Thirty-three pairs of supermassive black holes, each pair on a spiral of doom inside a galaxy, have been discovered by a team of US astronomers thanks to their unique ‘dancing’ styles.

When galaxies merge, their central giant black holes do too, their gravity pulling them gradually closer and closer until they collide to form an even larger black hole. Such black hole collisions are thought to emit a cascade of gravitational waves that it is hoped detectors such as LIGO (Laser Interferometer Gravitational Wave Observatory) and the upcoming LISA (Laser Inteferometer Space Antenna) mission will be able to detect. However, without knowing how many impending black hole collisions there are in the Universe, it is hard to guess how frequently we ought to be detecting gravitational waves.

Enter Dr Julia Comerford of the University of California, Berkeley, and her team. In her presentation today at the American Astronomical Society meeting in Washington, DC, she describes how 33 pairs of black holes have been identified by their waltz-like motion as they spiral around one another. Thanks to their gas-guzzling antics, some black holes flare brightly as active galactic nuclei, and by measuring the redshift of this light Comerford’s team were able to determine whether there was one black hole, or two. As they spin around each other, separated by thousands of light years, black holes moving away from us are redshifted and black holes appearing to move towards us have their light blueshifted. This ‘dance’ is the signature of a pair of black holes moving relative to one another at hundreds of kilometres per second.

Comerford’s team found 32 black hole pairs out of 2,000 within the 50,000-strong Deep Imaging Multi-Object Spectrograph (DEIMOS) survey conducted with the ten-metre Keck II telescope on Mauna Kea in Hawaii. Each pair is the result of a galaxy merger with look-back times of four to seven billion years (cosmological redshifts of 0.3–0.8). