  In this paper we study the infall, by dynamical friction, of rigid
"satellites" taken in a variety of initial configurations and models. We thus
measure how dynamical friction depends on the density concentration and on the
pressure anisotropy of the host galaxy and, separately, the response of the
galaxy during the process.
  The investigation is carried out by using a numerical laboratory set up in
previous papers of this series.
  We find that the pressure anisotropy present in the host galaxy has little
effect on dynamical friction. Instead, the shape of the galaxy density profile
is very important. The classical idealized description appears to be applicable
to galaxy models characterized by a small density gradient but not to
concentrated models. In contrast to the behavior found in small density
gradient models, the orbits of satellites captured in concentrated models are
not circularized by dynamical friction.
  The induced evolution in the host galaxy reflects the initial conditions
adopted for the captured satellite. Satellites spiraling in on quasi-circular
orbits tend to modify the pressure tensor of the host galaxy in the tangential
direction and the galaxy shape from spherical to oblate, while satellites
captured along quasi-radial orbits tend to induce pressure anisotropy in the
radial direction and to generate a prolate shape. This result suggests that the
shape of early-type galaxies may result from occasional mergers rather than
being directly related to the effectiveness of the radial-orbit instability
during the process of formation via collisionless collapse, as often argued in
the past.
