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A. C. Sadun (University of Colorado at Denver), P. Morrison (Massachusetts Institute of Technology)
Her A is a powerful double-lobe radio source that shows unusual features, from a double optical core to a string of radio intensity rings in one of its two lobes. Among scores of nearby strong radio sources, Her A is neither a hotspot FR II source nor a weak plume-lobed FR I, but rather shows strong resemblances to both types.
We model the development of Her A and its curious combination of features as follows. A rough billion years ago, at an epoch when little magnetized lobe plasma was in place, small clouds of denser gas from the double core reached the galaxy halo boundary, there to expand and radiate picohertz spherical acoustic wave trains. These grew as they drifted out in the wind, both the speed of sound and the bulk drift speed nearly uniform. Two small clouds near the core have been seen in optical absorption that fully resemble the radio features at the present lobe root. The simple kinematics of uniform acoustic drift shaped the old line of enlarging wave trains, now seen as remarkably circular radio features, their emitting plasma only some ten million years old.
The AGN jet has only recently filled in the pre-existing 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 only because it is viewed earlier in the process of magnetic plasma infill that built the radio-emitting lobes, evolving by dilution and decay. The eastern radio image is seen somewhat delayed by projection; the plasma we see there is still spreading. Recent X-ray observations have shown features around Her A whose explanation is not inconsistent with the aforementioned model.