Wednesday, January 16, 2008

A collision in store for our galaxy

Image Credit: Bill Saxton, NRAO/AUI/NSF

Today we continue on an expanding view of science from last week's meeting of the American Astronomical Society. Today, we'll expand our view from the sun's corner of our galaxy to look at the Milky Way as a whole.

One of the continuing mysteries of our Galaxy involves the so-called "high-velocity clouds", which are giant clouds of hydrogen gas that visible only by the radio light they emit and are moving far too fast to be part of the Milky Way. Some astronomers have thought that these clouds are raw materials for galaxies raining down on the Milky Way for the first time. Others have suggested that these clouds are actually hundreds of thousands of light-years away and not associated with out galaxy. And still others have suggested that these clouds are bits of material thrown out of the Milky Way by supernova explosions, the death throes of massive stars, that are just now falling back on to our galaxy.

Of course, as readers of this blog may suspect, probably all three explanations are right in specific cases.

The cloud pictured above is called "Smith's Cloud" after Gail Smith, an astronomer at the University of Leiden in the Netherlands in the early 1960s. Ms. Smith has since married and left the field of astronomy, but her cloud proved to be one of the enigmatic high-velocity clouds. Previous studies have found no stars in the cloud whatsoever, but these studies have detected the cloud glowing in optical light from hydrogen gas. This faint glow was consistent with the cloud being close to the Milky Way, as light from bright stars in the Milky Way can cause nearby hydrogen to glow.

Recently, a team of astronomers led by Jay Lockman used the new 300-foot-wide Byrd Green Bank Telescope , a telescope that looks at radio waves, to study Smith's Cloud. Their picture is above. Because the radio telescope allows them to get the speed of gas (from the Doppler Effect) as well as the image of the gas. From their work, Lockman and his team were able to piece together exactly how each part of the cloud is moving.

What they found was clear evidence that the cloud is indeed starting to come into contact with the Milky Way. Some parts of the cloud are being accelerated by the Milky Way's gravity, while other parts are slowing down as they run into gas from our own Milky Way Galaxy. All of these interactions allowed Lockman's team to do a complicated analysis that not only proved the cloud is falling toward the Milky Way, but also how far away it is, how big it is, and what the future holds for the cloud.

In summary, the cloud is about 8000 light-years away from our Galaxy and has enough gas to make a million stars the size of our sun. The cloud will hit the disk of the Milky Way face on in about 20 million years, which should cause a lot of that now-quiet gas to start to collapse and turn into stars. The rest of the gas will be mixed into our galaxy's private gas reserves to make stars long into the future. The impact site will be thousands of light-years away from us, though, so our descendants will have to travel quite a ways to watch the slow-motion collision in person.

An open question is, where did this gas come from? Maybe it is new material that has never been part of a galaxy before, or maybe it is a remnant of gas from a small galaxy that the Milky Way tore apart long ago. The gas is probably not the remains of old supernovae falling back into the Milky Way -- there's just too much of it, and it is a little too far away. It may not be possible to know for sure for a long time to come.

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