§15 Background to jet-cloud interactions

Jet-cloud interactions were first invoked to account for the sharp deflections observed in wide-angle tail (WAT) sources (Burns, 1986). Numerical simulations by de Young (1991) using the beam scheme (de Young, 1986; Sanders & Prendergast, 1974) showed that the jet can be considerably decelerated in a collision with the cloud. The cloud appears to be destroyed within a few million years and the jet adjusts itself in order to return to its original direction. The conclusion was that the interaction was not sufficiently long lived to produce a tail. Norman (1993) made a similar analysis using a RIEMANN code. He showed that a De Laval nozzle (see section §4) was formed which efficiently re-accelerated the jet in a different direction with respect to its original trajectory.

These contradictory results were reconciled by the 3D simulations performed by Higgins et al. (1999) using the Godunov method of Falle (1991). This reconciliation came about because in their models it was possible to vary two fundamental parameters: the Mach number of the jet and the density contrast $\eta$, defined as the ratio of the jet density to that of the environment. Higgins et al. were able to reproduce the structures shown by de Young using a fast, heavy jet model in their simulations (de Young used a Mach number $M \! \approx \! 25$ and a density contrast $\eta \! = \! 1$). On the other hand, for the case of light jets with low velocities, Higgins et al. reproduced the overall structure of the deflections shown by Norman (the simulations performed by Norman were for a jet with Mach number $M \! = \! 4$ and a density contrast $\eta \! = \! 0.2$).