Star Formation and Abundances in S0 Galaxies

A significant number of S0 galaxies with a detectable ISM show some level of on-going massive-star formation activity in the form of visible HII regions. A rich ISM, however, does not guarantee star formation: a similar number of gas-rich S0s have no detectable HII regions down to the level of regions ionized by single O stars. The H-alpha luminosities of star-forming S0s imply global star formation rates as much as 2-3 orders of magnitude lower than in spirals. Spectroscopy of the bright HII regions in the six star-forming S0s studied thus far reveal solar gas-phase oxygen abundances. In general, S0s follow the general trends of O/H with galaxy luminosity and Hubble Type seen for Spirals and Irregulars, and have O/H abundances typical of spirals of similar total stellar luminosity. The implications are that S0s are the low end of the Hubble Sequence of Spirals, and that their ISMs are primarily internal in origin. The surprise is that two counter-rotating S0s, which show direct evidence of a massive infall of gas and stars, also follow these trends.