THE H-ALPHA KINEMATIC OF THE CARTWHEEL GALAXY

We report on high spectral and spatial resolution kinematic observatio ns of Her emission from the Cartwheel system obtained with a scanning Fabry-Perot interferometer. Velocity fields for two galaxies were meas ured, for the main galaxy of the system, the Cartwheel galaxy, and for a spiral companion similar to 25 h(-1) kpc to the north. The velocity field and the rotation curve of the spiral companion shows classical differential rotation and indicates the mass of the galaxy to be in th e range 2.9 - 4.8 x 10(9) M circle dot. The Cartwheel galaxy contains strong H alpha in emission asymmetrically distributed along its outer ring, with some faint emission detected inside it. The velocity field and the rotation curve of the Cartwheel galaxy is clearly dominated by differential rotation although second-order effects cause distortions which can be understood in a scenario where expansion and density-wav e motions are also considered. Our best model-fit to the observed velo city field of the Cartwheel galaxy includes three components for the k inematic of its outer ring: a circular velocity of 217 km s(-1), an ex pansion velocity of 13-30+/-10 km s(-1) and a sinusoidal perturbation with an amplitude of 20+/-5 km s(-1). This expansion velocity, lower t han found in previous studies, implies that the ring could be older th an previously thought. The rotation curve of the Cartwheel galaxy is c onsistent with a progenitor which had the morphology of a late-type sp iral galaxy. Our measurements support the collisional models of ring g alaxy formation and should impose strong constraints to simulations.