Cloudy Findings: A new population shows up in the Milky Way

A radio telescope has detected hundreds of hydrogen clouds in the gaseous halo that surrounds the disk of our galaxy. This previously unknown population may have been lofted into the halo by a galactic fountain–powerful winds from supernova explosions within the disk.

CLOUD CROWD. An artist’s rendition of an edge-on view of the Milky Way (middle and left) shows hundreds of hydrogen clouds floating above the disk of the galaxy, some 15,000 light-years from Earth. Actual telescope data are shown in the cutaway (right). NSF/K. Woellert; NRAO/AUI

Astronomer Felix J. Lockman of the National Radio Astronomy Observatory in Green Bank, W. Va., discovered the clouds while searching the halo for 21-centimeter radio-wave emissions–a signature of atomic hydrogen. Using the Robert C. Byrd Green Bank Telescope, Lockman found the clouds and examined 38 of them in detail. They have an average diameter of 100 light-years and weigh between 50 to 100 times as much as the sun.

But it’s another property that intrigued Lockman. Although the clouds lie 5,000 light-years above the galaxy’s disk, they rotate in lockstep with it, he reports in an upcoming Astrophysical Journal Letters.

The similarity in rotation strongly suggests that the clouds originated in the disk, Lockman notes. The clouds would have different velocities if they were interlopers from beyond the galaxy. “These are home-grown objects,” he says.

If the clouds were driven from the disk into the halo, it’s possible that some are now raining back into the disk. If so, Lockman says, maintaining a steady population of halo clouds would require a succession of supernova explosions in the disk. The number of supernovas–the products of the collapse of massive stars–in the disk is enough to do the job, he calculates.

The new data support models in which “supernova explosions push stuff up in the halo and then things condense and rain down,” says Carl E. Heiles of the University of California, Berkeley. “The theory has been around awhile, [but] this is one of the few observational indications.”

Although observations with other radio telescopes had discovered hydrogen gas in the galaxy’s halo, “those measurements provided very little information about the actual spatial structure of the gas,” comments Blair D. Savage of the University of Wisconsin–Madison. Astronomers had speculated that the gas was like a mist held in place by magnetic fields or cosmic rays streaming out of the galactic disk.

Using the Green Bank telescope, Lockman found that the mist actually is hundreds of clouds whose density is about 100 times that of their surroundings. The clouds gave Lockman discrete structures he could analyze. “It’s like the difference between finding a pile of rocks and a pile of dirt,” Lockman says.

“The high sensitivity of the Green Bank telescope . . . should provide many new, interesting insights about the complete circulation of gas from the galactic disk into the halo and back,” says Savage. One puzzle is how material heated by supernovas, carried by their winds, and subsequently cooled in the halo could maintain the same rate of rotation as the material in the disk.

The clouds discovered by Lockman are so numerous that many are likely to absorb light from background stars, notes Savage. Measuring the specific wavelengths of light they absorb should make it possible to study the composition of the clouds. If the clouds are indeed driven by supernovas, they should contain an abundance of elements produced in those explosions. Spectrographs aboard the Far Ultraviolet Spectroscopic Explorer and the Hubble Space Telescope are ideal for such studies, says Savage.


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