This composite image shows the size and location of the Smith Cloud on the sky. The cloud appears in false-color, radio wavelengths as observed by the Robert C. Byrd Green Bank Telescope in West Virginia. The visible-light image of the background star field shows the cloud’s location in the direction of the summer constellation Aquila. The cloud is 15 degrees across in angular size — the width of an outstretched hand at arm’s length. The apparent size of the full moon is added for comparison. Credit: B. Saxton and F. Lockman (NRAO/AUI/NSF), and A. Mellinger
New Hubble telescope observations suggest a high-velocity H gas cloud was launched from the outer regions of our own galaxy around 70 million years ago. Now, the cloud is on a return collision course and is expected to plow into the Milky Way’s disk in about 30 million years at nearly 700,000 miles/ hour. Astronomers believe it will ignite a spectacular burst of star formation, perhaps with enough gas to make 2 million suns.
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. 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 an example of how the galaxy is changing with time,” explained Andrew Fox, Space Telescope Science Institute in Baltimore, Maryland. “It’s telling us that the Milky Way is a bubbling, very active place where gas can be thrown out of one part of the disk and then return back down into another….Our galaxy is recycling its gas through clouds, the Smith Cloud being one example, and will form stars in different places than before. Hubble’s measurements of the Smith Cloud are helping us to visualize how active the disks of galaxies are,” Fox said.
The 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 Smith Cloud might be a failed, starless galaxy, or gas falling into the Milky Way from intergalactic space. If these scenarios proved true, the cloud would contain mainly H and He, 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 team used Hubble to measure Smith Cloud’s chemical composition for the first time, to determine where it came from. They observed the UV from the bright cores of 3 active galaxies that reside billions of light-years beyond the cloud. Using Hubble’s Cosmic Origins Spectrograph, they measured how this light filters through the cloud. In particular, they looked for sulfur in the cloud which can absorb UV. “By measuring sulfur, you can learn how enriched in sulfur atoms the cloud is compared to the sun,” Fox explained. Sulfur is a good gauge of how many heavier elements reside in the cloud.
The astronomers found that the Smith Cloud is as rich in sulfur as the Milky Way’s outer disk, 40,000 light-years from the galaxy’s center (about 15,000 light-years farther out than our sun and solar system), ie 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.
Though this settles the mystery of the Smith Cloud’s origin, it raises new questions: How did the cloud get to where it is now? What calamitous event could have catapulted it from the Milky Way’s disk, and how did it remain intact? Could it be a region of dark matter – an invisible form of matter – that passed through the disk and captured Milky Way gas? The answers may be found in future research. http://hubblesite.org/newscenter/archive/releases/2016/04
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