Planetary Nebula Spectrograph survey of S0 galaxy kinematics. II. Clues
  to the origins of S0 galaxies
  The stellar kinematics of the spheroids and discs of S0 galaxies contain
clues to their formation histories. Unfortunately, it is difficult to
disentangle the two components and to recover their stellar kinematics in the
faint outer parts of the galaxies using conventional absorption line
spectroscopy. This paper therefore presents the stellar kinematics of six S0
galaxies derived from observations of planetary nebulae (PNe), obtained using
the Planetary Nebula Spectrograph. To separate the kinematics of the two
components, we use a maximum-likelihood method that combines the discrete
kinematic data with a photometric component decomposition. The results of this
analysis reveal that: the discs of S0 galaxies are rotationally supported;
however, the amount of random motion in these discs is systematically higher
than in comparable spiral galaxies; and the S0s lie around one magnitude below
the Tully--Fisher relation for spiral galaxies, while their spheroids lie
nearly one magnitude above the Faber--Jackson relation for ellipticals. All of
these findings are consistent with a scenario in which spirals are converted
into S0s through a process of mild harassment or "pestering," with their discs
somewhat heated and their spheroid somewhat enhanced by the conversion process.
In such a scenario, one might expect the properties of S0s to depend on
environment. We do not see such an effect in this fairly small sample, although
any differences would be diluted by the fact that the current location does not
necessarily reflect the environment in which the transformation occurred.
Similar observations of larger samples probing a broader range of environments,
coupled with more detailed modelling of the transformation process to match the
wide range of parameters that we have shown can now be measured, should take us
from these first steps to the definitive answer as to how S0 galaxies form.
