High-Velocity Clouds Between Galaxies Are Building Blocks of Milky Way
By Robert Sanders, Public Affairs
Several recent observations support a controversial theory that high-velocity clouds of hydrogen gas seen near our galaxy are left over from the formation of the Local Group of galaxies, which includes Andromeda and the Milky Way.
If these puzzling clouds are indeed remnants of the very early history of the formation of nearby galaxies, they may represent the earliest structures that formed in the universe some 15 billion years ago and should contain "dark matter." Dark matter is the missing mass astronomers know must exist in galaxies but can't see because it doesn't glow.
"These clouds are the building blocks of the Milky Way," said Leo Blitz, professor of astronomy and director of Berkeley's Radio Astronomy Laboratory. "They are interesting in and of themselves because they should be remnants of the very early history of the Local Group. The clouds tie cosmology and the origins of the universe into the history of our own local galaxies."
Blitz detailed the theory and recent observations that support it in an invited talk Jan. 14 at the national meeting of the American Astronomical Society in Atlanta, Ga.
Blitz and four other astronomers from the United States and Holland proposed three years ago that these enigmatic clouds are buzzing about the Local Group, colliding and merging with the galaxies there.
Discovered by radio astronomers in 1963, the gas clouds are moving at high velocity through space in orbits that don't conform to the nicely circular orbits of most other objects in the disk of the Milky Way. It became evident in 1972 that they are not simply falling into the Milky Way, either, since some have high velocities away from us.
Several hundred of these clouds have been mapped within the margins of the Local Group. Composed of atomic hydrogen, they have the mass of a small galaxy and are approximately 50,000 light years across.
According to the team's scenario, these clouds are what remain of several thousand such clouds that formed in this region early in the history of the universe and perhaps were the first large structures to form. Over the eons these clouds have collided and coalesced into the galaxies we see today. Andromeda and the Milky Way are still in the process of forming as they gobble up the remaining clouds.
No theory could explain the diverse observations of high-velocity clouds until Blitz and his colleagues at Princeton University, University of Maryland, College Park, the Harvard-Smithsonian Center for Astrophysics, and W. Butler Burton of the University of Leiden, the Netherlands proposed theirs in 1997. The theory was built on an analysis of radio observations of these clouds and computer simulations of the formation of the local group.