Properties of Polarized Synchrotron Emission from Fluctuation Dynamo Action -- II. Effects of Turbulence Driving in the ICM and Beam Smoothing

TL;DR

This study uses high-resolution simulations to explore how turbulence driving scales and observational effects influence polarized synchrotron emission in galaxy cluster environments, providing insights for future radio observations.

Contribution

It demonstrates the relationship between turbulence driving scale and polarization, accounting for observational smoothing and depolarization effects in the ICM.

Findings

Polarization fraction scales as the square root of the turbulence driving scale for scales > 60 kpc.

Beam smoothing reduces polarization by up to a factor of 2 at high frequencies.

Depolarization effects significantly diminish polarization signals at frequencies below 3 GHz.

Abstract

Polarized synchrotron emission from the radio halos of diffuse intracluster medium (ICM) in galaxy clusters are yet to be observed. To investigate the expected polarization in the ICM, we use high resolution ($1$\,kpc) magnetohydrodynamic simulations of fluctuation dynamos, which produces intermittent magnetic field structures, for varying scales of turbulent driving ($l_{\rm f}$) to generate synthetic observations of the polarized emission. We focus on how the inferred diffuse polarized emission for different $l_{\rm f}$ is affected due to smoothing by a finite telescope resolution. The mean fractional polarization $\langle p\rangle$ vary as $\langle p \rangle \propto l_{\rm f}^{1/2}$ with $\langle p \rangle > 20\%$ for $l_{\rm f} \gtrsim 60$\,kpc, at frequencies $\nu > 4\,{\rm GHz}$. Faraday depolarization at $\nu < 3$\,GHz leads to deviation from this relation, and in combination with beam depolarization, filamentary polarized structures are completely erased, reducing $\langle p \rangle$ to below 5\% level at $\nu \lesssim1$\,GHz. Smoothing on scales up to $30$\,kpc reduces $\langle p \rangle$ above $4$\,GHz by at most a factor of 2 compared to that expected at $1$\,kpc resolution of the simulations, especially for $l_{\rm f} \gtrsim 100$\,kpc, while at $\nu < 3$\,GHz, $\langle p \rangle$ is reduced by a factor of more than 5 for $l_{\rm f} \gtrsim 100$\,kpc, and by more than 10 for $l_{\rm f} \lesssim 100$\,kpc. Our results suggest that observational estimates of, or constrain on, $\langle p \rangle$ at $\nu \gtrsim 4$\,GHz could be used as an indicator of the turbulent driving scale in the ICM.