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Session 40 - Radiogalaxies & Jets.
Display session, Tuesday, January 14
Metropolitan Ballroom,
That VLBI-scale jets are variable and indeed episodic has long been established. More recently, extensive VLA observations of M87 (Biretta et al.\/, 1995, ApJ, 447, 582) show that extragalactic jets can be variable even on kpc scales. It is therefore reasonable to expect that some VLA-scale jets are also episodic to the point where jets may be completely turned off for a while, only to resume flow at a later time into the relaxing radio lobe.
Recent 3-D hydrodynamic simulations of a restarting jet with a passive magnetic field using ZEUS-3D are presented and compared with previous 2-D axisymmetric simulations (Clarke and Burns, 1991, ApJ, 369, 308). The 3-D simulations provide a more realistic comparison with candidate restarting jets such as 3C\,219 (Clarke et al.\/, 1992, ApJ, 385, 173). Line-of-sight integrations of the synchrotron emissivity accounting for Doppler favouritism are performed and it is found that unlike the observations of 3C\,219, the simulation reveals a well-defined bow shock leading the restarting jet as it advances into the magneto-ionic medium of the old radio lobe. In addition, the hot spot does not survive for any appreciable time without the ram-pressure confinement of a supersonic jet. Thus, it may be difficult to explain observations such as 3C\,219 in terms of a true restarting jet. Possible solutions and alternatives to this problem are investigated.
