The origin and evolution of cosmic magnetism are at present poorly understood. Answering the many open questions surrounding the physics of astrophysical magnetic fields is a difficult task since magnetic fields can be significantly affected by structure and galaxy formation and evolutionary processes. Their strength can be amplified, for example, in galaxy clusters through mergers and in galaxies through large-scale dynamos, invoking differential rotation and turbulence. Insights into the origin and properties of magnetic fields in the Universe could be provided by probing them on even larger scales. Along filaments and voids of the cosmic web, turbulent intracluster gas motions have not yet enhanced the magnetic field; its strength thus still depends on the seed field intensity, in contrast to galaxy clusters, where it probably mostly reflects the present level of turbulence (see, e.g., Donnert et al. 2009; Xu et al. 2010, 2011). Intervening magnetoionic media cause a difference in the phase velocity between the left-handed and right-handed circular polarization components of the linearly polarized synchrotron radiation emitted by a background radio source (e.g., Carilli & Taylor 2002; Govoni & Feretti 2004). This effect translates into a rotation of the intrinsic polarization angle, ψ0, (1)\begin{equation} \psi(\lambda^2)=\psi_0+\phi\lambda^2. \end{equation}ψ(λ2)=ψ0+φλ2.Following Burn (1966), the observed polarization angle, ψ, depends on the observation wavelength, λ, through the Faraday depth, φ, (2)\begin{equation} \phi=a_0\int_0^{z_{\rm s}}B_l(z)~n_{\rm e}(z)~\frac{\mathrm{d}l}{\mathrm{d}z}~\mathrm{d}z, \label{definition} \end{equation}φ=a0∫0zsBl(z)ne(z)dldzdz,where a0 depends only on fundamental constants, ne is the electron density, Bl is the component of the magnetic field along the line of sight, and zs is the redshift of the source. When the rotation is completely due to a foreground screen, the Faraday depth has the same value as the rotation measure (RM), defined by (3)\begin{equation} {\rm RM}\equiv\frac{\partial \psi}{\partial \lambda^2}\cdot \end{equation}RM≡∂ψ∂λ2·The Faraday depth is assumed to be positive when the line-of-sight average component of the magnetic field points toward the observer, otherwise it is negative for a field with an average component pointing away from the observer. The amount of Faraday depth measured by radio observations along a given line of sight is the sum of the contributions from the Milky Way, the emitting radio source, and any other source and large-scale structure in between hosting a magnetized plasma. The investigation of these contributions and of their possible dependence on redshift is essential to discriminate among the different scenarios of magnetic field formation and evolution and, therefore, crucial for the understanding of cosmic magnetism. Sensitive observations, a good knowledge of the Galactic Faraday foreground screen, and a statistical approach that is able to properly combine all of the observational information are necessary. An all-sky map of the Galactic Faraday rotation foreground and an estimate of the overall extragalactic contribution has been derived by Oppermann et al. (2012, 2015) in the framework of “Information Field Theory” (Enßlin et al. 2009) by assuming a correlated Galactic foreground and a completely uncorrelated extragalactic term. In this paper, we propose a new, fully Bayesian approach aiming at further disentangling the contribution intrinsic to emitting sources from the contribution due to the intergalactic environment between the source and the observer and at investigating the dependence of these contributions on redshift.
Citation: Govoni & Feretti 2004
Citation function: Motivation