Production of ringlike structure in the cocoon of Hercules A

The radio lobes of the radio galaxy Hercules A contain intriguing ringlike structures concentric with the jet axis. To investigate the occurrence of such features, we have used hydrodynamic simulations of jets with a range of Mach numbers (from M = 2 to 50) and densities (down to a ratio of 10^-4 relative to the background) to generate ray-traced images simulating synchrotron emission from the time-dependent shock structures. We compare these images with observations of Hercules A and consider the physical nature and temporal evolution of the most plausible configurations. We find that the observed ringlike structures are well explained as nearly annular shocks propagating in the backflow surrounding the jet. We infer that the jet is oriented at between 30° and 70° to the line of sight, consistent with radio depolarization observations of Gizani & Leahy. The observational lack of hot spots at the extremities of the radio lobes and the possible presence of a buried hot spot near the base of the western lobe are explained in terms of the intrinsic brightness fluctuations and dynamics of the terminal shock of an ultralight, low Mach number jet that surges along its axis as a result of intermittent pinching and obstruction by turbulent backflow in the cocoon. We conclude, from the appearance of both sides of Hercules A, that both jets are on the verge of becoming fully turbulent.