Some Star Clusters Go It Alone

Flying solo. Four of the newly discovered intergalactic clusters (in boxes).

After a half-century-long hunt, astronomers think they have finally detected rogue star clusters that roam the vast dark spaces between galaxies. The international team of scientists says these hundreds of vagabonds could just be the tip of the iceberg and could help solve long-standing galactic mysteries.

Most of the universes' galaxies, which each contain billions of stars, are surrounded by up to several thousands of so-called globular clusters, groups of up to a million suns packed into dense spheres by gravity. For decades, astronomers have also detected scattered light coming from the huge black void between galaxies, and they suspected it came in part from free-roaming clusters. It was just a guess, though, because no tools were sensitive enough to make out the origin of the light.

After searching through images obtained from the Hubble Space Telescope's brand-new Advanced Camera for Surveys and the 10-meter Keck Telescope in Mauna Kea, Hawaii, astronomer Michael West of the University of Hawaii, Hilo, and his colleagues in the United States and United Kingdom found what may be more than 300 intergalactic globular clusters, the farthest ones roughly 400 million light-years away. The number uncovered agrees well with predicted figures for the tiny patch of sky probed, "meaning there should be loads and loads of these things throughout the universe," West says. He presented his team's findings on 17 July at the International Astronomical Union's 25th General Assembly in Sydney, Australia.

"This is pretty heroic work,” says astronomer David Hanes of Queen's University in Kingston, Ontario. “You have to look at the emptiness between galaxies just for indistinguishable specks." For incontestable evidence that each faint dot spotted is an intergalactic globular cluster, Hanes says ground telescopes will need to gather precise details on each cluster's velocity, to confirm they are not actually orbiting galaxies.

The researchers speculate that the vagabond clusters were likely ripped loose from their parent galaxies by cosmic mayhem, such as the cannibalization of one galaxy by another. "This opens up whole new possibilities to study the death of galaxies, a mostly unexplored area," West says. Some clusters may even have formed by themselves in the intergalactic wasteland, he says. Learning more about the loners could help solve the decades-old puzzle of why some galaxies apparently possess far more clusters than expected for their size; these orphans could be adding to the count, West notes.

Related sites
Learn more about globular clusters
Michael West's page, including more pictures of the intergalactic globular clusters

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