High-velocity clouds (HVCs) are enormous, floating blobs of hydrogen gas that wander throughout the entire halo of our barred-spiral Milky Way Galaxy. Our Milky Way, like other galaxies inhabiting the Cosmos, was born at a very ancient time–less than a billion years after the Big Bang that occurred about 13.8 billion years ago. HVCs are important to our scientific understanding of the many mysteries of galactic evolution because they contain a large quantity of the atomic (baryonic) matter that exists in galactic halos. In January 2016, Hubble Space Telescope (HST) astronomers announced that they have observed an invisible high-velocity cloud racing toward our Galaxy at the almost incredible speed of 700,000 miles per hour. Even though literally hundreds of similar, immense HVC gas clouds zip around the outskirts of our Milky Way, this so-called “Smith Cloud” is one-of-a-kind because its trajectory is well-known–and HST observations indicate that this speed-demon of a cloud was evicted from the outer regions of the Galactic disk around 70 million years ago!
Spiral galaxies, like our Milky Way, are surrounded by nearly spherical regions composed of thinly scattered stars, globular clusters, and wispy, tenuous gas–which are known as halos. Galactic halos stretch far beyond the primary, visible component of galaxies, and they seem to be devoid of dust. Also, their shining stellar inhabitants are generally elderly, ancient stars.
The speedy Smith Cloud was first spotted in 1963 by the Dutch astronomer Dr. Gail Bieger, nee Smith. Then an astronomy student at Leiden University in the Netherlands, she discovered the radio waves emitted by its hydrogen. The giant cloud is on a return collision course with our Milky Way, and it is expected to crash into our Galaxy’s disk in approximately 30 million years. When this blast occurs, astronomers believe that it will trigger a spectacular and brilliant explosion of beautiful baby star birth–perhaps providing enough gas to create 2 million new and dazzling neonatal stars!
Therefore, the boomerang-like Smith Cloud is rushing back towards our Galaxy, after having been tossed out. Many astronomers think that this celestial speed-demon will blast into our Galaxy’s disk at a 45 degree angle, and that its impact will result in that predicted explosion of brilliant baby star birth. However, there is an alternative possibility. The gigantic crash may form a supershell of neutral hydrogen.
As they tracked the Cloud’s trajectory through time, the scientists were able to determine that it had traveled through our Galaxy’s disk about 70 million years ago–and in order for it to have survived this treacherous earlier encounter, they suggest that it is lodged within a massive halo of the mysterious dark matter–whose identity has not yet been determined. Dark matter is much more abundant than “ordinary” atomic matter, and it is generally thought to be composed of as yet unidentified non-atomic particles that do not interact with light, or any other form of electromagnetic radiation–which renders it invisible.
By using the National Science Foundation’s Robert C Byrd Green Bank Telescope in West Virginia, radio astronomers have discovered that the Smith Cloud measures 9,800 light-years long by 3,300 light-years wide in projection.
The bulk motions of these speedy clouds display velocities which are measured in excess of 70-90 kilometers per second. These huge hydrogen clouds can be massive in size–some are as large as millions of times the mass of our Sun. HVCs cover large regions of the sky–and they have been detected in other galaxies, as well as our own. In addition, when these clouds tumble into the disk of a galaxy, they provide the precious material that can give birth to baby stars, adding to the dilute star-forming material already swirling in the disk. The new material helps to maintain the star formation rate of the galaxy.
The origins of the HVCs remain undetermined. No one existing theory is able to provide an explanation for all of the HVCs in our Galaxy. However, it is known that some HVCs probably form as the result of interactions between the Milky Way and its satellite galaxies, such as the Large and Small Magellanic Clouds, which have produced a well-known HVC called the Magellanic Stream. Because of the various possible origins of these huge clouds, there are still many unanswered questions pertaining to HVCs for astronomers to answer.
Mysterious and dense clouds of gas were first discovered outside of the Galactic plane back in the middle of the 1950s. This proved to be a very important discovery because the then-current models of our Milky Way showed the density of gas decreasing with increasing distance from the Galactic plane–and the discovery of these strange clouds presented a puzzle. This is because, according to then-prevailing Galactic models, the dense gas clouds should have fallen apart a very long time ago. A solution to this puzzle was provided in 1956, that suggested the dense blobs of gas were being stabilized by a searing-hot, gaseous corona that surrounds the entire Milky Way. The Dutch astronomer, Jan Oort, of Leiden University in the Netherlands, proposed that cold gas clouds might be detected within the Galactic halo—far, far away from the Galactic plane.
These cold clouds of gas were soon discovered less than a decade later as a result of their neutral hydrogen radio emissions. They were zipping toward the Galactic disk at an amazingly high velocity relative to other entities inhabiting the disk. The first two clouds that were discovered were dubbed Complex A and Complex C. The two speed-demons were called “high velocity clouds” because of their anomalous velocities–distinguishing them from both gas wandering around at more normal velocities, as well as from their somewhat slower cousins already named “intermediate velocity clouds.” Several astronomers suggested explanations–which ultimately proved to be inaccurate–in respect to the mysterious nature of the HVCs.
A northern-sky survey of neutral sky radio emissions was completed in 1988 by astronomers using the Dwingeloo radio telescope in the Netherlands. This survey uncovered still more HVCs.
In 1997, a map of our Galaxy’s neutral hydrogen was reaching completion. This map helped astronomers to discover more HVCs. In the late 1990s, using data derived from the La Palma Observatory in the Canary Islands, the HST, and, later, the Far Ultraviolet Spectroscopic Explorer (FUSE), the distance to an HVC was calculated for the very first time. At about the same time, the chemical composition of HVCs was first measured. In addition, in 2000, a southern hemisphere survey of neutral hydrogen radio emissions was finished by astronomers using the Villa Elisa radio telescope in Argentina from which even more of these rather abundant LVCs were detected.
