NASA | STScI | HubbleSite | 2016 Jan 28
[img3="Illustration Credit: NASA, ESA, and A. Feild (STScI)Hubble Space Telescope astronomers are finding that the old adage "what goes up must come down" even applies to an immense cloud of hydrogen gas outside our Milky Way galaxy. The invisible cloud is plummeting toward our galaxy at nearly 700,000 miles per hour.
Science Credit: NASA, ESA, and A. Fox (STScI)"]http://imgsrc.hubblesite.org/hu/db/imag ... _print.jpg[/img3][hr][/hr]
Though hundreds of enormous, high-velocity gas clouds whiz around the outskirts of our galaxy, this so-called "Smith Cloud" is unique because its trajectory is well known. New Hubble observations suggest it was launched from the outer regions of the galactic disk, around 70 million years ago. The cloud was discovered in the early 1960s by doctoral astronomy student Gail Smith, who detected the radio waves emitted by its hydrogen.
The cloud is on a return collision course and is expected to plow into the Milky Way's disk in about 30 million years. When it does, astronomers believe it will ignite a spectacular burst of star formation, perhaps providing enough gas to make 2 million suns. ...
Astronomers have measured this comet-shaped region of gas to be 11,000 light-years long and 2,500 light-years across. If the cloud could be seen in visible light, it would span the sky with an apparent diameter 30 times greater than the size of the full moon.
Astronomers long thought that the Smith Cloud might be a failed, starless galaxy, or gas falling into the Milky Way from intergalactic space. If either of these scenarios proved true, the cloud would contain mainly hydrogen and helium, not the heavier elements made by stars. But if it came from within the galaxy, it would contain more of the elements found within our sun. ...
The astronomers found that the Smith Cloud is as rich in sulfur as the Milky Way's outer disk, a region about 40,000 light-years from the galaxy's center (about 15,000 light-years farther out than our sun and solar system). This means that the Smith Cloud was enriched by material from stars. This would not happen if it were pristine hydrogen from outside the galaxy, or if it were the remnant of a failed galaxy devoid of stars. Instead, the cloud appears to have been ejected from within the Milky Way and is now boomeranging back. ...
Giant Gas Cloud Boomeranging Back into Milky Way
University of Notre Dame | 2016 Jan 28
On the Metallicity and Origin of the Smith High-Velocity Cloud - Andrew J. Fox et al
- Astrophysical Journal Letters 816(1):L11 (2016 Jan 01) DOI: 10.3847/2041-8205/816/1/L11
arXiv.org > astro-ph > arXiv:1512.04957 > 15 Dec 2016