RM, if background sources are still significantly polarized at low frequencies (abriged).