A Flare From The Milky Way’s Supermassive Black Hole Was Likely Seen By Our Ancient Ancestors

Artist impression of our ancestors and the flared up gas in the night sky. NASA, ESA, G. Cecil (UNC, Chapel Hill), and J. DePasquale (STScI)

Sagittarius A* is the supermassive black hole at the center of the Milky Way, and it's mostly a quiet cosmic object. It does flare up occasionally, especially in recent years, but it has nothing on what some other supermassive black holes get up to.

Don’t be fooled by its current demeanor, though. Sagittarius A* is capable of dramatic releases of energy. Now, researchers have found evidence suggesting it produced a flash of light so powerful it lit up gas 200,000 light-years away.

As reported in an upcoming paper in The Astrophysical Journal, Sagittarius A* flared up 3.5 million years ago as a large cloud of hydrogen, about 100,000 times the mass of the Sun, entered the accretion disk surrounding the black hole. This led to a tremendous outburst. Intense light and plasma were released from the center of the Milky Way and we continue to see the effects today.

Illustration of the Magellanic Stream getting ionized. NASAESA, and L. Hustak (STScI)

The plasma makes up what we call the Fermi Bubbles, two large, hot structures above and below the plane of the Milky Way. The intense light stretched so far it hit the Magellanic Stream, the trail of gas left behind by the Large and Small Magellanic Clouds, the only companion galaxies to the Milky Way that are visible to the naked eye.

The light from around the black hole stripped the hydrogen in the Magellanic Stream of its electrons. This is a process known as ionization. The process made the stream bright, creating a halo-like structure visible in the night sky. This would have persisted for up to a million years, as our ancestors were moving through the African plains. It was likely visible as a glowing spot along the arc of the Milky Way, in the constellation Sagittarius.

"The flash was so powerful that it lit up the stream like a Christmas tree—it was a cataclysmic event!" lead author Andrew Fox of the Space Telescope Science Institute said in a statement. "This shows us that different regions of the galaxy are linked—what happens in the galactic center makes a difference to what happens out in the Magellanic Stream. We're learning about how the black hole impacts the galaxy and its environment."

There is a lot of gas in the stream, enough to make 100 million stars like the Sun, but it is very spread out so studying it is not an easy task. The Hubble Space Telescope has a way to do it by looking at how the light of distant galaxies is absorbed by the gas in the Magellanic stream. The researchers can then compare it with the gas that precedes the Magellanic Clouds, known as the Leading Arm.

The observations showed how the Magellanic stream was irradiated and ionized, while the Leading Arm doesn’t show this. Given the position of the stream, there’s only one culprit: Sagittarius A*

"We always thought that the Fermi Bubbles and the Magellanic Stream were separate and unrelated to each other and doing their own things in different parts of the galaxy’s halo," said Fox. "Now we see that the same powerful flash from our galaxy's central black hole has played a major role in both."

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