Space

Six million years ago, the Milky Way blew its lid

Artist's impression shows the Milky Way as it may have appeared 6 million years ago during a "quasar" phase of activity
Artist's impression shows the Milky Way as it may have appeared 6 million years ago during a "quasar" phase of activity
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Artist's impression shows the Milky Way as it may have appeared 6 million years ago during a "quasar" phase of activity
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Artist's impression shows the Milky Way as it may have appeared 6 million years ago during a "quasar" phase of activity
Map of the Milky Way galaxy
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Map of the Milky Way galaxy

Our galaxy hasn't always been quiet a neighborhood. Scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) say that during the Miocene epoch about six million years ago, the galactic core of the Milky Way may have been so active that it became a quasar and blasted away most of the galaxy's normal matter, equivalent to hundreds of billions of Suns.

The Milky Way galaxy is estimated to be as massive as somewhere between one or two trillion Suns, of which five-sixths is dark matter that can only be detected by the gravitational field it generates. The rest is of normal matter, which should weigh in at 150 to 300 billion solar masses. But when all the visible stars, dust, and gases are added together, they only come to 65 billion solar masses.

The question is, where is the rest?

"We played a cosmic game of hide-and-seek. And we asked ourselves, where could the missing mass be hiding?" says Fabrizio Nicastro, a research associate at the CfA 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."

By calculating the amount of X-rays absorbed, the CfA team could measure how much normal matter is in this gaseous fog. They discovered that it forms a gas bubble around the center of the galaxy that extends out for two-thirds of the distance to the Solar System. This bubble is not only incredibly massive, but unbelievably hot, which means that the sort of energy that would make a supernova look like a damp squib was involved.

Map of the Milky Way galaxy
Map of the Milky Way galaxy

The source of this energy is the supermassive black hole at the galactic center, which is estimated to have a mass of 4.1 to 4.5 million solar masses. That black hole is currently inactive, having run out of "food" in its vicinity. But six million years ago things were very different according to the CfA team – the galaxy was going through a quasar phase.

Quasars as the most active objects in the Universe, punching out 100 million times the luminosity of our galaxy today. During the Milky Way's quasar phase, the black hole at its center sucked in huge amounts of matter for four to eight million years. This generated enough energy to pump gas out of the galaxy at speeds of 2 million mph (1,000 km/s).

Eventually, the center of the galaxy was depleted of gas and the black hole quieted down, but not before it had expelled 130 billion solar masses of matter at a temperature of millions of degrees, forming a shock wave that is now 40,000 light years in diameter and growing. The extremely high temperature is particularly important because it renders the gas nearly invisible to direct observation. In addition, the presence of stars at the galactic center that are only six million years old provides corroborative evidence as to the date of the event's end.

The CfA team says that future space missions, such NASA's X-ray Surveyor and ESA's Athena X-ray Observatory, could provide more data on this elusive bubble.

These results were published in The Astrophysical Journal.

Source: Harvard-Smithsonian Center for Astrophysics

4 comments
Ingvar Olsson
They say "They discovered that it forms a gas bubble around the center of the galaxy that extends out for two-thirds of the distance to the Solar System. This bubble is not only incredibly massive, but unbelievably hot, which means that the sort of energy that would make a supernova look like a damp squib was involved." And this happened 6 million years ago. So the bubble has expanded to about 2/3 of the way to the Solar system in 6 million years. If I understand this correctly then the remaining 1/3 of the way to the Solar system will take another 3 million years and bubble will the arrive at the Solar system. What will happen then? If the bubble is "not only incredibly massive, but unbelievably hot, which means that the sort of energy that would make a supernova look like a damp squib was involved" then I guess that the Earth, as we know it, will cease to exist? Even if the planet remain I suppose it will be stripped of all atmosphere and all water. So man will go extinct in 3 million years unless we develop space travel that is faster than the speed of the bubble? The bubble speed is 1000 km/s (app 0,3% of light speed). That's pretty fast! Are these conclusions correct?
Tanstar
Ingavar Olsson: It would also severely limit the direction we could travel in. You can't go in, as that puts you closer? Since it's a bubble, travelling around the Milky Way would just have the same affect as staying put. Out is the only direction. This means not only space travel, but inter-galactic space travel.
ScottClarkbf96c44df5404e16
I had the same question, and how this will impact the functioning of Sol. But 3MM years is also enough to wonder whether it will be homo sapiens or a successor species that will need to tackle it, and what types of adaptations will have been evolved before hand. It's a fantastic source of energy to be channeled, for a considerable amount of time too, probably multiple generations by human lifespan measures.
Fretting Freddy the Ferret pressing the Fret
I don't have hard numbers on hand, but given the very large distance between the centre of the galaxy and the Solar system, I reckon it won't have a catastrophic effect on our Solar system by the time it reaches us. The expanding gas bubble will just be too spread thin to do serious harm. Perhaps, it won't even make a big indentation on the heliosphere when it crashes into it.