About 3.5 million years ago, the supermassive black hole at the center of our Milky Way galaxy unleashed an enormous burst of energy. Our primitive ancestors, already afoot on the African plains, likely would have witnessed this flare as a ghostly glow high overhead in the constellation Sagittarius. It might have persisted for 1 million years.
Now, eons later, astronomers are using NASA’s Hubble Space Telescope’s unique capabilities to uncover even more clues about this cataclysmic explosion. Looking to the far outskirts of our galaxy, they found that the black hole’s floodlight reached so far into space it illuminated a vast train of gas trailing the Milky Way’s two prominent satellite galaxies: the Large Magellanic Cloud (LMC), and its companion, the Small Magellanic Cloud (SMC).
The black hole outburst was probably caused by a large hydrogen cloud up to 100,000 times the Sun’s mass falling onto the disk of material swirling near the central black hole. The resulting outburst sent cones of blistering ultraviolet radiation above and below the plane of the galaxy and deep into space.
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. The flash lit up a portion of the stream, ionizing its hydrogen (enough to make 100 million Suns) by stripping atoms of their electrons.
Fox’s team used Hubble’s ultraviolet capabilities to probe the stream by using background quasars—the bright cores of distant, active galaxies—as light sources. Hubble’s Cosmic Origins Spectrograph can see the fingerprints of ionized atoms in the ultraviolet light from the quasars. 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 LMC and SMC in their orbit around the Milky Way.
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, Fox used Hubble’s ultraviolet spectroscopy to measure the expansion velocity and composition of the ballooning lobes.
Source: “Intense flash from Milky way’s black hole illuminated gas far outside of our Galaxy” Hubblesite, 2 June 2020. <https://hubblesite.org/contents/news-releases/2020/news-2020-33>