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 analyze the changes in gas
velocity and density far from the mass. We show analytically that the
steady-state friction force is Mdot*V, where Mdot is the mass accretion rate
onto 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 Mdot 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.
