Discovery of counter - rotating gas in the galaxies NGC 1596 and NGC 3203 and the incidence of gas counter - rotation in S 0 galaxies

We have identified two new galaxies with gas counter-rotation (NGC 1596 and NGC 3203) and have confirmed similar behaviour in another one (NGC 128), this using results from separate studies of the ionized-gas and stellar kinematics of a welldefined sample of 30 edge-on disc galaxies. Gas counter-rotators thus represent 10±5% of our sample, but the fraction climbs to 21 ± 11% when only lenticular (S0) galaxies are considered and to 27 ± 13% for S0s with detected ionized-gas only. Those fractions are consistent with but slightly higher than previous studies. A compilation from well-defined studies of S0s in the literature yield fractions of 15 ± 4% and 23±5%, respectively. Although mainly based on circumstantial evidence, we argue that the counter-rotating gas originates primarily from minor mergers and tidally-induced transfer of material from nearby objects. Assuming isotropic accretion, twice those fractions of objects must have undergone similar processes, underlining the importance of (minor) accretion for galaxy evolution. Applications of gas counter-rotators to barred galaxy dynamics are also discussed.