What are those mysterious gasses in our Milky Way?

The spiral galaxy M 83 as seen at radio wavelengths. Photo credit: University of Michigan and National Radio Astronomy Observatory Click for larger image

ANN ARBOR—University of Michigan astronomers have found evidence for clouds of high-speed gases around galaxies that may help scientists understand the mysteries of similar material in our own Milky Way galaxy. Postdoctoral fellow Eric Miller and astronomy professor Joel Bregman presented their results Jan. 8 at the 203rd meeting of the American Astronomical Society in Atlanta, Georgia, where they described their studies of the faint radio and optical signals from two nearby galaxies.

The Milky Way galaxy has a disk of stars and gas that rotates in an orderly fashion, but the galaxy also contains clouds of neutral gas that are not part of this motion. “These high velocity clouds have been a mystery for some time, mainly because we do not know their distance, which is necessary to determine their mass,” Miller said. Both mass and distance provide clues to the clouds’ origins. Observing clouds of this type in another galaxy solves the distance problem because the clouds are at nearly the same distance as the galaxy being observed. However, radio emissions from the clouds, which provide other essential information for calculating cloud mass, are dim, presenting a challenge.

Miller and Bregman met the challenge by carefully selecting two galaxies to study: Messier 51 (the Whirlpool galaxy) in the constellation Canes Venatici (the Hunting Dogs) and Messier 83 in the constellation Hydra (the Water Snake)—two of the nearest spiral galaxies that resemble our own Milky Way galaxy. In addition to being “magnificent” to view in optical light, “they are two of the very best galaxies to study in radio waves, where we have a great view of their disk of spinning neutral gas,” Miller said. Since many of the high velocity clouds appear projected on the disk, studying them is easier when the disk is readily observed.

The high velocity clouds of neutral hydrogen that the astronomers were seeking account for only about one percent of the gas in a galaxy. The rest of the gas in the galaxy is also neutral, but rotates quietly around the galaxy. Like the gas in the high velocity clouds, it gives off radio waves (similar to those used in cordless telephones) that can be picked up with large radio telescopes. Miller and Bregman used the Very Large Array of radio telescopes at the National Radio Astronomy Observatory’s site in New Mexico. These 27 radio dishes, arranged in the shape of a Y that is 1 km (0.62 mile) across, work together to produce images of the sky in radio light.

“One of the big challenges was to identify faint blobs of emission and make sure that we weren’t confusing it with radio noise,” Miller said. But by recording images of the gases at different velocities, the astronomers were able to distinguish clouds that were moving peculiarly from the rest of the calmly moving gas. “We were able to identify individual high velocity clouds in these galaxies for the first time, and we also found that many more smaller clouds are present,” said Miller. In the galaxy Messier 83, the clouds appear to be projected on the galaxy disk, which indicates that the galaxy is making the clouds, said Bregman. “These clouds are probably produced by a galactic fountain, which occurs when normal disk gas is heated by supernovae (exploding stars), rises out of the disk, cools and falls back as clouds of neutral gas. It’s like a galactic weather pattern where the clouds are like snow or hail.”

The astronomers’ observations of the Whirlpool galaxy help explain the origin of high velocity clouds in the Milky Way. “If the Whirlpool galaxy has the same amount of high velocity gas as the Milky Way—and there is evidence that this is the case—then we can use that information to solve for the distance of the high velocity clouds in the Milky Way galaxy,” Miller said. “When we do that, the distance we obtain places the clouds in the halo of the Milky Way,” suggesting that the clouds formed from the galaxy, possibly when another galaxy passed nearby, pulling off outer parts from each galaxy and creating long streamers of gas.

A competing idea is that the clouds are gas that never formed into stars and that these clouds lay far from galaxies. “Our observations argue against that picture,” Miller said.

Related links:

Joel Bregman > National Radio Astronomy Observatory’s Very Large Array > American Astronomical Society > Additional images >

American Astronomical Society >Additional images >