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The Smith Cloud: The Biggest Thing You've Never Heard of

by Sarah Scoles
Did you know that a cloud of gas containing enough material for 10,000,000 suns is flying toward the Milky Way? And that if you could see it, it would take up as much of the sky as the Orion constellation from shoulder to shoulder? For those of you who are counting, that's as big as a hero.


This cloud, called the Smith Cloud, was discovered in 1963 by Gail Bieger-Smith and has as much mass as a dwarf galaxy...except that it has no stars. Though astronomers have known of its existence for fifty years, they didn't know much about its details for a while. In 2008, though, Lockman, et al., reobserved this Smith Cloud with the Green Bank Telescope, the first telescope sensitive enough  and with high enough resolving power to make a detailed map. From this observation, Lockman's team could find out more about the cloud's shape and kinematics, and thus determine whether it was going away from us, coming toward us, or doing the Macarena.


Bulge: No HVCs there.Is this cloud unusual?

Nope. Clouds of fast-moving gas surround the Milky Way like a SWAT team. Scientists have named them high-velocity clouds, or HVCs, and they live outside of the Galaxy's disk, in the halo. We can't see them, as they are made of neutral hydrogen, which emits radio waves that you'd have to have gigantic telescope eyes to see. But if we could see these clouds, they would cover 40% of our sky.


Where do these clouds of gas come from?

The hypotheses:
They could be made of material from other galaxies, gas either ejected or stripped-off.
  • They could be made of material from the Magellanic Clouds, which are small satellite galaxies associated with the Milky Way.
  • They could be from other galaxies in our Local Group, possibly other satellite galaxies.
  • Did you know that we have a bunch of satellite galaxies that are all closer than what we call the "nearest galaxy," Andromeda?
They could be made of material from our own galaxy.
  • They could be leftovers from its formation.
  • They could be due to a "galactic fountain" gushing gas. In regions with a lot of massive stars, supernovae often go off. Supernovae are hot. Supernovae heat the gas around the stars, which causes the gas to rise and form "superbubbles," which leave the Galaxy's disk and go into the halo. As the gas rises, it cools, condenses, and eventually returns back toward the disk.
Now we know where Superbubble chewing gum got its name. Also, this particular bubble was discovered with the Green Bank Telescope. http://www.nrao.edu/pr/2006/plume/ (Credit: B. Saxton/NRAO/AUI).
Either way, or both ways, fast-moving clouds of gas are all around us, and the Smith Cloud is the one that looks most like it is intent on destroying us.


But here's the thing about collisions: they aren't bad and don't destroy us. They are awesome and make our galaxy awesomer. You'll have to keep reading to find out why.


But fear not the Smith Cloud.


What's up with the Smith Cloud?

Well, based on its kinematics, our galaxy either expelled it, or it came from the Sagittarius dwarf galaxy.


The Smith Cloud moves at about 300 km/s (relative to the Sun). For a reference point, the space shuttle used to (RIP) travel at a maximum speed of 1.4 km/s. And the shuttle certainly didn't contain 10,000,000 times the mass of the Sun inside its fuel tank. Nor was the shuttle 9,800 light-years long. Nor was it 3,200 light-years wide. But the Smith Cloud doesn't have NASA tattooed on its side. So who is really the winner here?
My name is Smith. Smith Cloud. (Credit: B. Saxton/NRAO/AUI)


Anyway, Smith is currently about 40,400 light-years away and appears (because of its cometary shape) to be moving toward us. You can see how the part at the top right looks like it's leading the motion, while the lower left looks like it's trailing. At this rate and this distance, it should arrive around 27 million years from now.


By retracing the gas's orbit, astronomers found that the Smith Cloud has already passed through the disk of the Galaxy once, about 70 million years ago.

What will happen when the Smith Cloud smashes into us?

Joyous will be the day. Without the introduction of new gas, our galaxy would not be able to maintain its star formation rate. Because of HVCs' entrances into the the Milky Way, we have enough gas to collapse into new stars.

Why is the Smith Cloud, and why are high-velocity clouds in general, important to us?

Without gas coming from outside the Milky Way, the cycle of star formation would cease more quickly, as all our star-forming material would be depleted. Infalling clouds can be like celestial storks, bringing (eventual) baby stars.


Before we had telescopes that could find these HVCs, determine their shape, determine their movement, and make predictions about their futures, we didn't know how the Milky Way--or any galaxy, really--got the gas it needed to form new stars. Now that we know for sure that the Smith Cloud is coming for us, not running away from us, we have more evidence that the HVC stork can deliver the amount of gas needed to form the number of stars we see forming.
Plop those babies down right there. On that X. (Credit: B. Saxton/NRAO/AUI).






Lockman, F., Benjamin, R., Heroux, A., & Langston, G. (2008). The Smith Cloud: A High-Velocity Cloud Colliding with the Milky Way The Astrophysical Journal, 679 (1) DOI: 10.1086/588838

Lockman, F. (2012). The Milky Way and its gas: Cold fountains and accretion EPJ Web of Conferences, 19 DOI: 10.1051/epjconf/20121908003

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