About 10% of nearby elliptical galaxies contain HI, with typical values of M(HI) ~ 5 × 108 M⊙ and M(HI)/LB ~ 0.03 M⊙/L⊙. The HI content is unrelated to the stellar content, (unlike the situation in spiral galaxies) suggesting that the HI in early-type galaxies has an external origin and is not produced by mass loss. This conclusion is strengthened by the distribution and kinematics of the HI structures, which lie outside the main optical body of the galaxies, have much larger specific angular momentum than do the stars, and are often highly inclined to the kinematic and distribution axes of the optical bodies.
The HI and stellar kinematics show that the rotation curves of E and SO galaxies are approximately flat from small (a few hundred pc) to large (10-20 kpc) radii, as is the case for spirals. Likewise, large mass-to-light ratios are found for some systems. Comparison with mass models derived from X-ray emission suggests that these may in some cases overestimate the mass.
The presence of HI is shown to enhance the likelihood that an E/SO galaxy has a nuclear radio continuum source, in agreement with models which suggest that the central engine is fuelled by cold gas. Current data suggest that the gas-to-dust ratio for the cold interstellar medium in ellipticals has a value similar to that found in the solar neighborhood.