Supermassive black holes are found in almost every galaxy in the universe. Some of them are very active, spewing out powerful jets of materials. Others are quiet, living passively. Sagittarius A*, the one at the center of the Milky Way, is in the latter group but it appears that it was briefly more active in the recent past.
A new study due to be published in The Astrophysical Journal provides evidence of a sudden explosion from our supermassive black hole roughly 3.5 million years ago. A flare erupted from the center of the Milky Way and it spread out in two enormous cone-shaped bursts of radiation through both the galaxy's poles and out into space. These “ionization cones” are called Seyfert flares, and start with a small diameter near the black hole, expanding as they escape the galaxy.
“The flare must have been a bit like a lighthouse beam,” said Professor Joss Bland-Hawthorn, co-author of the study from the University of Sydney in a statement. “Imagine darkness, and then someone switches on a lighthouse beacon for a brief period of time.”
Sagittarius A* is located 26,000 light-years from Earth and weighs a whopping 4.6 million times the mass of the Sun. The link with Sagittarius A* is not certain but the team argues that the amount of energy released by the event couldn’t have been produced by anything other than this supermassive black hole. The flare lasted for about 300,000 years, a very short time on a cosmic scale but enough for it to have an effect on the region surrounding our galaxy.
The Milky Way doesn’t float alone in the darkness of the cosmos. It's surrounded by smaller galaxies that orbit it like satellites orbiting Earth. The largest and closest are the Magellanic Clouds, a pair of interacting galaxies that are leaving in their trail a stream of high-velocity gas clouds. This Magellanic stream ribbons itself around the poles of the Milky Way and the team behind this study believe that the flare impacted the stream, pushing its gas further out.
An artist's impression of the massive bursts of ionizing radiation exploding from the center of the Milky Way and impacting the Magellanic Stream. James Josephides/ASTRO 3D
"These results dramatically change our understanding of the Milky Way," co-author Magda Guglielmo, also from the University of Sydney, said. "We always thought about our galaxy as an inactive galaxy, with a not-so-bright center. These new results instead open the possibility of a complete reinterpretation of its evolution and nature.
"The flare event that occurred 3 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."
There is still a lot that we do not understand about supermassive black holes and how they interact with and influence nearby galaxies, but research such as this provides new insights into one of astrophysics' outstanding problems.