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In a pre-print submitted on November 3, 2009, two astronomers revealed that Smith's Cloud may have more than 100 times the previously estimate of one to two million Solar-masses and so should be classified as a dwarf galaxy (with a current "tidal mass" around 300 million Solar-masses). Not only has Smith's Cloud avoided disintegration in colliding with the Milky Way, but its trajectory indicates that it may have survived a previous collision with our galaxy some 70 million years ago; however, it is unlikely to survive a future collision in about 27 million years. Hence, the normal gaseous matter of Smith's Cloud may be surrounded by a much more massive, dark matter halo. Model simulations of galaxy formation indicate that a galaxy of the Milky Way's should be accompanied by around 1,000 dwarf galaxies (Nichols and Bland-Hawthorn, 2009; and Unknown, New Scientist, November 23, 2009).
At the 211th meeting of the American Astronomical Society on January 11, 2007, a team of astronomers (Felix "Jay" Lockman, Robert A. Benjamin, A.J. Heroux, Travis Fischer, and Glen I. Langston) described how the leading edge of a giant cloud of mostly hydrogen gas is already colliding with the gas surrounding the spiral disk of the Milky Way galaxy. If visible with the naked eye, the cloud would span 30 times the width of the full moon (or 15 degrees) -- almost as much sky as the Orion Constellation. Within 20 to 40 million years, the cloud will interact more visibly with the denser gas of the galaxy's Perseus arm about a quarter turn of the way from Sol (and farther out from the galactic core) and become spectacularly visible on Earth. The collision will eventually create shock waves which should trigger rapid star formation in a ring-shaped region, whose most massive stars will blow up as supernovae a few million years later. Some theories suggest that a 2,000-light-year-wide ring of bright stars and gas near Sol called Gould's Belt was created by a similar collision between a giant gas cloud and the spiral disk (more discussion from Ken Croswell).
The cloud was discovered by American astronomy student Gail P. Smith in 1963 from data gathered with the Dwingeloo Radio Observatory while Smith was working at Leiden University in the Netherlands (Gail P. Smith, 1963), where she still lives today as Gail Smith Bieger. Smith's cloud is about 11 thousand light-years (ly) long and 2,500 ly wide and contains around a million Solar-masses of gas. It is located as close as 8,000 ly from the galactic disk and moving at more than 150 miles or 240 kilometers per second (over 540,000 miles or 869,000 km per hour) at an angle of about 45 degrees relative to the disk. Moving in rotation with the galaxy, the cloud's enlongated shape and cometary appearance indicate that it is already colliding with the comparatively rarified gas of the inner galactic halo and feeling a tidal force from the galaxy's gravity (which pulls the front parts of the cloud with greater force than regions on the far side), and so the cloud may already be starting to tear apart and "rain" gas into the spiral disk (NRAO press release and Lockman et al, 2007).
Smith's cloud appears to have no visible stars. It appears to be made of hydrogen and helium gas and to lack almost no heavier elements. It's low metallicity suggests that it is composed of intergalactic gas that is being pulled in gravitationally by the Milky Way's mass as a first-time visitor, as part of the billions-of-years-old formation process of the galaxy.
Many "high-velocity" gas clouds with low metallicity have been found in the vicinity of the Milky Way, within the halo surrounding its spiral disk (more discussion at Hubblesite.org and Wakker et al, 1999). Astronomers began to spot such clouds about half-century ago with the construction of radio telescopes that were able to detect cold, neutral hydrogen gas, but these early observations were not accurate enough to determine a cloud's distance, mass, or direction of motion. Today, however, powerful radio telescopes such as the 100-meter Robert C. Byrd Green Bank Telescope are finally able to provide more detail. Requiring nearly 40,000 radio brightness measurements, the first object to be spotlighted in extreme detail is Smith's Cloud, which was chosen, in part, because of its curiously elongated shape and relative proximity (more from Science Now and University of Wisconsin-Whitewater press release).
et al, 1999;
Larger composite image.
Intergalactic clouds of gas
in the vicinity of the Milky
Way are pulled in and torn
apart before "raining" down
their gas onto the accreting
galactic disk (more).
Intergalactic Gas Clouds in the Visible Universe
In 2002, four teams of astronomers using NASA's Chandra X-ray Observatory found evidence that most of the normal ("visible") matter in the universe lies hidden in vast, hard-to-detect gas clouds between galaxies. Over the past 14 billion years since the Big Bang, these clouds have gathered into a spidery network of filaments connecting galaxies and galaxy clusters. Observations of the quasars PKS 2155-304 and H1821+643 revealed various parts of the intervening system of hot gas, of which one appears to be a filament in which the Milky Way and Andromeda galaxies are embedded. The four teams using beacons of X rays from distant quasars to probe the contents of several intergalactic clouds and found that the clouds contain twice as much normal matter as galaxies do. On their way to Earth, X-rays from the quasars are absorbed by ionized oxygen and other ions within the intergalactic clouds, and so the strength of their absorption reveals the temperature, density, and mass of the intervening gas clouds. The gas clouds probed ranged in temperature from 300,000° to 5 million° C. Although ultraviolet detectors had been previously used to probe the coolest components of this gas, the x-ray studies confirmed computer simulations which had predicted that most of the normal matter in clouds actually has higher temperatures and can best be identified by X-ray detectors (CXC press release; Fang et al, 2002; Nicastro et al, 2002; and Zappocasta et al, 2002).
For more discussion of intergalactic and other high-velocity gas clouds, see astronomer Eric D . Miller's images of high-velocity gas clouds around nearby spiral galaxies M51 and M83, a related University of Michigan 2004 press release, AAS presentation, and thesis summary.
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