There's an Incoming Gas Cloud Straight Outta Intergalactic Space

gas cloud Smith Cloud
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 Space Telescope 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. NASA, ESA, and A. Feild (STScI)

You won't be able to see it when you step outside to go stargazing, but it's out there. Something quite invisible to the naked eye, but at the same, very interesting.

What is it?  According to astronomers, it's a cloud called Smith Cloud (after the astronomer Gail Smith, who discovered it in the early 1960s). At first astronomers thought it was purely hydrogen gas heading right for our galaxy at a speed of 700,000 miles (1,126,540 kilometers) per hour.

So, they used Hubble Space Telescope to measure its chemical composition using a specialized instrument called the Cosmic Origins Spectrograph. It studies light by breaking it down into its component wavelengths. What COS gives clues to the origins of objects in the universe, and the universe itself. 

How did they do that? 

The trick to looking at a cloud of gas in the cosmos is not to look FOR the cloud. Instead, you look at light as it travels through the cloud. In particular, astronomers studied it by looking at the ultraviolet light of three distant active galaxies as it passed through the cloud. The light gets absorbed by hydrogen and other elements, and astronomers look at spectra of the light to see which are missing due to the absorption.

Sulfur Gives away the Game

It turns out the cloud is extremely rich in sulfur along with the hydrogen. Its existence implies that the cloud was enriched by stars that blew their elements out to space.

Sulfur is created inside stars, and as they die, they eject that and other elements (such as carbon, nitrogen, oxygen, and even such heavy elements as iron). That provides a way to enrich such nearby "pristine" hydrogen clouds as Smith Cloud with the stuff of stars. 

Meet Smith Cloud

The existence of Smith Cloud (named for astronomer Gail Smith, who discovered it in the early 1960s) has been something of a mystery.

We know it's there, but why? The fact that it exists and can be traced back to the Milky Way tells astronomers that our galaxy is a pretty active place. It can throw gases out of one place and they'll end up somewhere else as the galaxy wheels through space. That also means the galaxy is dynamic — it's changing with time.

 Smith Cloud is pretty large — about 11,000 light-years long and 2,500 light-years across. However, since it's all gas, it's not something you can spy out with a telescope. Before the Hubble observations, astronomers thought this cloud might be a failed galaxy, one without any stars. That would make it a traveling cloud of gas, and for a while they thought it was coming from outside the Milky Way and was nearly purely hydrogen. 

Where Did it Come From?

Based on the Hubble observations, it's apparent that the cloud was once part of the Milky Way and somehow was ejected out to intergalactic space about 70 million years ago. Instead of continuing on out to enrich the environment between galaxies, the cloud is coming back, like a boomerang. What happened to send it out and what sent it back?  Was there some really large-scale event that somehow shoved the gas out of the galaxy?

 It would have to be pretty energetic, considering how fast the cloud is moving. Equally powerful would be whatever sent the cloud BACK to the Milky Way. Could dark matter and a galaxy collision have been part of the story? We don't know. 

The questions want to answer will give some clue to not just the Milky Way's past, but the history of Smith's Cloud. There's even a possibility that dark matter is somehow involved. Since this invisible "stuff" is everywhere, that's not surprising. But dark matter isn't just an answer. It's still a mystery, and it raises more questions than it answers.