Milky Way galaxy: Cloaked in a hot shroud?

Spacecraft observations indicate that a vast, unseen halo of hot gas envelops our home galaxy. Large enough that it literally could be brushing up against the Milky Way’s nearest galactic neighbors, the halo may be the remnants of the primordial material that built the Milky Way, researchers reported this week.

As gas clouds fall onto the Milky Way and encounter a vast corona of hot gas (light pink), oxygen atoms at the clouds’ edges heat up. STScI/NASA/JHU

Kenneth R. Sembach of the Space Telescope Science Institute in Baltimore and his colleagues base their findings on observations of gas clouds known to be raining down on our galaxy (SN: 1/25/97, p. 55). His team examined 60 clouds at far-ultraviolet wavelengths, which for the first time, revealed the clouds’ hot, outer edges.

Sembach’s group used distant quasars as a source of ultraviolet light. En route to Earth, some of the quasar light passes through a gas cloud and is absorbed. Dips in the quasars’ spectra recorded by the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, which was launched in 1999, show that the absorption comes from oxygen atoms stripped of five of their eight electrons.

The energy required to strip away so many electrons suggests that as the clouds fall toward the Milky Way, they plow through an extremely hot gaseous medium, Sembach says. When the clouds encounter this high-temperature region, which cloaks the galaxy, their outer edges become extremely hot and atoms become highly ionized.

The absorption dip indicates that the clouds barrel through the halo, known as the galactic corona, at 224 kilometers per second. Although the corona can’t be seen directly, the energy it imparts to the clouds indicates it has a temperature of at least 1 million kelvins and weighs as much as 100 million suns, Sembach reported at a meeting of the American Astronomical Society in Washington, D.C.

The corona extends at least 150,000 light-years above and below the plane of our spiral galaxy. By comparison, one of the Milky Way’s nearest neighbors, the Large Magellanic Cloud galaxy, lies about 170,000 light-years from Earth.

Other studies have shown that at least some of the infalling clouds contain fewer heavy elements than do gas clouds contained within the Milky Way. This suggests that the clouds examined by Sembach’s team originate outside the galaxy. In contrast, a much smaller and cooler halo previously discovered by FUSE may have originated in the Milky Way.

The origin of the newfound halo fits with a theory, proposed by Leo Blitz of the University of California, Berkeley and his colleagues, that the infalling clouds are leftovers from our galaxy’s formation. As a huge reservoir of gas condensed to build the Milky Way, its outer regions may have heated up, creating the high-temperature halo. Previous observations hint that some other galaxies also have hot coronas, which could be remnants of their own formation, Blitz notes.

If the Milky Way’s halo is large enough, it may map out an even bigger halo of dark matter that’s believed to surround our galaxy, Blitz notes. Dark matter–exotic, invisible material that would have clumped together much earlier in cosmic history than ordinary, visible matter did–may have served as the scaffolding for the gas and stars that became the Milky Way and all the other galaxies in the cosmos.

The new data could be the last that FUSE acquires for a while. The satellite shut down Dec. 10, 2001, after two of its four reaction wheels failed. Researchers are now trying to find alternative ways to steer it.

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