Studying the Kinematics of Faint Stellar Populations with the Planetary
  Nebula Spectrograph
  Galaxies are faint enough when one observes just their light distributions,
but in studying their full dynamical structure the stars are spread over the
six dimensions of phase space rather than just the three spatial dimensions,
making their densities very low indeed. This low signal is unfortunate, as
stellar dynamics hold important clues to these systems' life histories, and the
issue is compounded by the fact that the most interesting information comes
from the faintest outer parts of galaxies, where dynamical timescales (and
hence memories of past history) are longest.
  To extract this information, we have constructed a special-purpose
instrument, the Planetary Nebula Spectrograph, which observes planetary nebulae
as kinematic tracers of the stellar population, and allows one to study the
stellar dynamics of galaxies down to extremely low surface brightnesses. Here,
we present results from this instrument that illustrate how it can uncover the
nature of low surface-brightness features such as thick disks by studying their
kinematics, and trace faint kinematic populations that are photometrically
undetectable.
