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Session 22 - Radio - Loud AGN & Extragalactic Jets.
Display session, Wednesday, January 07
Exhibit Hall,
Her A (3C 348) is one of a few strong radio sources which shows unusually simple intensity features within its extended lobes. Such objects do not easily fit the Fanaroff-Riley scheme for jet and lobe sources. In the plane of the sky these features appear as circular rings, but they are in fact thin edge-brightened spherical shells. Her A displays a remarkably simple and uniform kinematics. We model the source by a two-stage account of its dynamics. First, acoustic waves were excited to form sphere after sphere drifting with a pre-existing thermal galactic wind. They arise at one spot along the jet axis at each edge of the galaxy, expanding uniformly at sound speed while they drift axially out at Mach 6. In a much later second stage, a faster flow of relativistic plasma forms the radio lobe and infuses it with radio electrons. These electrons decorate the pre-existing shells, now in steady motion. (Morrison and Sadun, 1996.) Although the two lobes do not look at all the same, they appear quite similar in structure after taking into account plausible orientations and the transit time delays they cause. Two small absorption rings have been found near the core with the HST (Baum, et al. 1996), apparently just what is needed to generate the initiating acoustic waves at the galaxy boundary. This source has an optically double active nucleus (Sadun and Hayes, 1993). Perhaps periodic tidal forces acting on the AGN core winds determine the modulations that gave repeated rise to the acoustic waves during the early outflow.
