Simulations of Narrow-Angle Tail Radio Galaxy Evolution and Shock Interactions

Abstract

We present the results of three-dimensional magnetohydrodynamic (MHD) simulations of radio galaxy (RG) outflows interacting with active galaxy cluster environments. The goal of these simulations was to analyze the consequences of these interactions such that observations of cluster radio emission can provide a more effective proxy for understanding the dynamical states of intracluster media (ICM). We conducted the first high-resolution numerical study dedicated to the evolution of a narrow-angle tail (NAT) RG utilizing 3D MHD and energy-dependent transport of cosmic ray electrons (CRe). We verify existing theoretical models of the jet bending process and, in an appendix, extend them into an empirical formalism that allows us to model the jet trajectories that develop from arbitrary jet-wind orientations. We observe that the early development of our NAT consists of an extended formation period, during which the evolving RG goes through a transient phase where it is more reminiscent of a more gently bent wide-angle tail RG. Once the jets are fully bent, we note the jet trajectories do not remain static, but episodically "flap" in response to instabilities that develop in the surrounding ICM wind. This occasional disruption of the jets enhances magnetic fields via dynamical stretching, and is critical to the state of the plasma in the tails that develops. Synthetic observations show that once plasma released early in the NAT's evolution has faded, the observed morphology of the NAT is very nearly steady-state with a roughly constant brightness distribution and a steady, self-similar, curved integrated spectrum. We then used the evolved NAT we created to run multiple simulations to explore the interactions between the tail plasma and shock fronts characteristic of those generated by a major cluster merger. Our primary motivation in conducting these simulations is to explore the possibility that CRe populations recently released from a NAT could be seed relativistic electrons necessary to explain observations of radio relics. We analyze the character the shock front develops within our simulated tails with a theoretical treatment based on a Riemann problem analysis. We note the development of vortical motions induced by the shock passage through the low-density plasma of the tails, and the resulting magnetic field amplification over and above amplification from shock compression. We examine synthetic observations of our shocked tails and compare to common properties of emission from radio relics.