The Origin of the Ionized Gas in S0 Galaxies

Abstract

Several pieces of evidence indicate that the ionized gas present in elliptical galaxies is in the majority of the cases of external origin. In particular, the counter-rotation of the gas with respect to the stars constitutes the most direct evidence of the acquisition phenomenon (see Caldwell, Kirshner & Richstone 1986 Astrophys. J., 305, 136; Bertola & Bettoni 1988 Astrophys. J., 329, 102; Bertola, Buson & Zeilinger 1988 Nature, 335, 705). There is a priori no reason why such accretion events should be limited only to elliptical galaxies. While in the case of spirals the pre-existing gas may hide the process, S0 galaxies seem to be the best candidates, a part from ellipticals, for detecting these events. Indeed, the acquisition has certainly taken place in those S0s which possess polar rings and in a few galaxies where the gas is found to be in counter-rotation with respect to the stars (Galletta 1987 Astrophys. J., 318, 531; Dettmar 1989 Proc. 2nd Wyoming Conf. “The interstellar medium in external galaxies”; Rubin 1988 in “Large Scale Motions in the Universe” p. 541). On the basis of the assumption that the gas in SO galaxies is of external origin, we expect to observe counter-rotation phenomena in 50% of the objects, where ionized gas has been detected.