Cataclysmic Flash From Supermassive Black Hole at Center of Milky Way Lit up Vast Stream of Gas Far Beyond Our Galaxy

A cataclysmic burst of energy unleashed by the supermassive black hole that lies at the center of the Milky Way around 3.5 million years ago illuminated a vast train of gas orbiting our galaxy, researchers have suggested.

Observations with the Hubble Space Telescope indicate that the light emitted by this titanic explosion—which may have been visible to our early hominid ancestors as a glowing region in Earth's sky—lit up the Magellanic Stream, a long trail of gas extending like a contrail from two of the Milky Way's neighboring dwarf galaxies: the Large and Small Magellanic Clouds.

This is an impressive feat, considering that the Stream is located at an average distance of around 200,000 light-years away from our galaxy.

Scientists think that the massive release of energy was the result of a vast hydrogen cloud—with a mass up to 100,000 times that of our sun—falling into the disk of material that orbits the supermassive black hole, known as Sagittarius A*.

This led to a powerful outburst of ultraviolet (UV) light that shot cone-shaped beams of electromagnetic radiation out of the Milky Way's two poles, almost like the twin beams of a lighthouse. The UV rays emitted from the galaxy's south pole eventually reached the Magellanic Stream where it ionized hydrogen atoms—meaning it stripped them of their electrons—thus causing them to light up, according to a study accepted for publication in the Astrophysical Journal.

"The flash was so powerful that it lit up the stream like a Christmas tree—it was a cataclysmic event," Andrew Fox, an author of the study at the Space Telescope Science Institute (STScI) in Baltimore, Maryland, 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."

Fox and colleagues came to their conclusion by conducting ultraviolet measurements of distant quasars behind the Magellanic Stream using an instrument on Hubble known as the Cosmic Origins Spectrograph.

Milky Way, explosion, supermassive black hole
An illustration of the massive burst of energy from the Milky Way's center. NASA, ESA, and L. Hustak STScI

Quasars are the incredibly bright cores of distant galaxies, which emit vast amounts of light as gas spirals at high speed into the extremely massive black holes at their hearts. These astronomical objects are among the most distant and luminous objects in the universe, with the brightest known to outshine all of the stars in the galaxies in which they lie.

In the study, the scientists analyzed how ultraviolet light from the quasars passed through the Stream, with Hubble recording the telltale fingerprints of atoms that had likely been ionized by the massive flash of light around millions of years ago.

"When the light from the quasar passes through the gas we're interested in, some of the light at specific wavelengths gets absorbed by the atoms in the cloud," Elaine Frazer, another author of the study from STScI, said in a statement. "When we look at the quasar light spectrum at specific wavelengths, we see evidence of light absorption that we wouldn't see if the light hadn't passed through the cloud. From this, we can draw conclusions about the gas itself."

The cataclysmic explosion also shot out vast amounts of plasma—one of the four fundamental states of matter consisting of superheated, charged particles—that now linger around 30,000 light-years above and below the plane of the Milky Way. These massive accumulations of plasma are known as the Fermi Bubbles, and the latest results shed new light on these structures that are only visible in X-ray and gamma-ray light.

"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," Fox said. "Now we see that the same powerful flash from our galaxy's central black hole has played a major role in both."

The explosion that rocked the Milky Way more than three million years was so powerful it may have temporarily outshone all the stars in the galaxy combined. Furthermore, researchers think that the event may have persisted for hundreds of thousands of years.

"This is a dramatic event that happened a few million years ago in the Milky Way's history," Lisa Kewley from the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3D, who was not involved in the latest paper, said last year after Fox and other scientists released a paper describing the explosion.

"A massive blast of energy and radiation came right out of the galactic center and into the surrounding material. This shows that the center of the Milky Way is a much more dynamic place than we had previously thought. It is lucky we're not residing there!" she said.