2003A&A...400...81V

The propagation velocity of the first gas ring in collisional ring galaxies, i.e. the velocity at which the maximum in the radial gas density profile propagates radially in the galactic disk, is usually inferred from the radial expansion velocity of gas in the first ring. Our numerical hydrodynamics modeling of ring galaxy formation however shows that the maximum radial expansion velocity of gas in the first ring (v_gas) is invariably below the propagation velocity of the first gas ring itself (v_ ring_). Modeling of the Cartwheel galaxy indicates that the outer ring is currently propagating at v_ring≃100km/s, while the maximum radial expansion velocity of gas in the outer ring is currently v_gas≃65km/s. The latter value is in marginal agreement with the measurements of Higdon (1996ApJ...467..241H) based on HI kinematics. Modeling of the radial B-V/V-K color gradients of the Cartwheel ring galaxy also indicates that the outer ring is propagating at v_ring≥90km/s for the adopted distance to the galaxy of 140Mpc. On the other hand, the azimuthally averaged Hα surface brightness profile of the Cartwheel's outer ring does not peak exterior to those in K- and B-bands, contrary to what would be expected for such a high propagation velocity. We show that a combined effect of 41° inclination, finite thickness, and warping of the Cartwheel's disk might be responsible for the lack of angular difference in the peak positions. Indeed, the radial Hα surface brightness profiles obtained along the Cartwheel's major axis, where effects of inclination and finite thickness are minimized, do peak exterior to those at K- and B-bands. The angular difference in peak positions implies v_ring=110km/s, which is in agreement with the model predictions. We briefly discuss the utility of radio continuum emission and spectral line equivalent widths for determining the propagation velocity of gas rings in collisional ring galaxies.