Dynamical friction in a gas: the subsonic case

We study the force of dynamical friction acting on a gravitating point mass that travels through an extended, isothermal gas. This force is well established in the hypersonic limit, but remains less understood in the subsonic regime. Using perturbation theory, we analyse the changes in gas velocity and density far from the mass. We show analytically that the steady-state friction force is (M) over dot V, where (M) over dot is the mass accretion rate on to an object moving at speed V. It follows that the speed of an object experiencing no other forces declines as the inverse square of its mass. Using a modified version of the classic Bondi-Hoyle interpolation formula for (M) over dot as a function of V, we derive an analytic expression for the friction force. This expression also holds when mass accretion is thwarted, e. g. by a wind, as long as the wind-cloud interaction is sufficiently confined spatially. Our result should find application in a number of astrophysical settings, such as the motion of galaxies through intracluster gas.