Galaxy dynamics and the mass density of the universe.

Abstract

Dynamical evidence accumulated over the past 20 years has convinced astronomers 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 observations concerning the distribution of luminous and nonluminous matter in the Milky Way, in galaxies, and in galaxy clusters. Observations of neutral hydrogen disks, some extending in radius several times the optical 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 enough to probe the outer mass distribution. But the diverse kinematics of inner cores, peripheral to deducing the overall mass distribution, offer additional evidence that ellipticals have acquired gas-rich systems 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. = 50 kmsec'lmegaparsec'11 the luminous matter constitutes only 1% of the closure density. On scales greater than binary galaxies (i.e., .100 kpc) all systems indicate a density -10% of the closure density, a density consistent with the low baryon density in the universe. If large-scale motions in the universe require a higher mass density, these motions would constitute the first dynamical evidence for nonbaryonic matter in a universe of higher density.Twenty years ago, astronomers started accumulating observations that indicated that orbital velocities of stars and gas in spiral galaxies remained high out to the limits of the observable galaxy. By 1982, after a decade of initial disquiet, most astronomers reluctantly accepted the conclusion that a galaxy consists of much more than the luminous stars, gas, and dust that can be observed at various wavelengths. In fact, at least 90% of a galaxy consists of dark matter, matter that is undetectable at any wavelength and that is deduced only by its gravitational influence on the orbits of the stars and the gas that we can see. But not all astronomers were reluctant to accept the evidence. "There are reasons, increasing in number and quality, to believe that the masses of ordinary galaxies may have been underestimated by a factor of 10 or more," stated Ostriker, Peebles, and Yahill (1) in a truly prescient sentence.This revolution in our understanding came about because theoretical arguments revealed the necessity of stabilizing the disks of galaxies with invisible halos (2), because observations with optical (3, 4) and with radio (5) telescopes showed uniformly high rotation velocities, and because an enormously influential review of the inferred distribution of

N-BODY SOLUTIONS AND COMPUTING GALACTIC MASSES

“…With observations of spiral galaxies, where stellar circular velocities tend to flatten or even increase with distance from the galactic center, the method predicts much larger M r ( ) values (the total mass to distance r) than are known to exist, particularly on galactic edges (e.g., Ostriker et al 1974;Rubin 1983Rubin , 1993. This supports the belief that huge amounts of unobserved mass must exist in a halo.…”

Kinematic and Photometric Evidence for a Bar in NGC 2683

“…), and cos 4θ profiles as functions of radius were presented by Shaw (1993), as were models of the observed luminosity distribution. Rubin (1993) found the Hα and [N ii] emission to show multi-valued, figure-of-eight velocity structure in the inner 60 of the galaxy. Light from bulge stars or foreground gas warping out of the galaxy plane and into the line of sight were suggested as possible explanations for the observed complex velocity structure.…”

A new formula for the rotation velocity – and density distribution of a galaxy

“…They found that M increased linearly with R out to at least 100 kpc. They now believe that around 90% of the mass in a spiral galaxy consist of dark and invisible matter, Rubin (6),with a mass to light (luminosity) ratio in solar units : M / L ≈ 200 ( M sun / L sun ). There is also another possible mass distribution according to the so called King´s model Combes et al (3) (p.87) where spiral galaxies follow a M α R 2 distribution between mass and distance to center, which is observed.…”