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Missing Milky Way mass blown away by bingeing supermassive black hole

Mystery of the missing baryons leads to a surprising find

The supermassive black hole at the heart of the Milky Way galaxy was bursting with nuclear activity when humans' first ancestors roamed the Earth, according to a team of astrophysicists.

On the hunt to find the galaxy’s missing mass, the astrophysicists stumbled across evidence that the black hole may have been much more active in the past.

The research, published in The Astrophysical Journal, shows that the total mass of the Milky Way including visible and dark matter is about one to two trillion solar masses. Five sixths of the total mass is dark matter, and one sixth is normal baryonic matter.

The mass locked up in the galaxy’s stars, gas and dust should be about 150 – 300 billion solar masses. But scientists have only found 65 billion solar masses of stuff and have been looking for the missing mass ever since.

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

"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 said.

The researchers measured the absorption levels to estimate the amount of matter distributed across the galaxy. The models suggested that a bubble of gas centred around the black hole, stretching two thirds of the way to the Earth, once existed more than six million years ago.

To burst the bubble, huge amounts of energy must have come from the monstrous black hole, the researchers believe. Some of the gas was swallowed by the gaping mouth of the black hole, and other parts were blasted away by at two million miles per hour.

The model fits together with the timeline of six-million-year old stars created near the galactic centre formed from the same material which flowed into the black hole.

Six million years later, the shock wave created by that galaxy’s active phase has finally crossed 20,000 light-years of space.

Researchers believe that the mass in the hot gas was too hot to observe and could make up 130 billion solar masses – large enough to account for the missing mass.

"The different lines of evidence all tie together very well," said Martin Elvis, co-author and researcher at the Harvard-Smithsonian Center for Astrophysics. "This active phase lasted for four to eight million years, which is reasonable for a quasar."

Nowadays, the supermassive hole is much more docile and only takes in small mouthfuls of hydrogen gas as its “food” reserves have dried up. ®

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