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Giant cloud's gas native to our galaxy.

A massive cloud falling toward our galaxy's disk is likely from the Milky Way itself, not a visitor, a new study shows. This object, called the Smith Cloud, lies 40,000 light-years away and contains 2 million Suns' worth of gas, primarily hydrogen. It's falling toward the galactic disk at nearly 300 km/s (660,000 mph).

The Smith Cloud is one of the best characterized high-velocity clouds, speedy wisps that rain down as a light drizzle onto our galaxy. This shower helps fuel star formation (S&T: Sept. 2015, p. 16). But despite these clouds' importance, astronomers know a paltry amount about what they're made of and where they come from.

Andrew Fox (Space Telescope Science Institute) and colleagues used the Hubble Space Telescope and the 91-meter (300-foot) Green Bank Telescope to better understand the Smith Cloud's makeup. The astronomers took advantage of three distant active galactic nuclei (AGN) that lie behind the cloud from our perspective. With Hubble, the team observed the AGN through the cloud to see how well the gas absorbed certain wavelengths from the accreting black holes' light. Which wavelengths the gas absorbs, and how deeply, reveals how tainted the cloud is by elements heavier than helium, known as metals in astrospeak. The researchers found that the Smith Cloud contains about half the concentration of metals that the Sun has.

But metals come from stars--they're synthesized in stellar cores or created when stars die--and astronomers don't know of a single star in the Smith Cloud. So it's highly unlikely that this object is a chewed-up dwarf galaxy or a pristine, first-time visitor from intergalactic space.

However, the heavy-element level is a close match for the gas in the Milky Way's own outer disk. The team argues in the January 1st Astrophysical Journal Letters that the cloud's gas was either spewed or torn out of our galaxy. And given the cloud's orbit, it either passed through or came from the outer disk about 70 million years ago. So the Milky Way might be feeding on itself.

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Title Annotation:MILKY WAY; Smith Cloud
Author:Carlisle, Camille M.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jun 1, 2016
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