Ancient humans could have seen the disk around our galaxy’s black hole

New Scientist : Sagittarius A*, the Milky Way’s central black hole, pulls in far less matter from its surroundings than active galactic nuclei (AGNs) do. That extra matter around AGNs forms a glowing accretion disk that’s visible in different wavebands, including the optical. It now appears that sometime between 1 million and 3 million years ago, Sagittarius A* was an AGN for a period of a few hundred thousand years, with an accretion disk bright enough to be seen by Neanderthals and other ancient members of the genus Homo. The first piece of evidence was the discovery of Fermi bubbles —structures of high energy x rays and gamma rays extending perpendicular to the galaxy’s plane. Simulations by Bill Mathews and Fulai Guo of the University of California, Santa Cruz, in 2011 suggested that the bubbles could have been caused by a burst of activity from Sagittarius A*. Now Joss Bland-Hawthorn of the University of Sydney and his colleagues have connected an usually bright section of the Magellanic stream to a burst of high-energy UV radiation that would have been released by a flurry of activity from Sagittarius A* at the same time. What caused Sagittarius A* to become an AGN isn’t clear. Although most AGNs are thought to be caused by the mergers of two galaxies, there is no other evidence suggesting the Milky Way experienced a recent merger.