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 (lf) to generate synthetic observations of the polarized emission. We focus on how the inferred diffuse polarized emission for different lf is affected due to smoothing by a finite telescope resolution. The mean fractional polarization ⟨p⟩ vary as ⟨p⟩∝lf1/2 with ⟨p⟩>20% for lf≳60 kpc, at frequencies ν>4GHz. Faraday depolarization at ν<3 GHz leads to deviation from this relation, and in combination with beam depolarization, filamentary polarized structures are completely erased, reducing ⟨p⟩ to below 5% level at ν≲1 GHz. Smoothing on scales up to 30 kpc reduces ⟨p⟩ above 4 GHz by at most a factor of 2 compared to that expected at 1 kpc resolution of the simulations, especially for lf≳100 kpc, while at ν<3 GHz, ⟨p⟩ is reduced by a factor of more than 5 for lf≳100 kpc, and by more than 10 for lf≲100 kpc. Our results suggest that observational estimates of, or constrain on, ⟨p⟩ at ν≳4 GHz could be used as an indicator of the turbulent driving scale in the ICM.

Highlights

  • Galaxy clusters are the largest known gravitationally bound systems which provide important clues on how structures were formed in the Universe

  • Our simulations were initialized with weak seed magnetic fields of the form B = B0[0, 0, sin(10 π x)] where the amplitude B0 was adjusted to a value such that the initial plasma β = Pth/PB ∼ 106, where Pth and PB are the thermal and magnetic pressures, respectively

  • In Sur et al [5], it was shown that the power spectrum computed from the 2-D map of Faraday depth (FD), the line of sight integral of B weighted by ne, could be directly used to infer the magnetic integral scale, i.e., the coherence scale of the magnetic fields, in the intracluster medium (ICM) see [11], provided FD is measured accurately

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Summary

Introduction

Magnetic fields are believed to play an important role in the evolution of the intracluster medium (ICM) of galaxy clusters which emit in the radio and X-ray wavebands. A large fraction of massive merging clusters show diffuse radio emission (radio halo) originating from relativistic electrons, possibly accelerated by turbulence in the ICM, illuminating the cluster magnetic fields via synchrotron radiation [1,2]. This emission is expected to be partially polarized, and its measurement provides insights into the statistical properties of magnetic field structure in the ICM [3,4,5]. A direct detection of polarized halo emission remains elusive and is a major science driver for the Square Kilometre Array (SKA) later this decade, e.g., [20]

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