New sky map: Look, Ma, no Milky Way!

Milky Way, begone!

Bright galaxies and gas clouds in the southern sky imaged by HIPASS. These galaxies, some newly visible, lie in strings and sheets rather than fill the sky uniformly. Images: B.S. Koribalski,

In this image, which records hydrogen gas from our galaxy and its neighbors, the Milky Way galaxy (bottom) pulls gas from two nearby galaxies (upper left).

Among the string of dots at the right is a previously unseen gas cloud.

On a clear night, the starlit swath of our home galaxy stretches across the sky like a diamond bracelet. But to

astronomers trying to look at galaxies beyond our own, the dust and stars of the Milky Way present a major nuisance, blotting out visible light from some 15 percent of the universe.

A new sky map provides the first unobscured view of an extensive portion of the heavens. Using the radio telescope in Parkes, Australia, astronomers have imaged the entire southern sky at the so-called HI radio wavelength emitted by atomic hydrogen gas. Unlike visible light, this radiation passes unimpeded through the Milky Way, revealing the myriad galaxies behind it.

Astronomers had already used this method to peer through the Milky Way’s murk, but the new survey is the first to provide positions and distances of galaxies and gas clouds over a wide area, out to 500 million light-years. “It certainly gives a better picture of the local universe,” comments Ofer Lahav of the University of Cambridge in England.

Although our galaxy is rich in hydrogen, astronomers can easily distinguish its emissions from those of other  galaxies.

Cosmic expansion shifts the radio waves emitted by external galaxies to longer wavelengths, notes Lister Staveley-Smith of the Australia Telescope National Facility in Epping. His team unveiled the map May 23 in Socorro, N.M., at a meeting on gas and galaxy evolution.

By detecting gas clouds that emit little visible light yet that each weigh as much as several hundred million suns, the team may have found building blocks left over from the formation of the Milky Way and its neighbors, Staveley-Smith says. The study, known as HIPASS (HI Parkes All-Sky Survey), thus is yielding a more accurate assessment of the cosmic amount of ordinary matter—material made of neutrons and protons—and in turn, the amount of unseen dark matter the cosmos contains, notes Lahav.

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