High‐Resolution Optical Rotation Curves of Low‐Luminosity Spiral Galaxies

We present optical longslit spectroscopic observations of 21 low-luminosity,extreme late-type spiral galaxies. Our sample is comprised of Sc-Sm LocalSupercluster spirals with moderate-to-low optical surface brightnesses and withluminosities at the low end for spiral disk galaxies (M_V>-18.8). For eachgalaxy we have measured high spatial resolution position-velocity (P-V) curvesusing the H alpha emission line, and for 15 of the galaxies we also derivemajor axis rotation curves. In ~50% of our sample, the P-V curves showsignificant asymmetries in shape, extent, and/or amplitude on the approachingand receding sides of the disk. A number of the P-V curves are still rising tothe last measured point, or reach a clear turnover on only one side. In mostinstances we find good agreement between the kinematic centers of extremelate-type spirals as defined by the global HI emission profile and by theiroptical continuum, although in a few cases we see evidence of possible realoffsets. In spite of their shallow central gravitational potentials, at least 6of the galaxies in our sample possess semi-stellar nuclei that appear to becompact nuclear star clusters; in 5 of these cases we see kinematic signaturesin the P-V curves at the location of the nucleus. Finally, we find that likegiant spirals, our sample galaxies have higher specific angular momenta thanpredicted by current cold dark matter models.Comment: 20 pages; accepted to ApJS; higher resolution version of Figure 1 available as .png fil