Star formation in the disks of H I-rich S0 galaxies

Abstract

We present the results of a Hα emission-line imaging survey of a sample of neutral-gas-rich S0 galaxies. We find evidence of disk H II regions in 14 of our sample of 32 galaxies, detect nuclear or faint diffuse circumnuclear Hα + [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 S0'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 S0'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 S0'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.