Ram-Pressure Confinement of a Hypersonic Jet

We present the results of a high-resolution, hydrodynamical simulation of an extremely supersonic jet with a Mach number (relative to the undisturbed ambient medium) of 4650. Both the Mach number and the final length of the jet are greater than those of any previously published simulation. We find that the hypersonic jet is highly dynamic in nature with varying head velocity and shape; the importance of the back action of the cocoon upon the jet is also clearly seen. Such hypersonically moving jets can account for the observations of luminous extragalactic radio jets, associated with classical double radio sources, which appear to be over-pressured with respect to the surrounding ambient medium. The pressure jump across the bow shock of a hypersonic jet can easily be a factor of tens or hundreds resulting in an overpressured cocoon which confines the overpressured jet. In our model the jet remains well collimated while the region behind the bow shock is overpressured by a factor oF ~1600 with respect to the quiescent ambient medium, thus establishing the validity of the ram-pressure confinement scheme. New VLA data are used to constrain the physical parameters of 3C 208 within the confines of our hypersonic jet simulation.