An immense, invisible cloud of hydrogen gas currently outside our Milky Way galaxy is plummeting toward our galaxy at nearly 700,000 miles (1,152,641 km) per hour. The cloud – known as the Smith Cloud – appears to have been ejected from within the Milky Way and is now boomeranging on a return collision course. It’s 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 new suns. That’s according to new research by a team of Hubble Space Telescope astronomers published in January, 2016 in The Astrophysical Journal Letters.
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.
It’s located in the direction to the constellation Aquila the Eagle in our sky.
Though hundreds of enormous, high-velocity gas clouds whiz around the outskirts of our galaxy, the 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.
Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland is the team leader. Fox said:
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.
Astronomers used to think 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 the cloud came from within our galaxy, it would contain more of the elements found within our sun.
The team used Hubble to measure the Smith Cloud’s chemical composition for the first time, to determine where it came from. They observed the ultraviolet light from the bright cores of three 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 ultraviolet light. Fox explained:
By measuring sulfur, you can learn how enriched in sulfur atoms the cloud is compared to the sun. 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, 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). The team suggests that 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 researchers say, the cloud appears to have been ejected from within the Milky Way and is now boomeranging back.
Though this might settle 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? Future research will try to answer these questions.
Bottom line: Research by a team of Hubble Space Telescope astronomers published January 1, 2016 in The Astrophysical Journal Letters suggests that the Smith Cloud came from within our galaxy and is now falling back toward it. It will plow into the Milky Way in about 30 million years and ignite a spectacular burst of star formation, perhaps enough gas to make 2 million suns.