“We always thought of our Galaxy as an inactive galaxy, with a not so bright center,” said Magda Guglielmo from the University of Sydney about 2019 Hubble Space Telescope data showing that a titanic, expanding beam of energy sprang from close to the SgrA*, the supermassive black hole in the center of the Milky Way, 3.5 million years ago, shooting a cone-shaped burst of radiation through both poles of the Galaxy and beyond into deep space.
In cosmic time, that is astonishingly recent. On Earth at that point, the asteroid that triggered the extinction of the dinosaurs was already 63 million years in the past, and humanity’s ancient ancestors, the Australopithecines, were afoot on the vast savannas of Africa.
A reinterpretation of the Milky Way’s evolution and nature?
“These new results instead open the possibility of a complete reinterpretation of its evolution and nature,” Guglielmo added. “The flare event that occurred three million years ago was so powerful that it had consequences on the surrounding of our Galaxy. We are the witness to the awakening of the sleeping beauty.”
That’s the finding arising from research conducted by a team of scientists led by Joss Bland-Hawthorn from Australia’s ARC Center of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and soon to be published in The Astrophysical Journal.
The Seyfert Flare -“We an only imagine what they saw”
The phenomenon, known as a Seyfert flare, created two enormous ‘ionization cones’ that sliced through the Milky Way – beginning with a relatively small diameter close to the black hole, and expanding vastly as they exited the Galaxy.
“In 2021, the evidence is now overwhelming that the huge x-ray and gamma ray bubbles emanating from the central regions of the Milky Way were powered by a massive outburst from the Galactic nucleus,” Joss Bland-Hawthorn, Director of the Sydney Institute for Astronomy, wrote in an email to The Daily Galaxy. “A supermassive black hole with 4 million times the mass of our Sun is known to lurk there, and indeed the 2020 Nobel Prize in Physics was awarded for work that confirmed this to be true. As material falls into the black hole, it heats up and creates jet-like explosions. These have now been seen with x-ray and radio telescopes. All this happened about 3-4 million years ago, when cave people walked the Earth. We can only imagine what they saw.”
So powerful was the flare that it impacted on the Magellanic Stream – a long trail of gas extending from nearby dwarf galaxies called the Large and Small Magellanic Clouds. The Magellanic Stream lies at an average 200,000 light years from the Milky Way.
The explosion was too huge, says the Australian-US research team, to have been triggered by anything other than nuclear activity associated with the black hole, known as Sagittarius A, or Sgr A*, which is about 4.2 million times more massive than the Sun.
Like a lighthouse beam
“The flare must have been a bit like a lighthouse beam,” says Bland-Hawthorn, who is also at the University of Sydney. “Imagine darkness, and then someone switches on a lighthouse beacon for a brief period of time.” The blast, the researchers estimate, lasted for perhaps 300,000 years – an extremely short period in galactic terms.
Using data gathered by the Hubble Space Telescope, the researchers calculated that the massive explosion took place little more than three million years ago. “This is a dramatic event that happened a few million years ago in the Milky Way’s history,” says Lisa Kewley, Director of ASTRO 3D. “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!”
In conducting the research, Bland-Hawthorn was joined by colleagues from the Australia National University and University of Sydney, and, in the US, the University of North Carolina, University of Colorado and the Space Telescope Science Institute in Baltimore.
The paper follows on from research also led by Professor Bland-Hawthorn and published in 2013. The earlier work looked at evidence of a massive explosive event beginning in the center of the Milky Way, ruled out a nuclear starburst as the cause and tentatively tied it to activity in SgrA*.
The latest work confirms Sgr A* as prime suspect, but, the researchers concede, there is still a lot more work to be done. How black holes evolve, influence and interact with their host galaxies, they conclude, “is an outstanding problem in astrophysics.”