Milky Way Hydrogen Clouds
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Milky Way Hydrogen Clouds

December 8, 2010
Artist's conception showing regions of the Milky Way studied by researchers, with hydrogen clouds more abundant in the region above the area where the central bar merges with the spiral arm; bright point at bottom center is location of our solar system.

This discovery of gas clouds in and above the Milky Way that have preferred locations has given astronomers a key clue about the origin of such clouds, which play an important part in galaxy evolution.

More about this Image The discovery of these abundant hydrogen gas clouds was made by H. Alyson Ford of the University of Michigan, whose Ph.D. thesis research from Swinburne University formed the basis for this work, working in collaboration Felix J. Lockman of the National Radio Astronomy Observatory (NRAO) and Naomi Mclure-Griffiths of CSIRO (Commonwealth Scientific and Industrial Research Organisation) Astronomy and Space Science.

"We've concluded that these clouds are gas that has been blown away from the galaxy's plane by supernova explosions and the fierce winds from young stars in areas of intense star formation," said Ford.

Ford and the team studied gas clouds in two distinct regions of the Milky Way, that were located between 400 and 15,000 light years outside the disk-like plane of the galaxy. The disk contains most of the galaxy's stars and gas, and is surrounded by a "halo" of gas more distant than the clouds the astronomers studied.

First detected with the National Science Foundation's Robert C. Byrd Green Bank Telescope, the clouds are in a transitional area between the disk and the halo, and their origin has been uncertain. The research team used data from the Galactic All-Sky Survey, made with CSIRO's Parkes radio telescope in Australia.

When comparing the observations of the two regions, the astronomers saw that one region contained three times as many hydrogen clouds as the other. In addition, that region's clouds are, on average, twice as far above the galaxy's plane. The astronomers believe this difference is due to the fact that the region with more clouds lies near the tip of the galaxy's central "bar," where the bar merges with a major spiral arm. This is an area of intense star formation, containing many young stars whose strong winds can propel gas away from the region. The most massive stars also will explode as supernovae, blasting material outward. In the other region they studied, star formation activity is more sparse.

The clouds consist of neutral hydrogen gas, with an average mass equal to that of about 700 suns. Their sizes vary greatly, but most are about 200 light-years across. The astronomers studied about 650 such clouds in the two widely-separated regions of the galaxy. (Date of Image: 2010)

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