Hercules A (3C 348): Phenomenology of an Unusual Active Galactic Nucleus

Hercules A is one of a few powerful double-lobe radio sources that show unusual features, from a double optical core to a string of radio intensity rings in one of its two lobes. Among scores of strong radio sources with z < 1, Hercules A is not a hot-spot FR II source, nor is it a weak plume-lobed FR I source; rather, it shows strong resemblances to both types. By undertaking a phenomenological study of the Hercules A system, we seek to explore further the FR I versus FR II dichotomy. As an initial effort, we model the development of Hercules A and its curious combination of features as follows. A rough billion years ago, at an epoch when little or no magnetized-lobe plasma was in place, small clouds of denser gas from the optical double core reached the galaxy halo boundary, there to expand and radiate picohertz spherical acoustic trains of waves. These grew larger as they drifted out in the wind, both the speed of sound and the bulk drift speed nearly uniform. The simple kinematics of uniform drift shaped the old line of such pulse spheres, now seen as remarkably circular radio features, their emitting plasma only some 10 million years old. The active galactic nucleus (AGN) jet has only recently filled in the preexisting density minima of the moving acoustic spheres. The periodic pattern was timed by successive orbital tidal excitations of the central AGN by its lighter companion galaxy. The eastern lobe can be understood as roughly symmetrical with the western lobe, differing in appearance only because it is viewed earlier in the process of magnetic plasma infill that built the radio-emitting lobes, to evolve by dilution and decay. The eastern radio image is seen somewhat delayed by projection; the plasma we see there is still spreading.