Solving the Mystery of the Milky Way’s Missing Mass

Smithsonian scientists have discovered a huge cloud of super hot gas expanding from the middle of our galaxy

Milky Way, artist's impression
An artist's impression of the Milky Way six million years ago, depicts an orange bubble at the galactic center and extending to a radius of about 20,000 light-years. Scientist think that outside of that bubble, a pervasive "fog" of million-degree gas might account for the galaxy's missing matter. Mark A. Garlick/CfA

At the center of our galaxy there is a black hole named Sagittarius A* (pronounced "Sagittarius A-star"). We have a good idea of what our galaxy, the Milky Way, is shaped like. It is a two-armed spiral reaching out from Sagittarius A* and constantly spinning. But as scientists observed the gravitational effects of the Milky Way, they realized that something was missing. Not including dark matter, the Milky Way weighs about 150 to 300 billion times the weight of the sun—but researchers found evidence of only about 65 billion solar masses.

That's a lot of missing mass.

A new research paper from the Harvard-Smithsonian Center for Astrophysics and published in The Astrophysical Journal, provides an explanation for much of that missing mass while also contributing to a better understanding of the recent history of the galaxy. By using X-ray telescopes, researchers have found a massive bubble of super-hot gas expanding from the neighborhood of Sagittarius A*. That gas is so hot (over a million degrees) that it was invisible to ordinary telescopes.

Martin Elvis, a Smithsonian astrophysicist with the Center for Astrophysics and one of the paper's authors, said it was a puzzle to figure out how the galaxy could hide that much mass. “Most of the ordinary matter is not found in anything we could easily see, so you have to think of ways to hide it,” he says. “One way to hide it is by heating it. As it gets hotter the electrons on the outside get stripped off. We only see things because of the effects on electrons, so as those disappear we can't see it."

The scientists used telescopes that observe X-rays rather than ordinary light. By studying X-rays using the XMM-Newton and Chandra spacecraft, the contours of a massive bubble of gas began to emerge.

Right now, Sagittarius A* is a relatively inactive black hole. But six million years ago, just as the first hominins were making their appearance on Earth, it was sucking in massive amounts of matter that was close to it. This created a bubble of relatively empty space. But Sagittarius A* also pumped out a low-density gas into that space, composed of lots of oxygen and probably also hydrogen and other elements.

“All we can really measure is oxygen,” says Elvis. “It's got the biggest chance of being seen. We'd love to do more but its very hard to see the other elements in there. It's clearly not pristine gas from the beginning of the universe because it's got this oxygen in it.”

The gas will probably never cool off. “The way gas would cool would be by radiating energy away,” says Elvis. “At low densities that doesn't happen much. It takes like the age of the universe to cool so it's not likely to happen.”

The bubble of gas is expanding at a rate of about a million miles an hour, and has already reached about two-thirds of the way to Earth. “When it gets here it will be less dense,” says Elvis. “It will be here in a few million years.”

Don't panic. While the gas is super hot, the particles are so diffused that they couldn't do any harm to humans. If you piloted a spaceship through this bubble, “basically you wouldn't notice,” says Elvis. “It's much less dense than the normal interstellar medium.”

The research appears to confirm a timeline of Sagittarius A*'s recent activity that was outlined in 2010 Douglas Finkbeiner, also with the Harvard-Smithsonian Center for Astrophysics.

Elvis hopes that a more powerful X-ray telescope will eventually allow a better picture of the distant bubble of gas to emerge. The Chandra X-ray telescope has a receiving area only the size of a serving dish to collect X-rays. NASA is currently considering a proposal to build a craft called The X-ray Surveyor.

“It would be a collecting area of a few square meters,” says Elvis. “No one is absolutely sure that we can build it but research is going on. We feel confident that we could build something for about the same cost as Chandra but hundreds of times better." With that kind of telescope, he adds, "we'd be able to see much finer detail over a much larger area . . . so we could map out this whole bubble very quickly.”

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