Structure and kinematics of edge-on galaxy discs - III. The rotation curves in the gas

A technique is introduced for deriving the gaseous rotation curves of edge-on spiral galaxies. The entire major axis position-velocity (XV) diagram is modelled with a set of rings in a least-squares sense, allowing for the effects of beam-smearing and line-of-sight projection. The feasibility of the technique is demonstrated by applying it to good quality HI XV diagrams of eight edge-on spirals. For seven additional spirals the XV diagrams are of insufficient quality, and the HI rotational velocities derived earlier using the envelope-tracing method are retained. The HI results are augmented with the optical emission line (HII) kinematics to arrive at estimates of the full rotation curves. A detailed comparison of the HI and HII kinematics shows that the discs in our sample are sufficiently transparent at the heights above the plane where we have taken our optical spectra to derive the stellar kinematics. In several of these spirals the HII is mainly confined to the spiral arms and does not extend out to the edge of the HI layer, which may have caused the HII velocity profiles to be significantly narrower than those of HI.