Scientists using the NASA/ESA Hubble Space Telescope have found faint chemical traces of our Milky Way Galaxy lurking inside the Smith Cloud, a gigantic streamer of hydrogen gas that will crash into the Galactic disk in about 27 million years.
This composite image shows the size and location of the Smith Cloud on the sky. 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. The Smith Cloud appears in false-color, radio wavelengths. The visible-light image of the background star field shows the cloud’s location in the direction of the summer constellation Aquila. Image credit: NASA / ESA / Z. Levay, STScI / B. Saxton / F. Lockman / NRAO / AUI / NSF / A. Mellinger.
The Smith Cloud, also known as WV 360 and HVC 040-15, is a so-called gaseous high-velocity cloud. It was discovered in 1963 by the Dutch astronomer Gail Bieger.
This object is approximately 40,400 light-years away, 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 is unique among other high velocity clouds because its orbit is well known, thanks to earlier studies with radio telescopes.
The comet-shaped starless gas cloud is plummeting toward our Galaxy at nearly 700,000 miles per hour. It is following a ballistic trajectory and will plow back into the Milky Way’s disk in 27 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.
Scientists long thought that the Smith Cloud might be some starless dwarf galaxy or gas falling into the Milky Way from intergalactic space.
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 measurements show that the cloud, because of its chemical composition, 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. Field & A. Fox, STScI.
New Hubble observations suggest it was launched from the outer regions of the Galactic disk 70 million years ago.
Dr. Andrew Fox of the Space Telescope Science Institute and co-authors used Hubble’s Cosmic Origins Spectrograph to determine for the first time the amount of heavier elements relative to hydrogen in the cloud.
The team observed UV light from the bright cores of three active galaxies that reside billions of light-years beyond the Smith Cloud.
The cloud absorbs some of the light in very small wavelength range, and by measuring the dip in brightness of these galaxies behind the cloud, the chemical makeup of the cloud can be estimated.
The astronomers looked specifically for absorption from the sulfur element, which is a good gauge of how many heavier elements reside in the cloud.
“By measuring sulfur, you can learn how enriched in sulfur atoms the cloud is compared to the Sun,” Dr. Fox explained.
He and his colleagues found that the Smith Cloud is as rich in sulfur as the Milky Way Galaxy’s outer disk, a region about 40,000 light-years from the Galactic center and about 15,000 light-years farther out than our Sun and Solar System are.
This means that the cloud was polluted by material from stars. This would not happen if it were pristine hydrogen from outside the Galaxy.
Instead, the cloud appears to have had an intimate relationship with the Milky Way, but was somehow ejected from the outer disk about 70 million years ago and is now boomeranging back onto its disk.
The infalling Smith Cloud does not emit light at wavelengths that Hubble is sensitive to. However, Hubble’s Cosmic Origins Spectrograph can measure how the light from distant background objects is affected as it passes through the cloud. These measurements yield clues to the chemical composition of the cloud. By using these intergalactic forensics, the astronomers trace the cloud’s origin to the disk of our Milky Way Galaxy. Combined UV 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. Field & A. Fox, STScI.
Though this settles the mystery of the 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 cloud has all the characteristics of a small, infalling dark-matter dominated galaxy, but its gas looks like it came from the outer parts of the Milky Way,” said team member Dr. Jay Lockman, of the National Radio Astronomy Observatory.
“There are theoretical calculations suggesting that a dark matter satellite could capture gas as it passes through the Milky Way disk and that may be the amazing circumstance we are witnessing.”
The findings were published December 30, 2015 in the Astrophysical Journal Letters (arXiv.org preprint).
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Andrew J. Fox et al. 2016. On the Metallicity and Origin of the Smith High-Velocity Cloud. ApJ 816, L11; doi: 10.3847/2041-8205/816/1/L11
