Properties of polarized synchrotron emission from fluctuation-dynamo action – I. application to galaxy clusters

Abstract

Using magnetohydrodynamic simulations of fluctuation dynamos, we perform broad-bandwidth synthetic observations to investigate the properties of polarized synchrotron emission and the role that Faraday rotation plays in inferring the polarized structures in the intracluster medium (ICM) of galaxy clusters. In the saturated state of the dynamo, we find a Faraday depth (FD) dispersion σ FD ≈ 100 rad m−2, in agreement with observed values in the ICM. Remarkably, the FD power spectrum is qualitatively similar to M(k)/k, where M(k) is the magnetic spectrum and k the wavenumber. However, this similarity is broken at high k when FD is obtained by applying rotation measure (RM) synthesis to polarized emission from the ICM due to poor resolution and complexities of spectrum in FD space. Unlike the Gaussian probability distribution function (PDF) obtained for FD, the PDF of the synchrotron intensity is lognormal. A relatively large σ FD in the ICM gives rise to strong frequency-dependent variations of the pixel-wise mean and peak polarized intensities at low frequencies ( 1.5 GHz). The mean fractional polarization p obtained at the resolution of the simulations increases from <0.1 at 0.5 GHz to its intrinsic value of ∼0.3 at 6 GHz. Beam smoothing significantly affects the polarization properties below 1.5 GHz, reducing p to 0.01 at 0.5 GHz. At frequencies 5 GHz, polarization remains largely unaffected, even when recovered using RM synthesis. Thus, our results underline the need for high-frequency ( 5 GHz) observations with future radio telescopes to effectively probe the properties of polarized emission in the ICM.