Things you can tell from Fermi bubbles

Faint clouds of gas tens of thousands of light-years above the disc of the Milky Way galaxy are revealing evidence of a giant explosion that ripped through the galactic centre only a few million years ago — recently enough that our ancestors would have seen it happening.

“This was a cataclysmic explosion,” says Andrew Fox of the Space Telescope Science Institute in Baltimore, US, “releasing an energy up to 10⁵⁷ ergs. That’s roughly one million times as much as the Sun will release in its entire 10-billion-year lifetime.”

The first traces of this explosion were discovered a decade ago in the form of two giant balloons of plasma, now called Fermi bubbles, stretching like giant dumbbells 25,000 light-years above and below the galactic centre. (Their name comes from the fact that they were discovered by scientists using NASA’s Fermi Gamma-ray Space Telescope.)

“We see them as a relic of a powerful outflow of gas from a long time ago,” Dhanesh Krishnarao, a graduate student in astronomy at the University of Wisconsin, said at this week’s online meeting of the American Astronomical Society.

But now one team of astronomers, led by Krishnarao, has finally been able to see the bubbles’ ghostly image with an Earth-based telescope, while another, led by Fox, is using the Hubble Space Telescope to find imprints of the explosion that created them in a long tail of gas known as the Magellanic Stream, escaping from the Magellanic Clouds.

Krishnarao’s team used an instrument called the Wisconsin H-Alpha Mapper, in Chile, to filter out background light from other sources and trace the expanding shell of the bubbles via a band of red light known as Lyman-alpha radiation emitted by ionized hydrogen.

By looking at tiny shifts in the frequency of that light, they were also able to determine how fast the gas forming the bubbles is moving and determine its density — important clues to understanding the explosion that created them.

Fox’s team, on the other hand, has turned its eyes farther away. Using Hubble, it has found signs that as the Magellanic Stream passed 200,000 or so light-years below the galactic centre, it was hit by an enormous blast of bright light at about the same time that the Fermi bubbles were probably formed.

That similarity in timing, he said at the AAS meeting, probably isn’t a coincidence. Rather, it suggests that the explosion that created the bubbles and the flash of light that ionised the Magellanic Stream were one and the same.

The most likely cause, he adds, was an event known as a Seyfert flash (or flare), which occurs when the supermassive black hole at a galaxy’s heart erupts violently as it consumes a large meal of gas and dust.

Looking at other galaxies, he says, it’s likely that our own has seen more than one such event. “We’re just seeing the evidence for the most recent one,” he says. Though, he adds, “it’s amazing to think we’re observing its effects several million years later”.

The flash, he says, would have been devastating to habitable planets within about 3000 light-years of the galactic centre.

But here on Earth, much farther away, it would simply have been dramatic, illuminating the heart of the Milky Way for 100,000 to 1 million years.

“When we look at the galactic centre [located in the constellation Sagittarius] on a dark night, we are treated to a pretty impressive sight,” Fox says.

“But just imagine what our ancestors three or four million years ago would have seen. The explosion would have created a glow that would have been visible.”

Both Fox’s and Krishnarao’s teams have papers pending, with preprints available on the arXiv server.