UNC-CH Physics and Astronomy Colloquium

Gerald Cecil, University of North Carolina at Chapel Hill

“When the Monster Awoke: Dating the Most Recent Major Flare of Our Galaxy’s Supermassive Black Hole”

The historical power output of mass accreted onto the 4 million solar mass black hole at the center of our galaxy 26,000 light years distant have been estimated from ionization/recombination X-ray spectra of its surroundings. These show that ~500 years ago it was ~200,000 times more luminous than today, but faded within the last century to its current feeble state. I will present our team’s evidence from ground- and Hubble-based spectra and photoionization modeling that this region was a billion times more luminous than at present for many hundred-thousand years ~3.5 million years ago. The principle “transceiver” of this powerful outburst is an angular arc of the Magellanic tidal Stream, 200,000 light years distant from us. Patches of this gas stream within the radiation pattern of the central black hole are too bright and too highly ionized to have been photo-energized by just Milky Way starlight. Model consistency points to a prolonged flare from the central black hole, not a sustained starburst. It occurred within the most recent 1/3000th history of our galaxy, so has repeated often to synchronize all Galactic inhabitants who are aware of electromagnetism. Being very nearby, this interaction will provide unique insights into the influences of the central black hole present in most galaxies.