2006ApJ...644..850S

The place of lenticular galaxies within the range of types of galaxies remains unclear. We previously reported the mass of molecular hydrogen for a volume-limited sample of lenticular galaxies, where we saw that the amount of gas was less than that predicted by the return of stellar mass to the interstellar medium. Here we report observations of atomic hydrogen (H I) for the same sample. Detections in several galaxies make more compelling the case presented in our earlier paper that the mass of cool gas in S0 galaxies cuts off at ∼10% of what is expected from current models of gas return from stellar evolution. The molecular and atomic phases of the gas in our sample galaxies appear to be separate and distinct, both spatially and in velocity space. We propose that the molecular gas arises mostly from the stellar mass returned to the galaxy, while the atomic hydrogen is mainly accumulated from external sources (infall, captured dwarfs, etc.). While this proposal fits most of the observations, it makes the presence of the upper mass cutoff even more mysterious.