Why Dominant Cluster Jets Are Different

D. Sumi and L. Smarr, in Physics of Energy Transport in Extragalactic Radio Sources, 168-181, ed. A. Bridle and J. Eilek (NRAO: Green Bank),  (1985).

Abstract

Recent numerical simulations by Norman et al 1983 have shown that the ability of axisymmetric hydrodynamic jets stabily to propagate through a pressure confining atmosphere depends on two parameters: 1) the ratio of the internal jet density to the external atmosphere’s density and 2) the jet’s internal Mach number. As the pressure of the external atmosphere changes along the jet’s path, these two parameters change accordingly, leading to the possibility that a stable jet can suddenly become unstable. Elliptical galaxies which are stationary with respect to any outside gas medium, can sustain two generic types of atmospheres, 1) an outflowing wind and 2) a radiatively cooling inflow. Isolated elliptical galaxies should have wind atmospheres, whereas dominant galaxies at the center of clusters of galaxies can have cooling inflow atmospheres. We find that jets emerging through wind atmospheres propagate stabily. In contrast, jets emerging through cooling inflow atmospheres rapidly evolve through parameter space, crossing over into the unstable region. The instability manifests itself as a strong planar shock which abruptly brings the.jet from a supersonic to a subsonic flow. After this shock we expect a subsonic plume to continue onward. The behavior of jets in these different atmospheres may help explain the three classes of radio morphology found associated with dominant cluster galaxies: close doubles, short jets, and Wide Angle Tail (WAT) radio sources. In particular, our picture explains why these sources are often smaller in size than comparably powered radio sources from isolated elliptical galaxies.