  The dynamical friction force experienced by a massive gravitating body moving
through a gaseous medium is modified by sufficiently strong large-scale
magnetic fields. Using linear perturbation theory, we calculate the structure
of the wake generated by, and the gravitational drag force on, a body traveling
in a straight-line trajectory in a uniformly magnetized medium. The functional
form of the drag force as a function of the Mach number (V_0/c_s, where V_0 is
the velocity of the body and c_s the sound speed) depends on the strength of
the magnetic field and on the angle between the velocity of the perturber and
the direction of the magnetic field. In particular, the peak value of the drag
force is not near Mach number 1 for a perturber moving in a sufficiently
magnetized medium. As a rule of thumb, we may state that for supersonic motion,
magnetic fields act to suppress dynamical friction; for subsonic motion,
magnetic fields tend to enhance dynamical friction. For perturbers moving along
the magnetic field lines, the drag force at some subsonic Mach numbers may be
stronger than it is at supersonic velocities. We also mention the relevance of
our findings to black hole coalescence in galactic nuclei.
