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)