Seventy million years ago, some unknown force blasted a tremendous amount of gas out of our galaxy. Known as the Smith Cloud, that gas is now arcing back toward the Milky Way, pulled in by its gravity. Thirty million years from now, it will return to our galaxy once more.

The cloud and its trajectory were already well-known, but the new study confirms its origin was inside the Milky Way.

The Smith Cloud was originally an amorphous blob of gas, but the forces it has been subjected to have shaped it into a comet-like form. It’s a whopping 11,000 light-years long and 2,500 across. At that size and its current distance, if it could be seen in visible light, it would appear 30 times bigger than the Moon in the night sky.

Taking home with you

It’s said that if you leave home, it will always be a part of you. That proves true at least in the case of the Smith Cloud, as it carries evidence of its origins in its composition.

Until recent observations by the Hubble telescope, researchers weren’t sure of the cloud’s origin. It was suggested that the cloud is a failed (starless) galaxy or that it's a cloud of gas heading for the Milky Way from elsewhere in space. But if that were the case, the cloud would match the composition of the gas between galaxies—it should be mostly hydrogen and helium.

If, however, the cloud originated in the Milky Way, it would match the galaxy’s gas, having heavier elements that were produced in the cores of the Milky Way’s stars.

Using Hubble’s Cosmic Origins Spectrograph (in conjunction with radio data from the Green Bank Telescope), researchers looked for signs of heavier elements in ultraviolet light passing through the cloud.(The UV originated in more distant galaxies that are behind the cloud from our perspective.)

Sulfur in particular can absorb UV light, so the researchers looked for signs of that. “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. And the amount of sulfur gives a good idea of the amounts of other heavy elements in the cloud.

Because there’s a lot of interference from dust within the Milky Way, it has been hard until now to detect any good sources of UV light behind the Smith Cloud. The researchers were the first to succeed in that endeavor, finding three active galaxies behind the cloud.

From this information, the researchers were able to determine a probable origin for it: the Milky Way’s outer rim, about 15,000 light-years farther out from the galaxy’s core than we are. This data puts its starting point a full 40,000 light-years from the center.

Additionally, by combining Hubble's data with radio data from the Green Bank Telescope, researchers were able to determine the cloud's infall velocity—well over a million kilometers per hour—which matches the hypothesis of its origin within the Milky Way.



Homecoming

Holding together Another mysterious characteristic of the cloud is its integrity. It has managed to hold together remarkably well over the past 70 million years, which you wouldn’t expect for something of the Smith Cloud’s mass. One possible reason for this integrity is the presence of dark matter. Just as the invisible material holds together galaxies (and the large-scale structure of the Universe as a whole), its presence in the cloud could help maintain its structure throughout its arcing journey. In previous work, this possibility was explored, but in that model it would have been a “dark galaxy”—a failed galaxy with no stars, only gas and dark matter. But the new data contradicts that model. An alternative is that it could be a cloud of dark matter that, when passing through the Milky Way, picked up some of our gas and then left. There are problems with this scenario as well, however: the researchers calculate that it’s unlikely such a dark matter cloud could have picked up enough gas to have formed the Smith Cloud. So for now, the Smith Cloud’s integrity remains mysterious.

When the cloud arrives home in 30 million years or so, its return will be greeted by fanfare in the form of new star formation. The researchers think the cloud has enough gas that some 2 million new suns will be formed. “The cloud is an example of how the galaxy is changing with time,” said Fox. “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.”

"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,” he added.

Indeed, the Smith Cloud is far from the only chunk of gas frothing and bubbling around the Milky Way’s edges (although it's the best-characterized one). Some are flowing into, others out of the galaxy. And these flows are absolutely key to understanding our galaxy's behavior. When the inflows are added up, their combined mass is similar to the Milky Way’s star formation rate. In other words, the inflows are the galaxy’s fuel supply, so studying the gas clouds can provide a more detailed understanding of the way stars form.

The Smith Cloud’s origin is now known, and that helps build a more complete model of the churning going on at the edges of the Milky Way and other galaxies. The biggest question that remains is what ejected the Smith Cloud in the first place.

But that's not the only unknown. The cloud seems to have more mass than it probably did when it was launched. However, recent theoretical work has suggested that the cloud could have picked up material from the galaxy’s corona as it left, so that might explain the discrepancy.

The cloud’s motion is also problematic, since it’s moving along its arc faster than the Milky Way’s rotation. This is not usually seen in clouds ejected from galaxies, which usually move slower than the rotation rates of their galaxies.

Nature, 2015. DOI: doi:10.3847/2041-8205/816/1/L11 (About DOIs)