NONLINEAR DYNAMICAL FRICTION IN A GASEOUS MEDIUM

Using high-resolution, two-dimensional hydrodynamic simulations, we investigate nonlinear gravitational responses of gas to, and the resulting drag force on, a very massive perturber M{sub p} moving at velocity V{sub p} through a uniform gaseous medium of adiabatic sound speed a{sub i}nfinity. We model the perturber as a Plummer potential with softening radius r{sub s} , and run various models with differing A=GM{sub p}/(a{sub i}nfinity{sup 2}r{sub s}) and M=V{sub p}/a{sub i}nfinity by imposing cylindrical symmetry with respect to the line of perturber motion. For supersonic cases, a massive perturber quickly develops nonlinear flows that produce a detached bow shock and a vortex ring, which is unlike in the linear cases where Mach cones are bounded by low-amplitude Mach waves. The flows behind the shock are initially non-steady, displaying quasi-periodic, overstable oscillations of the vortex ring and the shock. The vortex ring is eventually shed downstream and the flows evolve toward a quasi-steady state where the density wake near the perturber is in near hydrostatic equilibrium. We find that the detached shock distance delta and the nonlinear drag force F depend solely on eta=A/(M{sup 2}-1) such that delta/r{sub s} = eta and F/F{sub lin}=(eta/2){sup -0.45} for 100 >eta>2, where F {sub lin}more » is the linear drag force of Ostriker. The reduction of F compared with F{sub lin} is caused by front-back symmetry in the nonlinear density wakes. In subsonic cases, the flows without involving a shock do not readily reach a steady state. Nevertheless, the subsonic density wake near a perturber is close to being hydrostatic, resulting in the drag force similar to the linear case. Our results suggest that dynamical friction of a very massive object as in a merger of black holes near a galaxy center will take considerably longer than the linear prediction.« less