STAR-FORMATION IN THE DISKS OF H I-RICH S0-GALAXIES

We present the results of a Halpha emission-line imaging survey of a sample of neutral-gas-rich SO galaxies. We find evidence of disk H II regions in 14 of our sample of 32 galaxies, detect nuclear or faint diffuse circumnuclear Halpha + [N II] emission in another 11 galaxies without disk H II regions, and obtain upper limits for 8 galaxies. We find a striking dichotomy between SO's with and without H II regions; either a galaxy has a number of H II regions, most often distributed into distinct rings or ringlike structures, or there are none down to detection limits equivalent to a single unreddened H II region ionized by single O stars. We find that the SO's without disk H II regions have a lower median M(HI)/L(B) than those with disk H II regions, but the distributions have a large range of dispersion. Our data suggest that SO's may lie in a regime where local threshold effects, perhaps primarily kinematic in origin, are more important in determining the star formation in these galaxies than the global stability mechanisms that recent empirical models for large-scale star formation have suggested prevail in later-type spirals.