Constraining the role of star cluster mergers in nuclear cluster
  formation: Simulations confront integral-field data
  We present observations and dynamical models of the stellar nuclear clusters
(NCs) at the centres of NGC 4244 and M33. We then compare these to an extensive
set of simulations testing the importance of purely stellar dynamical mergers
on the formation and growth of NCs. Mergers of star clusters are able to
produce a wide variety of observed properties, including densities, structural
scaling relations, shapes (including the presence of young discs) and even
rapid rotation. Nonetheless, difficulties remain, most notably that the second
order kinematic moment V_rms = (V^2 + sigma^2)^(1/2) of the models is too
centrally peaked to match observations. This can be remedied by the merger of
star clusters onto a pre-existing nuclear disc, but the line-of-sight velocity
V is still more slowly rising than in NGC 4244. Our results therefore suggest
that purely stellar dynamical mergers cannot form NCs, and that gas dissipation
is a necessary ingredient for at least ~50% of a NC's mass. The negative
vertical anisotropy found in NGC 4244 however requires at least 10% of the mass
to be accreted as stars, since gas dissipation and in situ star formation leads
to positive vertical anisotropy.
