Rotation measures (RMs), derived using polarized radio sources embedded in clusters, are commonly used to estimate magnetic field strengths in intra-cluster media(ICMs). To obtain these field values, assumptions are often invoked that involve: no RM contributions from the RM source, adequate sampling of the cluster ICM, and power law distributions of the magnetic power spectra. We explore the impact of such assumptions using synthetic RM measurements of the ICM of a dynamical cluster extracted from a magneto-hydrodynamic (MHD) cosmological simulation. We first explore the viability of estimating cluster magnetic fields using both RMs fully sampling the cluster as well as limited sampling using non-interacting RM sources placed within the cluster.We then evolve the cluster both with and without an interacting central active galactic nucleus (AGN) to examine how a radio source interacting with the surrounding ICM modifies the observed RMs. We applying a density dependent RM coherence length to our models and find that the magnetic field estimates are improved over models with a constant RM coherence length. We also suggest that a magnetic field model only need be characterized by a coherence length, the central dispersion value, and the scaling with density to effectively produce observed cluster RMs. However, uncertainties- predominantly due to large scale anisotropies- produce errors in the estimates of cluster magnetic fields that under ideal conditions are a factor of 50%. For sources that interact with the surrounding ICM, we find that the motions induced in the local ICM do not significantly alter the overall distribution of RMs observed in a "foreground screen". However, if the magnetic fields introduced by the radio source are sufficiently strong, non-negligible RM contributions attributed to ICM entrainment with the radio source plasma could contaminate RM observations making estimates of the undisturbed ICM magnetic fields perilous.
University of Minnesota M.S. thesis. June 2014. Major: Astrophysics. Advisor: Thomas W. Jones. 1 computer file (PDF); viii, 54 pages.
Johnson, Andrew Richard.
Rotation measures in galaxy clusters from background and embedded sources: estimating the cluster magnetic fields.
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