GALAXY DYNAMICS AND THE MASS DENSITY OF THE UNIVERSE

Dynamical evidence accumulated over the past 20 years has convinced as tronomers that luminous matter in a spiral galaxy constitutes no more than 10% of the mass of a galaxy. An additional 90% is inferred by its gravitational effect on luminous material. Here I review recent obser vations concerning the distribution of luminous and nonluminous matter in the Milky Way, in galaxies, and in galaxy clusters. Observations o f neutral hydrogen disks, some extending in radius several times the o ptical disk, confirm that a massive dark halo is a major component of virtually every spiral. A recent surprise has been the discovery that stellar and gas motions in ellipticals are enormously complex. To date , only for a few spheroidal galaxies do the velocities extend far enou gh to probe the outer mass distribution. But the diverse kinematics of inner cores, peripheral to deducing the overall mass distribution, of fer additional evidence that ellipticals have acquired gas-rich system s after initial formation. Dynamical results are consistent with a low -density universe, in which the required dark matter could be baryonic . On smallest scales of galaxies [10 kiloparsec (kpc); H(o) = 50 km.se c-1.megaparsec-1] the luminous matter constitutes only 1% of the closu re density. On scales greater than binary galaxies (i.e., greater-than -or-equal-to 100 kpc) all systems indicate a density almost-equal-to 1 0% of the closure density, a density consistent with the low baryon de nsity in the universe. If large-scale motions in the universe require a higher mass' density, these motions would constitute the first dynam ical evidence for nonbaryonic matter in a universe of higher density.