Disentangling the stellar populations in the counter-rotating disc
  galaxy NGC 4550
  In order to try and understand its origins, we present high-quality long-slit
spectral observations of the counter-rotating stellar discs in the strange S0
galaxy NGC 4550. We kinematically decompose the spectra into two
counter-rotating stellar components (plus a gaseous component), in order to
study both their kinematics and their populations. The derived kinematics
largely confirm what was known previously about the stellar discs, but trace
them to larger radii with smaller errors; the fitted gaseous component allows
us to trace the hydrogen emission lines for the first time, which are found to
follow the same rather strange kinematics previously seen in the [OIII] line.
Analysis of the populations of the two separate stellar components shows that
the secondary disc has a significantly younger mean age than the primary disc,
consistent with later star formation from the associated gaseous material. In
addition, the secondary disc is somewhat brighter, also consistent with such
additional star formation. However, these measurements cannot be
self-consistently modelled by a scenario in which extra stars have been added
to initially-identical counter-rotating stellar discs, which rules out Evans &
Collett's (1994) elegant "separatrix-crossing" model for the formation of such
massive counter-rotating discs from a single galaxy, leaving some form of
unusual gas accretion history as the most likely formation mechanism.
