Filling the Gap

Based on his observation that clusters of galaxies do not have enough matter to remain gravitationally bound, Fritz Zwicky proposed (in 1933) the existence of dark matter to provide the needed gravity. Since then, there has been a growing body of supporting evidence, including flat rotation curves in large spiral galaxies, larger-than-expected velocity dispersion in elliptical galaxies, and certain measured characteristics of the cosmic microwave background, all of which require the presence of dark matter for their explanation.

Dark matter is matter of unknown composition that does not emit or reflect light, but can be detected by its gravitational effect on ordinary matter. It is also an important component of the hot Big Bang inflationary model of the universe.

Using this new component of mass and large computer simulations, astrophysicists have developed a cold dark matter (CDM) model for the evolution of the universe. It is a “bottom up” model in the sense that dwarf galaxies, largely made up of dark matter with some stars made from ordinary matter, form first in the early universe. These eventually coalesce into larger galaxies and then into clusters of galaxies. The model predicts that large galaxies like the Milky Way should be surrounded by hundreds of these very small dwarf galaxies.

Until now this prediction has not held up for the Milky Way Galaxy; only 12 such galaxies have been discovered. To explain this discrepancy astronomers have suggested that these galaxies may exist but have too few stars to be seen. If this is the case, they would have to be made up mostly of dark matter. Others prefer to abandon the dark matter model altogether and come up with alternative explanations, like modified Newtonian dynamics (MOND).

Recently, however, in a paper appearing in the November 10 issue of the Astrophysical Journal, authors Joshua D. Simon and Marla Geha report on observations of ultra-faint galaxies in the neighborhood of the Milky Way Galaxy that show promise for “filling the gap” in this chasm. They have performed detailed spectroscopy of stars in 8 of the 12 dwarf galaxies that were newly discovered in the Sloan Digital Sky Survey (SDSS). This process allowed them to determine the mass of these objects and come to the following conclusion: “It seems that very small, ultra-faint galaxies are far more plentiful than we thought. I’m astonished that so many tiny, dark matter-dominated galaxies have now been discovered,” says Geha.

After accounting for the fact that the SDSS covers only about 25 percent of the sky down to a certain level of faintness, they estimate that as many as 50 more dark matter-dominated dwarf galaxies orbiting the Milky Way may yet be discovered. This goes a long way toward filling the gap for the few hundred that are needed to confirm the CDM model. It also provides further confirmation of RTB’s creation model, which incorporates components of big bang cosmology.