Dark swarm. Clouds of invisible dark matter, loaded with cold hydrogen gas (orange), surround the gaseous disk of the Andromeda galaxy (blue).

Galaxies Still Growing Strong

The big just keep on getting bigger. That expression rings true for large galaxies like the Milky Way, now that astronomers report finding construction materials--clouds of hydrogen--piled near our galactic neighbor, Andromeda. Moreover, the discovery confirms predictions that cold, dark matter may be governing the formation of galaxies. "It's a very nice and important piece of work," says Butler Burton, a retired radio astronomer at Leiden University, the Netherlands.

Some 40 years ago, radio astronomers discovered clouds of neutral hydrogen gas whirling around the Milky Way galaxy. Some suspected that the clouds are relatively distant and massive and are falling into the Milky Way, which may have formed from such stuff. Others thought that the clouds are instead closer and smaller and are falling back after being blown away from our galaxy by stellar explosions. However, because the distances to the clouds are unknown, it has been impossible to verify their mass and origin.

Now, Robert Braun of the Netherlands Foundation for Research in Astronomy in Dwingeloo and David Thilker of Johns Hopkins University in Baltimore, Maryland, have found similar clouds around the Andromeda galaxy. Scanning the skies with the Dutch Westerbork Synthesis Radio Telescope, they identified 86 new clouds, 20 of which surround Andromeda. A larger team led by Thilker then studied these 20 clouds in more detail with the Robert C. Byrd Green Bank Telescope in West Virginia. The results are described in two papers: One will appear shortly in Astronomy & Astrophysics, and the other was published on 20 January in The Astrophysical Journal Letters.

Because the distance to Andromeda, and thus the clouds, is known, the team could deduce the masses of the Andromeda clouds. Most of them contain about 1 million times more hydrogen than our sun. In addition, the velocities of hydrogen atoms in the clouds suggests that they contain about 100 times more invisible dark matter than hydrogen. Such clouds have been predicted by computer simulations of hierarchical galaxy formation in a universe dominated by cold dark matter, but they have so far eluded observers. "It appears that we've finally detected them," says Braun.

Although some of the Andromeda clouds are likely the remains of satellite galaxies torn apart by tidal forces, many others likely represent the same small primordial building blocks from which the Andromeda galaxy accreted over the past 10 billion years or so. The same is probably true for the clouds swarming around the Milky Way. According to Braun, this confirms that galaxies are still forming. Burton warns that the final verdict is not yet in--that would take elaborate measurements of trace gases in the clouds--but, he says, "it's more and more difficult for alternative explanations to survive."

Related sites
Abstracts of papers, with links to full text
Astrophysics, abstract astro-ph/0311571
Astrophysics, abstract astro-ph/0312323
Westerbork Synthesis Radio Telescope
Robert C. Byrd Green Bank Telescope