|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.|
Though hundreds of enormous, high-velocity gas clouds whiz around the outskirts of our galaxy, this particular hydrogen gas cloud known as 'Smith Cloud' is unique because its trajectory is well known. New 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.
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.
|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.|
"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.
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 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.
|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.|
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.
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.
image credit:- nasa/esa
articale reference:- hubblesite.org