WIDE INTERMEDIATE SCALE STRUCTURES IN MASS-LOADED FLOWS

We showed in earlier work that a clump embedded in tenuous plasma, whi ch is flowing subsonically relative to the clump, will develop a long thin tail whether the gas in the tail behaves adiabatically or isother mally. This paper presents a criterion for determining if viscous diss ipation in a tail with result in heating that will cause such a tail t o be wide. We argue that the observed properties of the tails in NGC 7 293 imply that a very strong effective viscous couple exists across ea ch of them. Assuming a strong couple, we calculate the viscous energy dissipation rate per unit volume and derive an upper bound on the numb er density in a tail in which viscous heating raises the temperature s ignificantly and thus causes broadening. The densities in observed bro ad tails in Abell 30 exceed this upper bound, and we suggest that the broadening of those tails is due to a phase transition in the tail gas , caused by photo-absorption heating. We comment on the relationship o f intermediate-scale structures in clumpy media to the issue of whethe r or not a global tenuous flow can be considered to be mass-loaded.