The Dark Side of Cosmology

Ricardo Muñoz

By Charlie Feigenoff (Ph.D., English '83)
Ricardo Munoz

Ricardo Muñoz
Photo by Tom Cogill

The Milky Way is one of billions of galaxies in the universe, but because it is our own galaxy, it is the logical choice for astronomers looking for clues to the origins of the universe. These scientists are called near-field cosmologists—and graduate student Ricardo Muñoz is one of their number. “Our challenge is to reconcile theories about the formation of the universe with what we can observe about the Milky Way,” Muñoz explains.

One prevalent cosmological theory is called the cold dark matter model. A variant of the big bang theory, it asserts that most of the material in the universe cannot be observed by its electromagnetic radiation, which is why it appears dark, and that this material is moving slowly, and therefore, is cold. One difficulty with this theory is that it predicts that the Milky Way should have between 500 and 1,000 satellite galaxies rotating around it—but only 20 have been observed.

“If the known satellite galaxies contain a large amount of dark matter, that would help make up for the discrepancy,” Muñoz says. “It also raises the possibility that many of the apparently ‘missing’ satellites are made completely from dark matter.” Muñoz and his advisor, Professor Steven R. Majewski, are trying to determine if such dark matter exists—and if so, to account for its mass and position.

Muñoz was employed as an observing assistant at the La Silla Observatory in Chile, his home country, when he met Majewski. The President’s Fellowship Muñoz was offered by the University proved decisive in allowing him to come to Charlottesville and study with Majewski.

Using standard observational techniques, Muñoz has been able to measure the visible mass in the central portions of the satellite galaxies and compute the total mass in these regions by applying the standard laws of gravitational physics to the motions of their stars. By comparing the visible to the dynamical mass measurements, he is able to gauge what fraction of the mass in the central portions of these galaxies is dark.

“We find that the motions of stars in the satellites cannot be accounted for simply by the attraction of visible stars,” Muñoz says. “Our computations tell us that there must be a great deal of dark matter associated with these galaxies. The challenge is to find out how it is configured outside the central areas of these systems so that we can derive their true total mass.”

One difficulty is that satellite galaxies are very diffuse and must be observed through the Milky Way, so it is very difficult to determine their full expanse. Muñoz draws on a special filtering system that Majewski refined that allows him to discriminate giant stars belonging to the satellite galaxies from those belonging to the Milky Way. This knowledge is critical for determining the presence of dark matter at the fringes of the satellite galaxies.

Muñoz has discovered that the tidal pull of the Milky Way tends to strip stars from the satellite galaxies that surround it, affecting the distribution and dynamics of stars in the outer portion of the satellites. Using computer simulations based on his data, Muñoz has shown that these small galaxies lack the protective cocoon of dark matter long assumed to be fundamental to their structure—and that dark matter as well as tidal stripping is affecting the motions of the giant stars near the edge of the satellite galaxies. “That’s one of the reasons that science is so interesting,” Muñoz says. “It can surprise you.”