Evolution of magnetic fields in galaxies

Abstract

Faraday rotation of polarized radio sources is one of the most effective observational tools to study cosmic magnetic fields. A densely-sampled grid of Faraday rotation measures that covers the complete sky (the Rotation Measure Grid) will allow us to probe the magnetic field in the Milky Way Galaxy as well as in other nearby galaxies. First results from a new database of 37543 rotation measures that covers over 80% of the sky show the potential of an all-sky polarization survey with SKA path finders and the SKA. Structures in RM on angular scales of tens of degrees and systematic variation in the amplitude and slope of rotation measure structure functions across the sky are presented. A future all-sky RM grid of ∼ 5000 per square degree will allow detailed mapping of the magneto-ionic medium, and also probe ∼ 1000 nearby galaxies with at least 20 rotation measures per galaxy. Observations of magnetic field structure in more distant galaxies will rely on the detection of polarized synchrotron emission for which a deep field would have the necessary sensitivity. At high redshift, imaging the polarized structure of normal galaxies at a resolution of ∼0.1 ′′ or better requires sensitivity beyond the design specifications of the SKA. However, the polarization of unresolved disk galaxies as a function of redshift provides a way to probe the evolution of magnetic fields in disk galaxies over time.

Structure in the Rotation Measure Sky

“…Experience from the literature Haverkorn et al 2008;Roy et al 2008) and from the present data (Stil 2009) shows that the slope of the SF is not always constant, but sometimes changes slope at an angular scale δθ ≈ 1 • . The precise angular scale at which a break in the power law occurs is not necessarily the same everywhere, but visual inspection of SFs across the sky suggests that if a break occurs, the SF slope is considerably higher for δθ < 1 • than for δθ > 1 • .…”