Sometimes They Come Back: Giant Gas Cloud on Collision Course With the Milky
Way

In 27 million years, you’d better fasten your seat belt: Sometime around then, a gas cloud with enough mass to make 2 million stars like the Sun will come crashing into the Milky Way.

Given the time frame, I’m not too concerned personally over this galactic train wreck. Also, stuff like this happens pretty often in our galaxy, and we’ve made it this far.

Still, it’ll be quite an event, and there’s a funny twist to it: We don’t really know where this cloud came from.

It’s called the Smith Cloud (it was discovered by astronomer Gail Smith in 1963, who was mapping the location of hydrogen gas in the sky), and by all accounts it’s a bruiser: It’s more than 10,000 light-years across; so big that even at its distance of 40,000 light-years (almost halfway across the galaxy!) it appears 30 times wider than the full Moon on the sky.

It’s part of a class of objects called high-velocity clouds; generally pretty big clouds of gas whizzing around outside the body of the Milky Way. Quite a few have been seen, but at 2 million times the mass of the Sun, the Smith Cloud is one of if not the most massive (most are tens of thousands of solar masses). The Milky Way is, overall, a flattish disk, and Smith orbits at a slight (30°) angle to it. Right now it’s about 10,000 light-years below the disk and headed up into it.

Where did this thing come from? There are lots of possibilities: It could be a “dark galaxy,” a clump of gas and dark matter that never formed stars. Or it could be a clot of gas left over from the formation of the Milky Way, orbiting far outside the galaxy, which got disturbed and plunged inward. Or it could be a cloud ejected from the Milky Way itself, blasted out into deep space and now finally heading back.

To find out, astronomers were clever. If the gas cloud were primordial—that is, very very old—it should consist of just hydrogen and helium, the lightest elements. Heavier elements have only been around in the Universe since stars created them, so by looking at the cloud’s ingredients we might be able to eliminate a couple of origin stories.

Using an ultraviolet camera on Hubble (called COS, the Cosmic Origins Spectrograph) they looked for the fingerprint of sulfur in the cloud. That element absorbs a very specific wavelength (color) of UV light (you can learn more about how this works in an episode of Crash Course Astronomy). Helpfully, the cloud’s size betrayed it: It’s so big it happens to cover up several very distant galaxies that emit a lot of UV light. Using those galaxies as light bulbs, the astronomers looked to see if there were any anomalous absorption of that wavelength of UV.

… And there was! Careful analysis indicated that the amount of sulfur in the cloud was pretty high. Kilo for kilo, it has about half the sulfur the Sun itself does (we use the Sun as the standard for such things, because in principle its elemental composition is easy to measure). There’s no way it could have that much sulfur and be left over from the early Universe. So boom, right away we know it isn’t some leftover gas cloud that’s been lurking in the Milky Way’s rural areas.

And we also know it’s not a dark galaxy, either: You need stars to make sulfur, and dark galaxies wouldn’t have any stars.

That means it must be local in origin, a cloud somehow ejected from the disk of the galaxy, where heavier elements are abundant.

We do know of ways that can happen. In several places along the galactic disk are “fountains,” huge eruptions of material blasting out into near-galactic space. These can be generated by a series of exploding stars, or by the fierce winds blown by thousands of young stars all forming at the same time in galactic gas clouds. The vast outflow of material can burst through the galactic plane like a geyser, ejecting a lot of material upward and outward.

And, like a fountain, sometimes that material comes back. The Smith Cloud must have formed that way. It may have started off smaller, but as it plowed through the material located in our galaxy’s halo, it picked up mass. And now it’s ready to deliver all that stuff back to us.

Like I said, lots of high velocity clouds like this are known. What’s interesting is that if you add them all up, they deliver about one solar mass worth of material every year to the galaxy. That is very roughly the same amount the Milky Way uses up every year making stars! So these clouds are like fuel, keeping star formation in the galaxy going. How about that?

Not that this entire mystery is solved. Smith is moving pretty rapidly through space—about 300 kilometers per second, or a million kilometers per hour—and that’s actually faster than the rotation of the galaxy at its location! Most such clouds are actually moving slower than that, so how Smith got its high velocity still isn’t clear.

So what will happen when it does come back? Well, the disk of the galaxy is lousy with gas. When Smith comes barreling in, it may very well collide with that gas. This will generate vast shock waves and collapse the cloud (think two cars in a head-on collision). If it gets dense enough, star formation could be triggered inside it, with thousands of stars being born all at once.

There are multiple sites of star birth in our galaxy, and some are pretty rigorous. This may be just one among them, though I suspect the velocity at which it’s moving will make this somewhat more violent than normal. Two million solar masses moving at a million kilometers per hour …

The galaxy is a surprisingly violent place. In astronomy, though, violence usually means something interesting. It’s how they form, it’s how they die, and it looks like in this case it’s even how they make sure there’s fuel left to generate more. It’s the ultimate recycling program.