When it comes to dining, the Milky Way prefers to eat in.
Three new studies reveal that Earth’s home galaxy indulged in cannibalism to assemble its halo, the diffuse distribution of stars that surrounds the dense core and disk of the Milky Way.
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One team of astronomers recently analyzed a selection of blue, luminous stars recorded by the fledgling Sloan Digital Sky Survey, a cosmic census that over the next 5 years will map 200 million celestial objects. The researchers expected to see a uniform distribution of material swaddling the Milky Way. Instead, among the 1 percent of the Sloan survey completed, they found two huge, elongated clumps of stars.
The finding appeared so bizarre, recalls Heidi Jo Newberg of Rensselaer Polytechnic Institute in Troy, N.Y., that at first she and her colleagues didn’t believe it.
The study suggests that the clumps are streamers—the stretched-out remnants of objects captured by the Milky Way billions of years ago. Theorists have proposed that the Milky Way built itself up by merging with other galaxies or snaring their material, she notes.
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The Sloan discovery strengthens the argument that the Milky Way matured by swallowing its neighbors, conclude Newberg and her colleagues, including Brian Yanny of the Fermi National Accelerator Laboratory in Batavia, Ill. They recently posted their analysis on the Los Alamos National Laboratory’s preprint server (http://xxx.lanl.gov/abs/astro-ph/0004128), a site for physical-sciences research articles.
“It’s an unexpected result,” comments Rosemary F.G. Wyse of Johns Hopkins University in Baltimore. “People have been searching for streamers for a long time.” It remains unclear, she adds, whether these streamers represent wholesale capture of material from small, mature galaxies several billion years ago or the much earlier incorporation of stars when both the Milky Way and its neighbors were still forming.
If such streamers are distributed uniformly across the sky, the halo could contain between 40 and 200 of them, Newberg and her collaborators estimate. Several streamers could come from the same captured galaxy.
Newberg, Yanny, and their collaborators set their sights on so-called A-colored stars, which are old and bright. Another team, which includes Željko IveziÄ of Princeton University, used the Sloan survey to examine a second group of elderly objects, known as RR Lyrae stars. In the northern part of the halo, the astronomers found the same pattern of clumping that Newberg’s team identified. Ivezic and his colleagues also report their work on the Los Alamos site (http://xxx.lanl.gov/abs/astro-ph/0004130).
In a third article posted on the site, researchers analyzed streamers to probe the invisible dark matter of our galaxy. Rodrigo Ibata of the Max Planck Institute for Astronomy in Heidelberg, Germany, and his colleagues describe their own large-scale survey of middle-aged stars known as carbon giant stars (http://xxx.lanl.gov/abs/astro-ph/0004011).
This team finds that more than half of the 75 stars it examined orbit the Milky Way along a great circle. This orbit must be shaped by the vast distribution of dark matter believed to engulf both our galaxy and its visible halo of material, the researchers say. The finding indicates that the dark matter must form a sphere rather than a football shape. Otherwise, the orbits of the carbon stars would spread out rather than remain in a circle, Ibata says.
He and his colleagues note that the stream of stars lies near the center of the Sagittarius dwarf galaxy, a small galaxy that is currently being torn apart by the Milky Way. “The carbon stars we discovered were almost certainly dumped into the halo from the Sagittarius dwarf galaxy,” he says.
Ibata says that he was surprised to find but a single stream among the carbon stars, which are about 6 billion years old—roughly half the age of the Milky Way. In contrast, the Sloan data suggest a multitude of streams.
The stars in the Sloan survey, however, are about twice as old as the carbon stars. Taken together, Ibata notes, the studies suggest that the cannibalistic Milky Way did most of its gorging when it was a youngster.
Yanny cautions, however, that the clumps of stars in the three studies could be part of the same streamers.