Magnetic field structures of galaxies derived from analysis of Faraday rotation measures, and perspectives for the SKA

Abstract

Context. The forthcoming new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Aims. Our analysis looks at what can be learned from these sources concerning the structure and evolution of magnetic fields of external galaxies. Methods. Recognition of magnetic structures is possible from Faraday rotation measures (RM) towards background sources behind galaxies or a continuous RM map obtained from the diffuse polarized emission from the galaxy itself. We constructed models for the ionized gas and magnetic field patterns of different azimuthal symmetries (axisymmetric, bisymmetric and quadrisymmetric spirals, 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. Assuming extrapolated number density counts of polarized sources, we generated a sample of RM values within the solid angle of the galaxy. Applying various templates, we derived the minimum number of background sources and the minimum quality of the observations. For a large number of sources, reconstruction of the field structure without precognition becomes possible. Results. Any large-scale regular component of the magnetic field can be clearly recognized from RM data with the help of the χ 2 criterium. Under favorite conditions, a few dozen polarized sources are enough for a reliable result. A halo field with a vertical component does not affect the results of recognition. The required source number increases for small inclinations of the galaxy's disk and for larger RM turbulence. A flat number density distribution of the sources can be overcome by more sensitive observations. Application of the recognition method to the available RM data in the region around M 31 indicates that there are significant RM contributions intrinsic to the background sources or due to the foreground of the Milky Way. A reliable reconstruction of the field structure needs at least 20 RM values on a cut along the projected minor axis. Conclusions. Recognition or reconstruction of regular field structures from the RM data of polarized background sources is a powerful tool for future radio telescopes. Measuring RM at frequencies around 1 GHz with the SKA, simple field structures can be recognized in galaxies up to about 100 Mpc distance and will allow to test dynamo against primordial or other models of field origin. The low-frequency SKA array and low-frequency precursor telescopes like LOFAR may also have good RM sensitivity if background sources are still significantly polarized at low frequencies.

The large-scale magnetic field in the fourth Galactic quadrant

“…Our best-fit model of the large-scale field seems to resemble most closely the quadrisymmetric (QSS) spiral model illustrated, e.g., in Fig. 2 of Stepanov et al (2008). How a field with such a configuration, i.e., with many reversals, is formed is a question that still needs to be answered, in spite of the general opinion that a dynamo is involved.…”

“…In addition, the reversals that are observed seem to have a different character from those in the Galaxy (see, e.g., Beck 2009). However, only future studies of the distribution of rotation measures of extragalactic background sources observed through the disk of inclined nearby, well-resolved spiral galaxies, as discussed e.g., by Stepanov et al (2008), can tell whether the Galaxy is indeed exceptional as far as the large-scale magnetic field is concerned.…”

Using rotation measure grids to detect cosmological magnetic fields: A Bayesian approach

“…3.2.2, but with an uncertainty drawn from a Gaussian with σ noise = 6.0 rad m −2 . This should be a reliable approximation of the maximum uncertainty in Faraday depth for a polarized signal with a S /N > 5 (Stepanov et al 2008). The results are shown in Fig.…”

“…We select the HBA region of the spectrum with less contamination from radio frequency interference (120-160 MHz; Offringa et al 2013). In this frequency range, the uncertainty on Faraday depth values is expected to be ≤0.05 rad m −2 for a signal-to-noise ratio larger than 5 (Stepanov et al 2008). Ionospheric Faraday rotation can degrade the results at LOFAR frequencies; the same holds for SKA1 -band 1 observations; see the next section.…”

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

“…Weak regular magnetic fields can also be measured by their Faraday rotation of polarized emission from background sources, but also requires deep integrations to obtain a sufficiently large number of sources (Stepanov et al 2008). The observations presented in this paper detected only one source behind the inner disk of IC 342 and another three behind the outer disk (see Sect.…”