Faint γ-rays indicate recent activity for the galaxy’s supermassive black hole.
Astronomers have found the best evidence yet that a pair of jets recently emanated from the dormant gravitational monster that lies at the centre of the Milky Way, a supermassive black hole known as Sagittarius A*.
Feeding on stars and clouds of gas that stray too close, black holes in the cores of other galaxies create bright jets that can be seen across cosmic distances. But the Milky Way’s black hole is quiet. The discovery of the faint γ-ray jets by the Fermi γ-ray space telescope suggests that Sagittarius A* has not always been so tranquil. It could have been active as recently as 50,000 years ago, after it gulped down a gas cloud with a mass about 100 times that of the Sun, says Douglas Finkbeiner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
“If these features truly are γ-ray jets from [the black hole], it would tell us that our galactic supermassive black hole has not always been as feeble as we currently observe it to be,” says Fred Baganoff of the Massachusetts Institute of Technology in Cambridge. And that in turn would solve an enduring puzzle, he notes. Sagittarius A* is currently growing at a rate that is about 1,000 times too slow to have reached its current mass of about four million solar masses in the time since our galaxy formed about 13.2 billion years ago. “If confirmed, the existence of γ-ray jets would provide the strongest evidence to date for the existence of higher activity in the past,” he says.