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The mystery of the million-mass cloud.


GAIL BIEGER-SMITH IS 69. She lives a quiet life in Wassenaar, a small, wealthy town west of Leiden, the Netherlands. She never expected to be haunted again by her brief astronomy career 45 years ago, but in early January she started getting phone calls from reporters and radio astronomers. The extragalactic cloud she discovered in 1963 had been found to be on a collision course with our Milky Way Galaxy. Some 20 to 40 million years from now, a million Suns' worth of hydrogen gas will plow into the galactic plane, likely setting off a huge burst of star formation in the Perseus Arm about a quarter of the way around the galaxy from us. Long forgotten, Smith's Cloud was suddenly headline news.

The news media always love a collision. The fact is, many other "high-velocity clouds" are also falling toward the galactic plane with no one noticing except specialists. But this one really is special, and its story highlights how the modern picture of our galaxy's evolution has been coming together.

Smith's Cloud

Gail Smith grew up in Vermont but decided to pursue a master's degree in astronomy in the Netherlands. "Leiden was an astronomical Mecca," she recalls. Under the leadership of Jan Oort, Dutch astronomers were at the forefront of probing and mapping our galaxy with the first generation of radio telescopes. Smith used the venerable 25-meter Dwingeloo radio dish, which was briefly the largest in the world after its inauguration in 1956. With it she discovered a huge, extragalactic cloud of hydrogen in Aquila that did not share the Milky Way's general motion, but was receding from us by 100 km per second.

Around that time she also married, got pregnant, and left astronomy. "It was my own decision," she says, "but Oort had never left any doubt about his idea that motherhood was incompatible with a career in astronomy."

In 2003 and 2004, Felix J. Lockman of the National Radio Astronomy Observatory used the new 100-meter Green Bank Telescope (GBT) in West Virginia to study the 21-cm radiation from neutral hydrogen in Smith's Cloud in much better detail than before. Last January he and his colleagues presented their results at the semiannual meeting of the American Astronomical Society. By measuring the radio spectrum at nearly 40,000 positions in the cloud, the team was able to deduce its properties and dynamics.

The cloud has a cometary shape extending more than 15[degrees] across the sky. In particular, Lockman's team confirmed its distance: 40,000 light-years from the Sun. Accurate distances are known for only a handful of high-velocity clouds. With the distance known, it was straightforward to calculate the cloud's dimensions--11,000 light-years long by 2,500 wide--and its position with respect to the Milky Way: some 9,000 light-years south of the galactic plane and 25,000 from the galactic center. And though the cloud is receding from the solar system due to our own motion, it's closing in on the Milky Way's plane at 70 km per second.

Team member Robert A. Benjamin (University of Wisconsin, Whitewater) says Smith's Cloud is the closest known high-velocity cloud to the Milky Way's disk. It has a total space velocity close to 300 km per second (much of it sideways with respect to the galaxy's plane). Lockman says its elongated shape probably results from tidal distortion as it falls deep into the Milky Way's gravity. It also seems to be starting to feel the drag of the galaxy's outer envelope of gas.

Despite the Green Bank observations, mysteries remain about the origin of high-velocity clouds. They may be blown into intergalactic space by multiple supernova explosions in the Milky Way's disk, only to fall back later. Or they may be primordial blobs of extragalactic gas that are just now falling in--a late, scaled-down version of the accretion of dwarf-galaxy-size gas clumps by which the entire Milky Way got built up starting not long after the Big Bang (S&T: October 2007, page 20).

Smith's Cloud may help settle the question--or not. It's probably too massive for the supernova explanation to work, says Lockman. Lifting a million solar masses high out of the Milky Way's plane would require an awful lot of energy. On the other hand, the cloud appears to share much of the Milky Way's rotational velocity, something unlikely in the pristine-infall scenario.

Future observations may reveal its heavy-element abundances, and that should settle the debate. Supernova-blasted stuff is rich with heavy elements. Primordial gas, little altered since the Big Bang, is not.

In all likelihood, both of these processes are separately in play. Other spiral galaxies have supernova-blown clumps and tendrils lofted from all across the faces of their disks, as in the photo at bottom left, and these must be either rising or falling or both. At the same time, astronomers keep finding ever more "star streams" of disrupted dwarf galaxies that have fallen into the Milky Way recently enough in cosmic history that they have not yet fully come apart.

Meanwhile, Gail Smith is still excited about the sudden interest in "her" cloud. "No, I've never regretted my decision to leave the field," she says. "But I'd love to witness the collision!"


Govert Schilling's latest book, The Hunt for Planet X, will be published in English later this year by Springer.
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Title Annotation:Milky Way Collision
Author:Schilling, Govert
Publication:Sky & Telescope
Geographic Code:4EUNE
Date:May 1, 2008
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