Later observations of Complex C revealed that the speedy cloud, originally believed to have a low metallicity, contained some sections that possessed a higher metallicity compared to the rest of the cloud. This indicated to astronomers that it had begun to mix with other gas in the Galactic halo. In the terminology that astronomers use, metals are all of the atomic elements heavier than helium. The inflationary Big Bang birth of our Universe, almost 14 billion years ago, produced only the lightest atomic elements–hydrogen, helium, and some lithium and beryllium. All of the heavy metals were manufactured in the searing-hot cores of stars–or else in the supernova blasts heralding the raging stellar death throes of the most massive stars. Therefore, the higher percentage of metals in some sections of the gas cloud indicates that these sections have been more heavily “polluted” by heavy metals–manufactured by stars–than the rest of the cloud. Using observations of the highly ionized oxygen and other ions, astronomers were able to determine that the hot gas in Complex C is an interface between hot and cold gas.
HVCs generally represent the coldest and most dense constituents of the Galactic halo. However, the halo itself also sports a multi-phase structure: frigid, dense neutral hydrogen, warm and warm-hot gas, and searing-hot hydrogen gas. Because of this, cool clouds floating through the diffuse halo medium run the risk of being ionized, during the course of their travels, by the warmer and hotter gas. This can form a pocket of ionized gas that surrounds a neutral interior in an HVC. There is evidence of this cool-hot gas interaction occurring in the Galactic halo.
There are more than 100 billion galaxies dwelling within our observable Universe. The observable, or visible, Universe, is that relatively small region of the entire, and unimaginably enormous Universe, that we are able to observe. Most of the Cosmos is situated very far beyond what we can see, because the light emitted from those incredibly remote regions–residing so very far beyond our visibility–has not had sufficient time to reach us since the Big Bang.
In the primordial Universe, opaque clouds, composed primarily of hydrogen gas, congregated together along massive filaments of the Cosmic Web, which is made up of invisible, transparent dark matter. So-called “ordinary” atomic matter accounts for only 4.6% of the Universe, while the transparent, ghostly dark matter accounts for about 24% of it. Most of the Universe is actually composed of the bizarre dark energy. Indeed, it is currently proposed that dark energy accounts for about 71.4% of the Universe, and this weird stuff is causing the Universe to accelerate in its expansion.
Spiral galaxies, like our Milky Way, are composed of flat, rotating disks, that are set on fabulous fire by the sparkling brilliant light of a host of stars. The disk also harbors a rich abundance of both gas and dust. In addition, a spiral galaxy hosts a central concentration of stars–called a bulge–which is surrounded by a much dimmer halo of stars, many of which are the sparkling constituents of globular clusters. Spirals got their name from their pin-wheel-like spiral arms that reach out from the center of the disk.
The Strange Return Of A Wandering Cloud
“The (Smith) 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 HST team leader, Dr. Andrew Fox, in a January 28, 2016 HST Press Release. Dr. Fox is of the Space Telescope Science Institute in Baltimore, Maryland.
“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,” Dr. Fox continued.
This comet-shaped region has been measured by astronomers to be 11,000 light-years long and 2,500 light-years across. If this speedy Cloud could be observed in visible light, it would stretch across the sky with an apparent diameter 30 times greater than the size of a full Moon.
For years, many astronomers have thought that the Smith Cloud might either be a starless failure of a galaxy, or gas tumbling into our Milky Way from intergalactic space. If either of these two theories proved to be correct, the Cloud would be composed mainly of hydrogen and helium–and be bereft of the heavy metals created by stars. However, if it originated within our Milky Way, it would contain more of the heavier atomic elements found within our Sun.
The team of astronomers used HST to measure the Smith Cloud’s atomic chemical composition for the first time. This was done in order for them to determine where it really originated. The astronomers observed the ultraviolet light emanating from the brilliant cores of a trio of active galaxies that dwell billions of light-years beyond the Cloud. Using HST’s Cosmic Origins Spectrograph, the scientists then measured how this light filters through the Cloud.
In particular, the astronomers went on the hunt for signs of sulfur in the Cloud which can absorb ultraviolet light. “By measuring sulfur, you can learn how enriched in sulfur atoms the Cloud is compared to the Sun,” Dr. Fox continued to comment in the January 28, 2016 HST Press Release. Sulfur is a valuable indicator of how many heavier elements–metals–are contained in the speedy Cloud.
The astronomers ultimately discovered that the Smith Cloud is as richly laden with sulfur as our Milky Way’s outer disk. Our Galaxy’s outer disk is a region approximately 40,000 light-years away from the Milky Way’s center (approximately 15,000 light-years farther out than our own Sun and its family of planets, moons, asteroids, and comets). This indicates that the Smith Cloud was enriched by material manufactured in the searing-hot hearts of stars. This would not happen if it were pristine hydrogen originating beyond our Galaxy, or if it were the tattered remains of a failed, starless galaxy. Instead, this mysterious Cloud seems to have originated in our Galaxy, but was booted out of it–and it is now in the process of speedily boomeranging back home.
Even though this provides a solution to the intriguing and captivating mystery of the Smith Cloud’s origin, it raises some new questions that need to be answered. How did the Cloud get to where it is now? What catastrophic event could have shot it out of our Milky Way’s disk, and how did it manage to stay intact after such a violent eviction? Could it be that a region of dark matter floated through the disk and snared Milky Way gas? The answers may finally be found in the future.
The team’s research appears in the January 1, 2016 issue of The Astrophysical Journal.