Galactic de Gustibus

Milky Way's snacks shed light on dark matter and galaxy growth

Thirteen billion years after its birth, the Milky Way is still packing on the stars. Astronomers have discovered two dwarf galaxies that are being devoured by the Milky Way. They’ve also found two vast, streams of stars that were most likely torn from star clusters or small galaxies that long ago came too close to our much bigger galaxy.

STARRY GOURMAND. Researchers propose that the Milky Way and other galaxies were built by gobbling up nearby dwarf galaxies and incorporating them into the large galaxy’s gaseous outskirts, as in this artist’s depiction. Johnston, J. Bullock

The studies are giving researchers a new tool for mapping our galaxy’s dark matter, the vast, invisible halo of material that astronomers say provides the Milky Way’s gravitational glue.

The dwarf galaxies join 10 previously known, nearby, small-fry galaxies, also called satellites, snared by the Milky Way. Two star streams had already been reported. Because of their faintness and diffuse structure, the Milky Way’s dwarf galaxies and star streams show up only when astronomers look for groups of stars that have similar color, velocities, and distances from Earth.

Streaming data

Several teams of astronomers contributed to the discoveries, but all used the same raw data—the Sloan Digital Sky Survey, a map of stars and galaxies that covers one-fourth of the sky (SN: 2/4/06, p. 78: Available to subscribers at Galactic cannibalism).

An inspection by Daniel Zucker of the University of Cambridge in England revealed one of the new dwarf galaxies in a region that’s called the Field of Streams because it’s crisscrossed with streams of stars. About 640,000 light-years from Earth in the direction of the constellation Canes Venatici, the galaxy is one of the most remote Milky Way satellites ever found.

Zucker immediately e-mailed his discovery to his Cambridge colleague Vasily Belokurov. Within hours, Belokurov replied that he had found yet another Milky Way satellite, a dwarf galaxy in the constellation Bootes. One of the faintest Milky Way satellite ever discovered, it lies about 200,000 light-years from Earth. Zucker, Belokurov, and their colleagues posted their findings in the June 1 Astrophysical Journal Letters and on the Internet (

The team has just reported what appears to be another faint dwarf galaxy, about 100,000 light-years from Earth, in the constellation Ursa Major (

In a separate set of findings, astronomers looking at the outskirts of the Milky Way found two new star streams, remnants torn from dwarf galaxies or star clusters. Two independent teams—Zucker’s group and one led by Carl Grillmair of the California Institute of Technology in Pasadena—discovered one of these ribbons of stars within the Field of Streams. Because its parent body isn’t obvious, the scientists dubbed the ribbon the orphan stream.

Separately, Grillmair and Odysseas Dionatos of the Astronomical Observatory in Rome found a second star stream, nicknamed the galactic highway, which is 30,000 light-years from Earth. Grillmair presented the findings in early June at a meeting of the American Astronomical Society in Calgary, Alberta.

Dark tracers

Streams arise when the Milky Way tugs on a nearby galaxy or star cluster. The gravity of various parts of our galaxy pulls individual stars ahead of or behind the passing galaxy or cluster. “So you get leading and trailing streams bracketing the orbital path of the parent cluster,” Grillmair notes. The stream may eventually encircle the Milky Way.

By tracing back in time the positions and velocities of stars in a stream, theorists can figure out the distribution of matter—mostly dark matter—that drew the stars out of the cluster, comments Kathryn Johnston of Wesleyan University in Middletown, Conn.

In the case of the recently discovered galactic highway, says Grillmair, “the fact that the stream is very smooth, without any serpentine wiggles, tells us that there aren’t any local concentrations of dark matter affecting its path.” On the other hand, “the overall curvature of the stream suggests that there’s a lot dark matter in our galaxy,” he adds.

The newfound star streams “are clearly another indicator that our model of hierarchical structure formation—big galaxies growing by eating smaller ones—is going in the right direction,” says Johnston.

The dwarf galaxies could help astronomers solve a long-standing problem with dark matter theory, Johnston adds. It predicts that there ought to be a few hundred satellite galaxies—rather than the 12 now known—circling close to the Milky Way. The newly discovered dwarfs suggest that as astronomers scour larger parts of the sky, they could detect another 60 or so dwarfs.

“It’s not clear yet if this will solve the missing-satellite problem,” says Johnston, but “this is the exciting new result everyone is talking about.”

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