Simulations and synthetic observations of active galactic nuclei jets in galaxy clusters: numerical tools and experiments.

Abstract

We 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.