The cool interstellar medium in S0 galaxies. I. A survey of molecular gas

Lenticular galaxies remain remarkably mysterious as a class. Observations to date have not led to any broad consensus about their origins, properties, and evolution, although they are often thought to have formed in one big burst of star formation early in the history of the universe and to have evolved relatively passively since then. In that picture, current theory predicts that stellar evolution returns substantial quantities of gas to the interstellar medium; most is ejected from the galaxy, but significant amounts of cool gas might be retained. Past searches for that material, though, have provided unclear results. We present results from a survey of molecular gas in a volume-limited sample of field S0 galaxies selected from the Nearby Galaxies Catalog. CO emission is detected from 78% of the sample galaxies. We find that the molecular gas is almost always located inside the central few kiloparsecs of a lenticular galaxy, meaning that in general it is more centrally concentrated than in spirals. We combine our data with H I from the literature to determine the total masses of cool and cold gas. Curiously, we find that, across a wide range of luminosity, the most gas-rich galaxies have similar to10% of the total amount of gas ever returned by their stars. That result is difficult to understand within the context of either monolithic or hierarchical models of evolution of the interstellar medium.