A Huge Gas Cloud Is Speeding Towards The Milky Way At 310KM Per Second

A composite false color image of the Smith Cloud. Saxton/Lockman/NRAO/AUI/NSF/Mellinger

Scientists have discovered where a giant gas cloud that's on a collision course with the Milky Way comes from, and the answer is closer to home than you might think.

Astronomers used the Hubble Space Telescope to determine for the first time the composition of the Smith Cloud, a giant gas cloud which will merge with the Milky Way in 30 million years. And they found the cloud is rich in heavy elements indicating that it originated in the Milky Way, not from intergalactic space.

The Smith Cloud, discovered in the 1960s, is travelling at over 310 kilometers (190 miles) per second, and if it were visible in the sky it would be 30 times larger than the full Moon. It is the only high-velocity cloud near our galaxy with a well-known orbit. Astronomers believe that when the cloud plows through us, it will generate 2 million new stars. These findings were published in the Astrophysical Journal Letters.  

The cloud doesn’t emit any light, so its composition cannot be estimated directly. The team was able to discover the composition by using background galaxies: as their light passes through the cloud some of the light gets absorbed at specific wavelengths which correspond to specific elements. By looking at how significant the absorption is, scientists can estimate the abundance of elements. They looked specifically at the absorption of sulfur.

“By measuring sulfur, you can learn how enriched in sulfur atoms the cloud is compared to the Sun,” said team leader Andrew Fox of the Space Telescope Science Institute in Baltimore in a statement.

It was long believed that the Smith Cloud originated in intergalactic space, away from stars. For this reason, scientists expected it to be only rich in hydrogen and helium. But they discovered that the cloud has a sulfur abundance similar to the Milky Way's outer disk, which indicated that the cloud was enriched by supernovae. The team was able to discover that the Smith Cloud had been expelled by the Milky Way about 70 million years ago, and it is now coming back.

The 100-million-year-long trajectory of the Smith Cloud. Credit: NASA, ESA and A. Feild 

This discovery has a large impact on understanding galaxy evolution, as it could help explain how galaxies continue to produce a large number of new stars for a long period of time. “We have found several massive gas clouds in the Milky Way halo that may serve as future fuel for star formation in its disk, but, for most of them, their origins remain a mystery,” said co-author Nicolas Lehner, from the University of Notre Dame, in the statement.

“The Smith Cloud is certainly one of the best examples that shows that recycled gas is an important mechanism in the evolution of galaxies.” 

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