Planetary nebulae as kinematic tracers of galaxy stellar halos
  The kinematic and dynamical properties of galaxy stellar halos are difficult
to measure because of the faint surface brightness that characterizes these
regions. Spiral galaxies can be probed using the radio HI emission; on the
contrary, early-type galaxies contain less gas, therefore alternative kinematic
tracers need to be used. Planetary nebulae (PNe) can be easily detected far out
in the halo thanks to their bright emission lines. It is therefore possible to
map the halo kinematics also in early-type galaxies, typically out to 5
effective radii or beyond. Thanks to the recent spectroscopic surveys targeting
extra-galactic PNe, we can now rely on a few tens of galaxies where the
kinematics of the stellar halos are measured. Here, I will review the main
results obtained in this field in the last decades.
