Mysterious clouds of gas hovering above the plane of the Milky Way may be the fractured remnants of superbubbles blown by stellar winds and exploding stars.

"There's a fundamental, interesting connection between gas far away from the Milky Way and the amount of star formation below it in the galactic plane," F. Jay Lockman of the National Radio Astronomy Observatory told Wired Science in a phone interview. The results could provide insight into how heavy elements traverse the galaxy and get incorporated into later generations of stars, planets and, perhaps, life.

The bulk of the matter in the Milky Way, including stars, hot star-forming regions and the gas and dust between stars called the interstellar medium, lies in a relatively flat disk called the galactic plane.

"It's a flattened system, kind of like a pierogi," Lockman said today at the American Astronomical Society in Miami.

The Milky Way also has a gaseous halo that extends above and below the galactic pastry. For years, astronomers expected the density of that gas to get thinner as it got farther from the Milky Way, the way Earth's atmosphere thins out at high altitudes. But earlier observations Lockman made at the Green Bank Telescope in West Virginia showed dense clouds hundreds of times more massive than the sun floating between the disk and the halo, hundreds to thousands of light-years above the galactic plane.

"This turned my whole conception of what was going on upside down," he said. "It's very much like seeing, all your life, a distant hillside that's covered in green fuzz. Then one day you get a pair of binoculars and you look and say, 'My God, there's trees!'"

To investigate these clouds further, Lockman and colleagues used data from the Parkes Observatory radio telescope in Australia of two regions of the Milky Way, one on either side of the galactic center from Earth's point of view.

Individual clouds in both regions looked about the same, Lockman said – on average they were 600 times the mass of the sun and spanned 30 to 40 light-years. But the region on the "northern" side, to the left of the galactic center, had three times as many clouds as the "southern" region.

"I thought we would see a difference between north and south, but I thought it would be pretty subtle," Lockman said. "It's not subtle at all."

It turned out the northern region included part of the bar of the Milky Way. A new survey of hydrogen gas in the galaxy that was presented at the same meeting confirmed this region is an active stellar nursery.

"At the end of the galactic bar emanating from the galactic center, there is a huge complex of star forming regions," said Tom Bania of Boston University.

The southern region, by contrast, fell between two spiral arms, "not particularly associated with anything at all," Lockman said.

Lockman and his colleagues concluded that the gas clouds were blown away from the galactic plane by stellar winds from these intense star-forming regions. When the more massive of these stars die, they explode as supernovas, blowing enormous bubbles of gas "like supersonic lava lamps," Lockman said. These bubbles pop like soap after they rise, leaving behind the mysterious clouds.

"There are still lots of questions," Lockman said. "But I think we've finally solved the question of their origin."

These clouds could also be responsible for transporting heavy elements around the galaxy. All elements heavier than hydrogen and helium are built in nuclear reactions inside stars, and are blown off into the interstellar medium when stars explode as supernovas. That material later condenses into new, metal-rich stars – and ultimately planets.

"When our galaxy formed, planets like the Earth could not form," Bania said. "Put the Earth in a blender and you've got silicon, magnesium, the stuff that's formed in supernovas."

If the clouds are in fact the remnants of supernova bubbles, Lockman said, then "it's quite possible that these clouds as they fall back to the Milky Way are the way that metals get mixed in through the disk, and this controls the overall evolution of the interstellar medium and the next generation of stars."

Image: Bill Saxton, NRAO/AUI/NSF

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