Astronomers Uncover New Clues about Ancient Explosion in Milky Way’s Center

About 3.5 million years ago, a so-called Seyfert flare from Sagittarius A*, Milky Way’s supermassive black hole, created two enormous ionization cones that sliced through our Galaxy, beginning with a relatively small diameter close to Sagittarius A* and expanding vastly as they exited the Milky Way. Now, astronomers using the NASA/ESA Hubble Space Telescope have found that the radiation cone that blasted out of the Milky Way’s south pole lit up a massive ribbon-like gas structure — called the Magellanic Stream — trailing the Milky Way’s two satellite galaxies: the Large Magellanic Cloud and the Small Magellanic Cloud. The flash lit up a portion of the Magellanic Stream, ionizing its hydrogen by stripping atoms of their electrons.

About 3.5 million years, a tremendous explosion rocked the center of our galaxy. Our distant hominid ancestors, already afoot on the African plains, likely would have seen the resulting flare as a ghostly glow high overhead in the night sky. Image credit: NASA / ESA / G. Cecil, UNC, Chapel Hill & J. DePasquale, STScI.

“The flash was so powerful that it lit up the stream like a Christmas tree — it was a cataclysmic event,” said Dr. Andrew Fox, an astronomer at the Space Telescope Science Institute.

“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.”

Dr. Fox and colleagues used Hubble’s Cosmic Origins Spectrograph to probe the stream by using background quasars as light sources.

The astronomers studied sightlines to 21 quasars far behind the Magellanic Stream and 10 behind another feature called the Leading Arm, a tattered and shredded gaseous arm that precedes the Large Magellanic Cloud and the Small Magellanic Cloud in their orbit around the Milky Way.

“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,” said Dr. Elaine Frazer, also from the Space Telescope Science Institute.

“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.”

An enormous outburst from the vicinity of the Milky Way’s central black hole sent cones of blistering ultraviolet radiation above and below the plane of the Galaxy and deep into space. Image credit: NASA / ESA / L. Hustak, STScI.

The team found evidence that the ions had been created in the Magellanic Stream by an energetic flash.

The burst was so powerful that it lit up the stream, even though this structure is about 200,000 light-years from the Galactic center.

Unlike the Magellanic Stream, the Leading Arm did not show evidence of being lit up by the flare. That makes sense, because the Leading Arm is not sitting right below the south galactic pole, so it was not showered with the burst’s radiation.

The same event that caused the radiation flare also burped hot plasma that is now towering about 30,000 light-years above and below the plane of our Galaxy.

These invisible bubbles, weighing the equivalent of millions of Suns, are called the Fermi Bubbles.

Their energetic gamma-ray glow was discovered in 2010 by NASA’s Fermi Gamma-ray Space Telescope.

In 2015, the researchers used Hubble’s ultraviolet spectroscopy to measure the expansion velocity and composition of the ballooning lobes. Now they managed to stretch Hubble’s reach beyond the bubbles.

“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,” Dr. 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 findings will be published in the Astrophysical Journal.