Smallest Galaxies Solve a Big Problem

PASADENA, Calif.--An unusual population of the darkest, most lightweight galaxies known has shed new light on a cosmic conundrum. Astronomers used the W. M. Keck Observatory in Hawaii to show that the recently uncovered dwarf galaxies each contain 99 percent of a mysterious type of matter known as dark matter. Dark matter has gravitational effects on ordinary atoms but does not produce any light. It accounts for the majority of the mass in the universe.

New observations of eight of these galaxies now suggest that the "Missing Dwarf Galaxy" problem--a discrepancy between the number of extremely small, faint galaxies that cosmological theories predict should exist near the Milky Way, and the number that have actually been observed--is not as severe as previously thought, and may have been solved completely.

"These new dwarf galaxies are fascinating systems, not only because of their major contribution to the Missing Dwarf problem, but also as individual galaxies," says Josh Simon, a Millikan Postdoctoral Scholar at the California Institute of Technology and the lead author of the study. "We had no idea that such small galaxies could even exist until these objects were discovered last year."

The Missing Dwarf Galaxy puzzle comes from a prediction of the "Cold Dark Matter" model, which explains the growth and evolution of the universe. The theory predicts that large galaxies like the Milky Way should be surrounded by a swarm of up to several hundred smaller galaxies, known as "dwarf galaxies" because of their diminutive size. But until recently, only 11 such companions were known to be orbiting the Milky Way. To explain why the missing dwarfs were not seen, theorists suggested that although hundreds of the galaxies indeed may exist near the Milky Way, most have few, if any, stars. If so, they would be comprised almost entirely of dark matter.

In the past two years, researchers struggling to prove the existence of nearly invisible galaxies were aided by images from the Sloan Digital Sky Survey that revealed as many as 12 additional very faint dwarf galaxies near the Milky Way. The new systems are unusually small, even compared to other dwarf galaxies; the least massive among them contain only 1 percent as many stars as the most minuscule galaxies previously known.

"When they were discovered, we didn't know if all of these ultrafaint objects were actually galaxies," says Marla Geha, a Plaskett Research Fellow at the National Research Council Canada's Herzberg Institute of Astrophysics. "We thought some of them might simply be globular star clusters, or that they could be the shredded remnants of ancient galaxies torn apart by the Milky Way long ago. To test these possibilities, we needed to measure their masses."

Simon and Geha used the DEIMOS spectrograph on the 10-meter Keck II telescope at the W. M. Keck Observatory in Hawaii to conduct follow-up studies of eight of the new galaxies. The duo used the Doppler effect--a shift in the wavelength of the light coming from the galaxies caused by their motion with respect to the earth--to determine the speeds of stars within the dwarf galaxies. "Because stars in galaxies move only under the influence of gravity, their speeds are determined by the total mass of the galaxy," says Simon. To the researchers' surprise, each system was among the smallest ever measured, more than 10,000 times less massive than the Milky Way.

"It seems that very small, ultrafaint 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.

"The formation of such small galaxies is not very well understood from a theoretical perspective," adds Simon. "Explaining how stars form inside these remarkably tiny galaxies is difficult, and so it is hard to predict exactly how many star-containing dwarfs we should find near the Milky Way. Our work narrows the gap between the Cold Dark Matter theory and observations by significantly increasing the number of Milky Way dwarf galaxies and telling us more about the properties of these galaxies."

Numerous, repeated measurements of 814 stars in the eight dwarf galaxies were obtained at W. M. Keck Observatory. The speeds of the stars, ranging from about 4 to 7 km/s, were much slower than the stellar velocities in any other known galaxy. For comparison, the sun orbits the center of the Milky Way at about 220 km/s. The astronomers measured precise speeds for 18 to 214 stars in each galaxy, three times more stars per galaxy than any previous study.

"This is a significant paper," says Taft Armandroff, director of the W. M. Keck Observatory, whose research includes the study of dwarf galaxies. "It is a compelling example of how large, ground-based telescopes can precisely measure the orbits of distant stars on the sky to just a few kilometers per second. I expect DEIMOS will soon tell us about the chemical composition of these stars to help us better understand how star formation takes place in such small galaxies."

Some parameters of the Cold Dark Matter theory can now be updated to match observed conditions in the local universe. Based on the masses measured for the new dwarf galaxies, Simon and Geha conclude that the fierce ultraviolet radiation given off by the first stars, born just a few hundred million years after the Big Bang, may have blown away all of the hydrogen gas from dwarf galaxies also forming at that time. The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or, in many cases, completely dark. When this effect is included in theoretical models, the number of expected dwarf galaxies agrees with the number of observed dwarf galaxies.

"One implication of our results is that up to a few hundred completely dark galaxies really should exist in the Milky Way's cosmic neighborhood," says Geha. "If the Cold Dark Matter model is correct they have to be out there, and the next challenge for astronomers will be finding a way to detect their presence."

Although the Sloan Digital Sky Survey was successful in finding a dozen ultrafaint dwarfs, it covered only about 25 percent of the sky. Future surveys that scan the remainder of the sky are expected to discover as many as 50 additional dark matter-dominated dwarf galaxies orbiting the Milky Way. Telescopes for one such effort, the Pan-STARRS project on Maui, are now under construction.

The paper, "Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem," will be published in the November 10 issue of The Astrophysical Journal. Funding for the project was provided by Caltech under the Millikan Fellowship program and by the Herzberg Institute of Astrophysics of the National Research Council Canada, and through a grant from the National Science Foundation (AST 0071048).

# # #

Scientists available for comment:

Joshua D. Simon California Institute of Technology Pasadena, California USA Phone: (626) 395-3693 [email protected]

Marla Geha National Research Council Canada, Hertzberg Institute of Astrophysics Victoria, BC Canada Phone: (250) 363-8103 [email protected]

Taft Armandroff Director W. M. Keck Observatory Phone: (808) 881-3855 [email protected]