DISTRIBUTION AND MOTIONS OF ATOMIC-HYDROGEN IN LENTICULAR GALAXIES .11. A SUMMARY OF H-I OBSERVATIONS AND EVOLUTIONARY SCENARIOS

The observed distribution and kinematics (as derived from 21-cm radio synthesis observations) of the H I gas in a sample of 24 gas-rich early-type (S0 to Sa) disk galaxies are summarized and compared to those of later type (Sb to Sm) spiral galaxies. Lenticular galaxies more frequently have inner or outer H I rings (4 and 6 out of 9, respectively) than spirals. The inner rings have a typical radius of 0.7 times the de Vaucouleurs radius, R25, and half of them appear to be slightly tilted out of the stellar disk. The outer rings have a typical radius of 1.8 R25, and half of them are tilted some 25-degrees to 55-degrees out of the plane of the stellar disk. Barred SB0 galaxies have a pronounced central hole in their H I distribution, about 3 or 4 times as large as the bar, and similar holes are found in non-barred S0 galaxies; in SBa galaxies the central holes are about twice as large as the bar. The maximum azimuthally averaged H I surface density in S0 galaxies is only approximately 1 M. pc-2 too low to sustain large-scale star formation; in S0/a galaxies we find a value of approximately 3 M . pc-2.

The H I velocity fields in most early-type galaxies can be well described by circular rotation, sometimes in a considerably warped H I distribution. The derived rotation curves are essentially flat, sometimes out to 10 optical disk scalelengths, indicating the presence of massive dark haloes, which is substantiated by detailed disk/bulge/halo mass model fits to the surface photometry and rotation curves of 3 galaxies. Simple spherical mass models show that within R = R25 the total mass-to-blue luminosity ratio M(T)/L(B) of the early-type disk galaxies is similar to that of the later-type spirals, but that at R = R25 about half (3 out of 5) of the lenticulars have exceptionally large M(T)/L(B) ratios, approximately 30 M./L.. These objects seem to be "supermassive", rather than "underluminous". In the Tully-Fisher relation between rotation velocity and blue luminosity we do not find the reported separation between early and later-type spirals, but rather a similar relation for lenticular and spiral galaxies.

A comparison of the observed properties of the H I in early-type disk galaxies to predictions made by various scenarios for the origin and evolution of the gaseous component in lenticular galaxies shows that none of the processes considered can readily explain all the observed properties. The most likely origins of the H I gas in S0 galaxies appear to be either (1) accretion from another galaxy (or galaxies), though in the not too recent past, say more than (a few times) 10(9)yr ago, or (2) a burnt-out disk scenario, for the lenticulars with an outer H I ring, in which the denser inner H I disk has been consumed in star formation. Also, in barred galaxies redistribution of gas by the bar potential may have been important.