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Disruption of galactic radio jets by shocks in the ambient medium


Observations show that jets in moderate luminosity (<1025 W Hz−1 at 20 cm wavelength) radio galaxies can flare dramatically in a few jet diameters and with opening angles up to 90° into diffuse lobes or tails1–4 (Fig. 1). These morphologies are difficult to reproduce in numerical simulations of supersonic jets that move outward in constant5,6 or smoothly varying atmospheres (N. Norman, unpublished results). By analogy with structures seen in the laboratory7, one could interpret the collimated jets as moderately supersonic (Mach number 2–5) fluid flows, and the lobes or tails as subsonic plumes that are subject to turbulent broadening and entrainment of the ambient medium. Our studies indicate that the transition from supersonic to subsonic flow can occur suddenly only if a strong planar (normal to the flow direction) shock or Mach disk forms within the jet. Here we show that such an internal shock is produced as the jet crosses a shock wave in the external medium. The external shock could form, for example, by a galactic wind encountering the intergalactic medium. We find that for jet disruption the jet Mach number must be less than the wind-shock Mach number, a result also understandable from analytic arguments. We apply this model to Cen A and wide-angle-tailed radio galaxies.

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Norman, M., Burns, J. & Sulkanen, M. Disruption of galactic radio jets by shocks in the ambient medium. Nature 335, 146–149 (1988).

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