Browsing by Subject "MHD"
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Item Cluster Weather Vanes: Radio Galaxies as Indicators of Galaxy Cluster Dynamics(2020-07) Nolting, ChrisWe report the results of multiple three-dimensional magnetohydrodynamic (MHD) simulation studies focusing on the dynamics of Active Galactic Nuclei (AGN) jets. The goal of these studies is to understand the interactions of such jets with dynamical features of galaxy cluster media such as winds and shock waves, and to use the recognizable observed features as diagnostic tools when examining real radio sources in clusters to better understand the state of the environment surrounding radio galaxies (RG). We describe in detail the physics of jet propagation in a wind, including the rate of advancement of the jet head in the presence of a head or tail wind, the bending of a jet by ram pressure due to a cross wind, and the combination of the two effects when there is some intermediate alignment. We show simulation results that confirm analytic predictions for the jet head advancement and bending, as well as the effects on a RG jet due to an encounter with a shock wave at various angles and shock strengths. In a perfectly aligned case, a sufficiently strong shock can cause a jet to be stripped of its cocoon material, its forward progression slowed, and in some cases the jet can be reversed entirely. Shocks propagating through low density cocoons produce vorticity which can result in a vortex ring, possibly disrupting the jet and creating a distinct ring structure. These rings can remain visible for significant time due to magnetic field amplification, but if they are overlapping active jets in projection, the emission from the fresh cosmic ray electrons (CRe) in the jets may overwhelm the aged electrons in the ring. The dynamics of vortex rings is discussed, and theoretical predictions are confirmed in simulation. Re-energization of aged CRe by shocks is examined, and adiabatic compression is sufficient to explain the brightened and spectrally flattened sources, while Diffusive Shock Acceleration is not required. Lastly, looking to future simulations, we outline an ongoing comparison study of two methods for solving the MHD equations, a 2nd order Total Variation Diminishing (TVD) method and a 5th order Weighted Essentially Non-Oscillating (WENO) method. Appendices include other work on numerical methods used in the completion of this dissertation. We find that RG jets make excellent "weather-vanes" for understanding the dynamics of their surrounding media, and useful tools for understanding the motions in galaxy clusters.Item Simulations and synthetic observations of active galactic nuclei jets in galaxy clusters: numerical tools and experiments.(2011-07) Mendygral, Peter JohnWe present the results and analysis of three-dimensional magnetohydrodynamic (MHD) simulations of jets from active galactic nuclei (AGN) in galaxy cluster environments. The purpose of these simulations was to investigate the interaction of AGN outflows with their environments and to understand the observational consequences that interaction produces. Synthetic X-ray observations of a set of simulations with AGN jets embedded in an analytically defined galaxy cluster were used to examine the reliability of common observational techniques. We explored the accuracy of measuring the enthalpy of X-ray cavities produced by AGN jets and found that observational techniques are accurate to within a factor of 2. We also tested observational methods for estimating the cavity age and mechanical power of AGN. Despite the simplified nature of the models these techniques are based on, estimates of cavity ages and jet powers were also accurate to within a factor of two. To expand our ability to produce more realistic simulations of AGN outflows we developed a new MHD code for numerical astrophysics called WOMBAT. This code was specifically designed and optimized for high performance and scaling on modern supercomputers. It has several additional physics modules for including optically thin radiative cooling, the effects of gravity and the transport and evolution of cosmic rays. We also present a new total energy conserving method for including the effects of gravity that was implemented into WOMBAT. We demonstrate that a non-conservative gravity scheme negatively impacts the accuracy and convergence of numerical Riemann solvers. Finally, we present WOMBAT simulations of AGN outflows in a galaxy cluster extracted from a cosmological simulation. We explore the effects of cluster "weather" on AGN jets and lobes. Although we chose a relatively relaxed cluster for these simulations, we find that bulk flows in the intra-cluster medium (ICM) were sufficient to significantly deflect the jets and lobes. We present synthetic X-ray observations that show highly asymmetric X-ray cavity structures. Synthetic radio images reveal similarities between "double-double" radio galaxies and our intermittent jets as well as morphologies similar to wide-angle tail galaxies. We show that variations in ram pressure in the ICM dominated both magnetic stresses and pressure variations on large scales. On smaller scales, however, magnetic stresses were dominant in localized regions around the jets and lobes. Through this work we find that observational methods used to estimate critical characteristics of AGN outflows, such as the energy released by the central engine, are reliable to within a factor of a few. We also found that AGN outflows can be significantly influenced ICM "weather". These findings reenforce the reliability of our understanding of these systems, and demonstrate how AGN outflows might be used to probe many of the properties of their environments.