Mysterious "high-velocity clouds" of hydrogen that scientists have puzzled over for more than 30 years may be the remnants of a primordial mass of hydrogen--perhaps among the first structures in the universe--that eventually collapsed to form the Milky Way and its neighbors. That theory, presented earlier today at the American Astronomical Society meeting in Toronto, supports current notions about how galaxies form.
The clouds have defied explanation since they were first observed in 1963. They move too fast to be orbiting the Milky Way, and some are so large that they stretch from one horizon to the other when observed from earthbound telescopes.
To try to solve the puzzle of how the clouds might have formed, a team led by astronomers Leo Blitz of the University of California, Berkeley, and David Spergel of Princeton University examined recent surveys and older observations of the clouds. They found that the data best fit a model placing the clouds in orbit around a group of galaxies that includes the Milky Way, its larger neighbor Andromeda, and about two dozen smaller galaxies. Although the new location--about a million light-years away--is a thousand times farther than some astronomers had suspected, the team says it best explains the motions of the clouds and their positions relative to the Milky Way and Andromeda. The team has also developed a computer model of a giant gas cloud collapsing to form the two galaxies that produces leftover clouds with motions resembling those of the high-velocity clouds. The simulation also suggests that the leftover material is still fueling new stars in the two galaxies.
Such an explanation "starts to fit in with current theories of galaxy formation and the origin of the universe," in which huge clouds of gas condensed to form galaxies, says team member Peter Teuben of the University of Maryland, College Park. Previous theories have suggested that the clouds are much closer and much smaller.
Ray Carlberg, an astronomer at the University of Toronto, agrees that the latest theory jibes with the "tremendous amount of information that [suggests] galaxies build up over time." More information on the molecular composition of the clouds would help make a stronger case, says Carlberg: A predominance of pristine hydrogen and helium gas would buttress the new theory by suggesting that the cloud was formed in the big bang, while the presence of heavier elements would indicate that the material was derived from stars.