Milky Way bubble a ‘distant echo’ of supermassive black hole binge

The supermassive black hole in the centre of the Milky Way, currently a sleeping giant, woke around eight million years ago to partake in a four-million-year guzzling binge before slipping back into hibernation, new research suggests.

Astronomers from Italy, Mexico and the US were looking for missing galactic mass and found an enormous bubble, 40,000 light-years across.

When the black hole fed voraciously, the researchers say, it churned surrounding gas and dust which was spat out at 1,000 kilometres a second to create the spherical shockwave.

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Their work, which will be published in The Astrophysical Journal and can be found in Arxiv, began with a search for the Milky Way’s missing matter.

Astronomers “weigh” galaxies by measuring their radius and clocking how fast their stars move. This is called dynamical mass and includes baryonic matter – which is all the “normal” matter such as gas and stars – and the mysterious dark matter. 

The Milky Way’s baryonic mass is around 150 to 300 billion times the mass of the sun. But when you add the mass of all visible baryonic matter, you come up with only 65 billion solar masses or so.

And it’s not a problem unique to the Milky Way. Other nearby galaxies also suffer mass discrepancies.

So where’s all the missing stuff? 

This is the question Fabrizio Nicastro from the Italian Institute of Astrophysics and colleagues set out to answer. 

“We played a game of cosmic hide-and-seek,” Nicastro says.

They examined X-ray data from 31 lines of sight taken by the European Space Agency’s space observatory XMM-Newton and simulated the distribution of gas in the middle of the galaxy.

They found evidence of a million-degree fog of gas – so hot we can’t see it – permeating our galaxy outside what appeared to be a bubble, which extended two-thirds the way to Earth from the centre of the galaxy.

The model that best fit the observations was, they write, a “radially expanding blast-wave or a shock-front generated in the centre of the galaxy and travelling outwards, so acting as a piston onto the ambient gas, and compressing the material at its passage, while pushing it (or a fraction of it) outwards”.

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And in the centre of the galaxy sits a supermassive black hole.

Black holes are pretty sloppy eaters. As matter falls in, dragged along by the gravitational pull, some is caught in the swirling morass and fired away.

Around eight million years ago, the researchers calculated, the Milky Way’s supermassive black hole started feeding on surrounding dust and gas. 

The feeding frenzy probably triggered star formation too – seen as a population of six-million-year-old stars today.

And the vast expanse of super-heated gas, an echo of the black hole’s binge, is up to 130 billion solar masses – and could account for the galaxy’s missing matter.