Milky Way's Supermassive Black Hole Solves Mystery of Missing Mass

New research argues that the supermassive black hole at the center of our galaxy is responsible for some of the Milky Way's missing mass, and determines the time when it exploded.

Astronomers believe that supermassive black holes (SMBH) are at the heart of almost every galaxy in the universe, including the Milky Way. The one in our galaxy is known as Sagittarius A*. It lies around 26,000 light years from Earth, and has a mass about four million times that of our Sun.

This SMBH is currently dormant, but new research has pinpointed its explosion to six million years ago, a time when the first human ancestors began to walk the Earth.

Analyzing data from the ESA's XMM-Newton spacecraft, an international team of scientists from Italy, Mexico and the US set out to find matter that is believed to be "missing" from the Milky Way.

Scientists have calculated that there is about 150-300 billion solar masses of normal matter in the galaxy. However, counting up all the stars, gas and dust we can see only gives about 65 billion solar masses, leading scientists to search for the missing matter.

"We played a cosmic game of hide-and-seek. And we asked ourselves, where could the missing mass be hiding?" said Fabrizio Nicastro, lead author of the paper published in The Astrophysical Journal Letters, and a research associate at the Harvard-Smithsonian Center for Astrophysics and astrophysicist at the Italian National Institute of Astrophysics.

The researchers believe the supermassive black hole at the center of the universe holds the key to the whereabouts of this missing mass.

"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," Nicastro explained in a press release.

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© NASA

The scientists think the fog is powered by the high-energy explosion of the star that went on to become the SMBH. Since the shock wave of the explosion has traveled 20,000 light years of space, they calculated that the black hole exploded six million years ago. 

According to their observations and computer models, this newly-observed million-degree gas can account for up to 130 billion solar masses of material, and therefore explain the mystery of the galaxy's missing mass.

"This active phase lasted for 4 to 8 million years, which is reasonable for a quasar," said Martin Elvis, co-author of the paper.

While scientists have calculated the total mass of our galaxy to be about 1-2 trillion times as much as our Sun, only about one sixth, or 150-300 billion solar masses, of that is normal matter.

About five sixths of the total mass of our galaxy, 1-2 trillion times the mass of the Sun, is in the form of dark matter, an unidentified type of matter which does not have an electric charge and therefore cannot be observed by shining a light on it. 

Scientists can only infer the presence of this 'invisible' matter by observing the motions of stars which react to its gravitational mass.