Unravelling the origins of S0 galaxies using maximum likelihood analysis
  of planetary nebulae kinematics
  To investigate the origins of S0 galaxies, we present a new method of
analyzing their stellar kinematics from discrete tracers such as planetary
nebulae. This method involves binning the data in the radial direction so as to
extract the most general possible non-parametric kinematic profiles, and using
a maximum likelihood fit within each bin in order to make full use of the
information in the discrete kinematic tracers. Both disk and spheroid kinematic
components are fitted, with a two-dimensional decomposition of imaging data
used to attribute to each tracer a probability of membership in the separate
components. Likelihood clipping also allows us to identify objects whose
properties are not consistent with the adopted model, rendering the technique
robust against contaminants and able to identify additional kinematic features.
  The method is first tested on an N-body simulated galaxy to assess possible
sources of systematic error associated with the structural and kinematic
decomposition, which are found to be small. It is then applied to the S0 system
NGC~1023, for which a planetary nebula catalogue has already been released and
analyzed by (Noordermeer et al., 2008). The correct inclusion of the spheroidal
component allows us to show that, contrary to previous claims, the stellar
kinematics of this galaxy are indistinguishable from those of a normal spiral
galaxy, indicating that it may have evolved directly from such a system via gas
stripping or secular evolution. The method also successfully identifies a
population of outliers whose kinematics are different from those of the main
galaxy; these objects can be identified with a stellar stream associated with
the companion galaxy NGC~1023A.
