Free Astronomy Magazine November-December 2016

SPACE CHRONICLES NGC 1068 is actually the source of its own dusty torus of dust and gas, forged from ma- terial flung out of the black hole’s accretion disk. This newly discovered cosmic fountain of cold gas and dust could re- shape our understand- ing of how black holes impact their host gal- axy and potentially the intergalactic medium. “Think of a black hole as an engine. It's fu- eled by material falling in on it from a flat- tened disk of dust and gas,” said Jack Galli- more, an astronomer at Bucknell University in Lewisburg, Pennsyl- vania, and lead author on a paper recently published in Astrophys- ical Journal Letters . “But like any engine, a black hole can also emit exhaust.” That ex- haust, astronomers dis- covered, is the likely source of the torus of material that effec- tively obscures the re- gion around the gal- axy's supermassive black hole from optical telescopes. NGC 1068 (also known as Messier 77) is a barred spiral gal- axy approximately 47 million light- years from Earth in the direction of the constellation Cetus (aka the ‘Whale’). At its center is an active galactic nucleus, a supermassive black hole that is being fed by a thin, rotating disk of gas and dust known as an accretion disk. As ma- terial in the disk spirals toward the central black hole, it becomes super- heated and blazes bright with ultra- these clouds, enabling them to adhere to pow- erful magnetic field lines that wrap around the disk. Like water being flung out of a rapidly rota- ting garden sprinkler, the clouds rising above the accretion disk get accelerated centrifu- gally along the mag- netic field lines to very high speeds − approxi- mately 400 to 800 ki- lometers per second (nearly 2 million miles per hour). This is up to nearly three times faster than the rotational speed of the outer accretion disk, fast enough to send the clouds hur- tling further out into the galaxy. “These clouds are trav- eling so fast that they reach 'escape velocity' and are jettisoned in a cone-like spray from both sides of the disk,” said Gallimore. “With ALMA, we can for the first time see that it is the gas that is thrown out that hides the black hole, not the gas falling in. This sug- gests that the general theory of an active black hole is oversimplified,” he adds. With future ALMA obser- vations, the astronomers hope to work out a fuel budget for this black hole engine: how much mass per year goes into the black hole and how much is ejected as ex- haust. “These are fundamental quantities for understanding black holes that we really don't have a good handle on at this time,” con- cludes Gallimore. A LMA image of the central region of galaxy NGC 1068. The to- rus of material harboring the supermassive black hole is high- lighted in the pullout box. This region, which is approximately 40 light-years across, is the result of material flung out of the black hole's accretion disk. The colors in this image represent the mo- tion of the gas: blue is material moving toward us, red moving away. The areas in green are low velocity and consistent with ro- tation around a black hole. The white in the central region means the gas is moving both toward and away at very high speed, the conditions illustrated in the artist impression. The outer ring area is unrelated to the black hole and is more tied to the structure of the central 1,000 light-years of the host galaxy. [Gallimore et. al; ALMA (ESO/NAOJ/NRAO); B. Saxton (NRAO/AUI/NSF)] violet radiation. The outer reaches of the disk, however, are consider- ably cooler and glow more appre- ciably in infrared light and the mil- limeter-wavelength light that AL- MA can detect. Using ALMA, an international team of astronomers peered deep into this region and discovered a sprin- kling of cool clouds of carbon mon- oxide lifting off the outer portion of the accretion disk. The energy from the hot inner disk partially ionizes n