A long time ago, but actually far, far away from any galaxy, stars formed by the bushel within a gigantic cloud of gas. The discovery of this intergalactic stellar hatchery helps settle a long-standing debate by showing that not all wandering stars are castoffs from galaxies. At least some are born in the middle of nowhere.
The current theory of star formation is tied closely to the evolution of galaxies. Although the first stars obviously preceded the first galaxies in the early universe--forming around clumps of dark matter (ScienceNOW, 13 September)--for the most part, new stars are supposed to coalesce within gas clouds contained by a galaxy's gravity. Anything less dense than a galaxy is not supposed to possess enough gravity to compress gas and form a stellar nursery. Hence not many stars are thought to call the space between galaxies home.
Astronomers occasionally have found some rogue stars zipping through intergalactic space on their own, however. But they generally attribute these wanderers to the gravitational actions of black holes, which can whiplash unsuspecting stars entirely out of galaxies--or to galactic collisions, which can strip off whole spiral arms.
Now, researchers have proof positive that at least some wandering stars emerge in intergalactic space. Ming Sun, an astronomer at Michigan State University in East Lansing, and colleagues spotted the free-forming stars in an enormous plume of gas trailing a galaxy called ESO 137-011, which is plunging headlong into the giant Abell 3627 cluster. They studied the tail, which is 200,000 light-years long--twice the width of our Milky Way galaxy--with NASA's orbiting Chandra X-ray Observatory and the Southern Astrophysical Research (SOAR) Telescope in Chile. The telltale spectrum of hydrogen reveals that 29 regions within the tail are full of stars that are very young, perhaps less than 10 million years old, the researchers report in the 10 December issue of Astrophysical Journal. The Chandra spacecraft also located two x-ray sources within the tail, another sign of star-forming activity.
The stars' youth means they cannot have formed in a galaxy. Instead, they must have formed in the cloud itself, which was ripped out of ESO 137-001 by so-called ram pressure, when the galaxy plunged through a zone of high-temperature gas within the Abell 3627 cluster. That also means much of the raw material for new stars was removed from the galaxy itself, severely inhibiting its own ability to form stars. "Our observations have revealed a new mechanism for populating intergalactic space with stars," says Michigan State University astronomer and co-author Mark Voit.
It's an intriguing find, says astrophysicist Edwin Turner of Princeton University. But he cautions that because it's only a single example, "it's difficult to judge" whether this is a common phenomenon or an isolated case "due to rare circumstances of some sort."
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