Ringing in a new estimate for dark matter.

Ringing in a new estimate for dark matter

Amid the starlight, tenuous dust and empty space in the voids between galaxies, an exotic structure discovered earlier in the decade stands alone: a halo of hydrogen gas heavier than 2 billion suns, the only primordial intergalactic cloud ever identified. Past measurements of this elliptical ring in the constellation Leo have yielded important information about its structure and the pair of galaxies it orbits.

But new observations indicate astronomers have overestimated by about a factor of 10 the amount of "dark" matter--mass hidden from view because it does not radiate at any observed wavelength -- in the M96 galaxy group that contains the unusual ring. In addition, statistical analysis of 155 other small galactic groups suggests scientists have similarly misjudged the amount of dark matter in these systems, asserts Stephen E. Schneider of the University of Massachusetts at amherst, who originally discovered the ring (SN: 3/5/83, p.148).

Although Schneider's findings may not apply to larger galaxy clusters, several astronomers say the revised estimates fit with other observations indicating the heavens hold only enough total mass to generate a relatively weak gravitational tug. Some theories contend the universe contains enough matter to force its eventual gravitational collapse. But without sufficient mass, it might expand forever.

The ring, reports Schneider in the aug. 1 ASTROPHYSICAL JOURNAL, "provides some of the most direct dynamical evidence yet for the lack of dark matter -- at least to the extent supposed by many -- in one group of galaxies." Using the Arecibo telescope in Puerto Rico to measure the velocity of radio-wave-emitting hydrogen at different points around the ring, Schneider deduced how much mass the galactic pair at the ring's center must have to keep the ring in orbit. That amount, he says, is only about twice as large as the mass within the luminous regions of the two galaxies. The lower-than-expected ratio between the two mass values yields a dark-matter estimate that is one-tenth the amount calculated for M96 and other small galactic groups before the ring's discovery.

Moreover, Schneider says his statistical analysis of data from small galaxy groups indicates astronomers may be assigning too many distant, high-velocity galaxies to such groups. This, he says, may have led to inflated estimates of the dark matter required to hold member galaxies together. Schneider's statistical argument, notes Morton S. Roberts of the National Radio Astronomy Observatory in Charlottesville, Va., could account for the uneven spread of velocities among individual galaxies in a group.