Two-dimensional Hα kinematics of bulgeless disc galaxies

We present two-dimensional Hα velocity fields for 20 late-type, disc-dominated spiral galaxies, the largest sample to date with high-resolution Hα velocity fields for bulgeless discs. From these data, we derive rotation curves and the location of the kinematic centres. The galaxy sample was selected to contain nucleated and non-nucleated galaxies (as determined from prior Hubble Space Telescope imaging), which allows us to investigate what impact the gas kinematics in the host disc 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, which in nearly all cases either can be attributed to the presence of a bar or is connected to a rather patchy distribution of the Hα emission and the stellar light. There appears to be no systematic difference in the kinematics of nucleated and non-nucleated discs. 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 modelling of the velocity fields relatively straightforward. Due to the complexities of the velocity fields, we obtain reliable determinations of the kinematic centre for only six of our 20 sample galaxies. For all of these, the locations of the nuclear star cluster/photometric centre and the kinematic centre agree within the uncertainties. These locations also agree for seven more objects, despite considerably larger uncertainties as to the accuracy of the kinematic centre. If we disregard all kinematically irregular galaxies, our study concludes that nuclear star clusters truly occupy the nuclei, or dynamical centres, 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-centre cluster formation and subsequent sinking of clusters due to dynamical friction. In that case, our results imply that dynamical friction in the centres of bulgeless galaxies must be very effective in driving massive clusters to the kinematic centre.