Quasars are the light fantastic. These brilliant cores of active galaxies blaze with the radiance of a hundred billion stars compressed into a region of space not much larger than our solar system. Supermassive black holes, with millions or billions of times the mass of our sun, are the only imaginable powerhouse behind these tsunamis of raw energy.
Hubble Space Telescope astronomers set their sights on the nearest quasar to Earth, Markarian 231, located 581 million light-years away. Black holes – even supermassive ones – are too compact to be resolved by any present-day telescope. So, astronomers did the next best thing, measure all the light from a disk of infalling material around the black hole. The ultraviolet radiation – only measurable by Hubble – revealed evidence for a curious gap in the disk. Instead of being pancake shaped, it looks more like it has a big donut hole. The best explanation for the gap is that two black holes are orbiting each other in a dizzying dance that powers the quasar fireworks. This carves out the gap. The second black hole must have come from a smaller galaxy that merged with Markarian 231 to ignite the quasar about 1 million years ago.
Astronomers using NASA's Hubble Space Telescope have found that Markarian 231 (Mrk 231), the nearest galaxy to Earth that hosts a quasar, is powered by two central black holes furiously whirling about each other.
The finding suggests that quasars – the brilliant cores of active 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 skaters, the black-hole duo generates tremendous amounts of energy that makes the core of the host galaxy outshine the glow of the galaxy's population of billions of stars, which scientists then identify as quasars.
Scientists looked at Hubble archival observations of ultraviolet radiation emitted from the center of Mrk 231 to discover what they describe as "extreme and surprising properties."
If only one black hole were present in the center of the quasar, the whole accretion disk made of surrounding hot gas would glow in ultraviolet rays. Instead, the ultraviolet glow of the dusty disk abruptly drops off towards the center. This provides observational evidence that the disk has a big donut hole encircling the central black hole. The best explanation for the observational data, based on dynamical models, is that the center of the disk is carved out by the action of two black holes orbiting each other. The second, smaller black hole orbits in the inner edge of the accretion disk, and has its own mini-disk with an ultraviolet glow.
"We are extremely excited about this finding because it not only shows the existence of a close binary black hole in Mrk 231, but also paves a new way to systematically search binary black holes via the nature of their ultraviolet light emission," said Youjun Lu of the National Astronomical Observatories of China, Chinese Academy of Sciences.
"The structure of our universe, such as those giant galaxies and clusters of galaxies, grows by merging smaller systems into larger ones, and binary black holes are natural consequences of these mergers of galaxies," added co-investigator Xinyu Dai of the University of Oklahoma.
The central black hole is estimated to be 150 million times the mass of our sun, and the companion weighs in at 4 million solar masses. The dynamic duo completes an orbit around each other every 1.2 years.
The lower-mass black hole is the remnant of a smaller galaxy that merged with Mrk 231. Evidence of a recent merger comes from the host galaxy's asymmetry, and the long tidal tails of young blue stars.
The result of the merger has been to make Mrk 231 an energetic starburst galaxy with a star-formation rate 100 times greater than that of our Milky Way galaxy. The infalling gas fuels the black hole "engine," triggering outflows and gas turbulence that incites a firestorm of star birth.
The binary black holes are predicted to spiral together and collide within a few hundred thousand years.
Mrk 231 is located 581 million light-years away.
The results were published in the August 14, 2015, edition of The Astrophysical Journal.