The Neutral Hydrogen Gas Content of Star-forming and Quiescent S0 Galaxies

Lenticular (or S0) galaxies that remain actively star-forming (SFS0s) are a unique transition population among galaxies. We investigated the HI (21cm) neutral hydrogen content of a sample of lenticular galaxies identified from within the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey and which have been classified as star-forming or quiescent (qS0s). We compared these populations to each other and to a sample of star-forming spiral galaxies in MaNGA. We found that the HI Mass fraction and HI Deficiency of the SFS0s were intermediate between the quiescent S0s and the star-forming spirals, but most similar to the spirals. The HI depletion times of the quiescent S0's is very much longer than that of the SFS0 and spirals, which have roughly similar times. We also compared the molecular gas fractions of the three groups which shows that the lowest fraction of the three populations, followed by the star-forming S0s and then the star-forming spirals. A comparison of total baryonic mass comparisons indicates that the star-forming S0s have significantly lower masses than the qS0s in the sample. This suggests that the star-forming S0s must form from lower-mass progenitors (unobserved by MaNGA) which either accrete external gas, or loose spiral arms. The more massive quiescent S0s are a completely separate population, most likely to be formed either from fading star-forming spirals or from medium- to high-mass spirals undergoing mergers. The SFS0s in this sample cannot form from the quiescent S0s in this sample that receive in-falling gas, nor can they fade to create these qS0s.