Numerical Simulations of Restarted Jets -- I. Dynamics

We performed high-resolution three-dimensional relativistic magnetohydrodynamic (RMHD) simulations of restarted jets evolving within the cavity of a previous jet episode, using the PLUTO code. The simulations cover a spatial domain of $50\;\mbox{kpc}$ with a resolution of $40\;\mbox{pc}$. Three suites of simulations were performed to understand the impact of jet power, magnetic fields and quiescence time on the evolution of restarted jets. The restarted jets undergo a complex, multi-stage evolution, with the remnant cocoon relaxing from an initially conical structure to a mushroom-shaped morphology via an intermediate cylindrical phase. As the cocoon of the initial jet expands, dense material entrained by fluid instabilities such as Kelvin-Helmholtz and Rayleigh-Taylor significantly alter the conditions within it. As a result, the interaction of the restarted jet with the cocoon is markedly different from that of the initial jet with the ambient medium. In particular, we find that the restarted jet propagates ballistically through the rarefied remnant cocoon, without creating prominent backflows. Deceleration of the jet and associated backflows are observed only when it encounters dense structures. The structure and strength of the shocks in restarted jets are affected by jet power, magnetic field strength, and quiescence time. Finally, we discuss the implications of the dynamics for observed properties of radio galaxies.