The violent past of our galaxy's black hole could help us solve a cosmic mystery

Source: Mark A. Garlick/CfA

The supermassive black hole at the center of our galaxy is pretty quiet now, but new research suggests that 6 million years ago it was raging and gobbling up matter. Scientists think it could explain where some of the missing matter in our galaxy is. 

Our Milky Way has a mass that's 1 trillion to 2 trillion times the mass of the sun, according to the Harvard-Smithsonian Center for Astrophysics. Scientists think that five-sixths of all that mass is made of a mysterious substance called dark matter. The rest, about 150 billion to 300 billion times the mass of the sun, is normal matter. The problem is that we can only account for 65 billion solar masses of it. 

The new research suggests that the missing mass is tied up in million-degree fog that's spread all across the galaxy.

"We played a cosmic game of hide-and-seek," lead author Fabrizio Nicastro said in a statement. "We analyzed archival X-ray observations from the XMM-Newton spacecraft and found that the missing mass is in the form of a million-degree gaseous fog permeating our galaxy. That fog absorbs X-rays from more distant background sources."

Where did the fog come from? The team used computer models and X-ray observations and realized matter wasn't distributed evenly in the background fog. They discovered a giant empty bubble at the center of our galaxy. It seems the black hole, called Sagittarius A*, that sits in the center of our galaxy attacked that bubble of gaseous matter 6 million years ago. Some of the gas was swallowed up by the black hole, and the rest was ejected from the galaxy at two million miles per hour, according to the Harvard-Smithsonian Center for Astrophysics.

The center of the Milky Way shown in X-rays
Source: NASA/UMass/D.Wang et al/STScI

The theory has more support, too. A patch of 6 million-year-old stars sit at the center of our galaxy. Those stars likely formed from the same bubble that the black hole was feeding on, according to the Harvard-Smithsonian Center for Astrophysics. 

The math makes sense too. All that gas swallowed or ejected by the black hole adds up to about 130 billion solar masses. Add that to the 65 billion solar masses we can account for, and it brings the total amount of normal matter up to 195 billion solar masses. That number falls within the predicted range.

We'll need more research before scientists know for sure there that this background fog accounts for the missing Milky Way gas. There are two future X-ray missions planned that might reveal more about the missing mass in our galaxy.