Big galaxies, it seems, eat little ones

The serene beauty of the Milky Way in the night sky belies our galaxy's predatory nature. It grows by devouring smaller companion galaxies and star clusters. A new star map shows that this is going on to a far greater extent than astronomers had realized.

The map charts the outer regions of our Milky Way galaxy. This perspective gives insight into what astronomers believe is a cosmic theme. The map reveals "a striking demonstration of multiple merger events going on in the Milky Way galaxy right now," says Brian Yanny, a survey team member at the Fermi National Accelerator Laboratory in Batavia, Ill. "This is happening all over the universe as big galaxies grow by tearing up smaller ones into [star] streams."

A global collaboration of astronomers called the Sloan Digital Sky Survey reported the findings last week as part of an ongoing program to map more than a quarter of the sky. The Milky Way map was constructed by Vasily Belokurov and Daniel Zucker at Cambridge University in England. While they knew that stars from companion galaxies are streaming into the Milky Way, they found far more influx than expected. Multiple star streams are flowing. Some wrap around the galaxy several times. Amazed, they call the area of sky they have mapped the "Field of Streams," according to the Sloan Survey announcement.

The abundance of star streams suggests that there probably are more dwarf galaxies orbiting the Milky Way than astronomers have yet found. These star groupings are dim, and so they are hard to find. Trying to find dwarf galaxies and small clumps of stars in the vast sky is "like panning for gold," says Wyn Evans, a Sloan team member at Cambridge University.

Backtracking a star stream is one way to hunt them down. Drs. Belokurov and Zucker now have found two such "nuggets." One of them is the farthest satellite dwarf galaxy yet known. It lies at a distance of 640,000 light years. The other one shows the distortion caused by the Milky Way's tidal forces, Belokurov notes.

In this kind of study, astronomers are after more than the dynamics of galaxy growth. Many galaxies, including the Milky Way, seem to have massive halos of so-called "dark matter." This is matter of an unknown nature that can be detected only by its gravitational effect on visible matter. Careful study of the star flows in the Field of Streams should enable astronomers to chart that gravitational influence in detail. That should give a better estimate of dark-matter amounts and distributions. It should also help resolve the question of whether dark matter is hot, fast-moving material or cold, slow-moving stuff. Preliminary study favors the cold, slow option.

"The fact that we can see a Field of Streams like this suggests that dark-matter particles are very cold, or slow-moving. If the dark matter were made up of warm, fast-moving particles, we wouldn't expect these thin streams to hang around long enough for us to find them," explains James Bullock, an astronomical theorist at the University of California in Irvine.

What we call the Milky Way remains an enchanting spectacle. But remember, it's a jungle out there.