A large cloud of hydrogen gas is on a return collision course to the Milky Way galaxy, running towards it with a speed of almost 1 126 540 km/h (700 000 mph) and is expected to hit it in about 30 million years. The new Hubble Space Telescope observations suggest the giant, high-velocity hydrogen gas cloud, was launched from the outer regions of the galactic disk around 70 million years ago and is now boomeranging back.
The so-called Smith Cloud was discovered in the early 1960s by Gail Smith, a doctoral astronomy student at the time, who recorded the radio waves emitted by the cloud's hydrogen.
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 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 constellation Aquila. Image credit: Saxton/Lockman/NRAO/AUI/NSF/Mellinger
When the cloud hits our galaxy it will most likely ignite a spectacular burst of star formation, perhaps with enough gas to produce 2 million suns.
"The cloud is an example of how the galaxy is changing with time. 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," explained team leader Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland.
This diagram shows the 100-million-year-long trajectory of the Smith Cloud as it arcs out of the plane of our Milky Way galaxy and then returns like a boomerang. Hubble Space Telescope measurements show that the cloud came out of a region near the edge of the galaxy's disk of stars 70 million years ago. The cloud is now stretched into the shape of a comet by gravity and gas pressure. Following a ballistic path, the cloud will fall back into the disk and trigger new star formation 30 million years from now. Image credit: NASA/ESA/A. Feild (STScI)
"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."
The comet-shaped region of gas is about 11 000 light years long and 2 500 light years across. If the cloud could be seen in visible light, it would spread across the sky in an apparent diameter 30 times greater than the size of the full moon.
The scientists have previously thought the Smith Cloud could be a failed, starless galaxy, or gas, falling into the Milky Way from intergalactic space. However, in that case, it would mainly contain hydrogen and helium, not heavier elements made of stars.
Hubble's Cosmic Origins Spectrograph can measure how the light from distant background objects is affected as it passes through the cloud, yielding clues to the chemical composition of the cloud. Astronomers trace the cloud's origin to the disk of our Milky Way. Combined ultraviolet and radio observations correlate to the cloud's infall velocities, providing solid evidence that the spectral features link to the cloud's dynamics. Image credit: NASA/ESA/A. Feild (STScI)
Observations conducted by the Hubble Space Telescope detected the ultraviolet light from the bright cores of three active galaxies billions of light-years beyond the cloud. The scientists have used Hubble's Cosmic Origins Spectrograph to measure how the light filters through the cloud, searching for sulfur, in particular, as it absorbs ultraviolet light.
"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," Fox said.
The measurements showed that the cloud is as rich in sulfur as the Milky Way's outer disk, located approximately 40 000 light years away from the galaxy's center. According to the results, the cloud seems to have been ejected from within the Milky Way and is now boomeranging back.
Now that the Smith Cloud's origin has been resolved, new questions arise, such as how did the cloud get to its current location, what calamitous event could have catapulted it out from the disk of our galaxy, and how did it remain intact? One other question is whether the Milky Way's gas could have been captured by a region of dark matter that passed through the disk. Hopefully, further research will provide us with more answers.
Featured image: Hubble's Cosmic Origins Spectrograph can measure how the light from distant background objects is affected as it passes through the cloud, yielding clues to the chemical composition of the cloud. Astronomers trace the cloud's origin to the disk of our Milky Way. Combined ultraviolet and radio observations correlate to the cloud's infall velocities, providing solid evidence that the spectral features link to the cloud's dynamics. Image credit: NASA/ESA/A. Feild (STScI)
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