Free Astronomy Magazine November-December 2015

SPACE CHRONICLES The nearest quasar is powered by a double black hole A stronomers us- ing NASA's Hub- ble Space Tele- scope have found that Markarian 231 (Mrk 231), the nearest gal- axy to Earth that hosts a quasar, is powered by two central black holes furiously whirl- ing about each other. The finding suggests that quasars — the brilliant cores of ac- tive galaxies — may commonly host two central supermassive black holes that fall into orbit about one another as a result of the merger between two galaxies. Like a pair of whirling skat- ers, the black-hole duo generates tremendous amounts of energy that makes the core of the host galaxy outshine the glow of the galaxy's popula- tion of billions of stars, which scientists then identify as qua- sars. Astronomers looked at Hubble archival observations of ultraviolet radiation emitted from the center of Mrk 231 to discover what they de- scribe as "extreme and surprising properties." If only one black hole were present in the center of the quasar, the whole accretion disk made of surround- ing hot gas would glow in ultraviolet rays. Instead, the ul- traviolet glow of the dusty disk abruptly drops off towards the center. This provides obser- vational evidence that the disk has a big donut hole encircling the central black hole. The best explanation for the observational data, based on dynam- ical models, is that the center of the disk is carved out by the ac- tion of two black holes orbiting each other. The second, smaller black hole or- bits in the inner edge of the accretion disk, and has its own mini- disk with an ultravio- let glow. "We are extremely ex- cited about this find- ing because it not on- ly shows the existence of a close binary black hole in Mrk 231, but also paves a new way to systematically search binary black T his Hubble Space Telescope image reveals a bright starlike glow in the center of the interacting galaxy Markarian 231, the nearest quasar to Earth. Located 581 million light-years away, we are seeing the galaxy as it looked before multicelled life first appeared on Earth. Quasars are powered by a central black hole that heats the gas around it to unleash tremendous amounts of energy. [NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (Univer- sity of Virginia, Charlottesville/NRAO/Stony Brook University)] by NASA