Two-dimensional H_alpha kinematics of bulgeless disk galaxies
  We present two-dimensional H_alpha velocity fields for 20 late-type,
disk-dominated spiral galaxies, the largest sample to date with high-resolution
H_alpha velocity fields for bulgeless disks. From these data we derive rotation
curves and the location of the kinematic centers. The galaxy sample was
selected to contain nucleated and non-nucleated galaxies, which allows us to
investigate what impact the gas kinematics in the host disk have on the
presence (or absence) of a nuclear star cluster. In general, we find that the
velocity fields span a broad range of morphologies. While some galaxies show
regular rotation, most have some degree of irregular gas motions. There appears
to be no systematic difference in the kinematics of nucleated and non-nucleated
disks. Due to the large fields of view of the integral field units we use, we
are able to observe the flattening of the rotation curve in almost all of our
sample galaxies. This makes modeling of the velocity fields relatively
straight-forward. Due to the complexities of the velocity fields, we obtain
reliable determinations of the kinematic center for only 6 of our 20 sample
galaxies. For all of these the locations of the nuclear star
cluster/photometric center and the kinematic center agree within the
uncertainties. If we disregard all kinematically irregular galaxies, our study
concludes that nuclear star clusters truly occupy the nuclei, or dynamical
centers, of their hosts. Our results are thus consistent with in-situ formation
of nuclear star clusters. Yet, many well-motivated formation scenarios for
nuclear clusters invoke off-center cluster formation and subsequent sinking of
clusters due to dynamical friction. In that case, our results imply that
dynamical friction in the centers of bulgeless galaxies must be very effective
in driving massive clusters to the kinematic center. (abridged)
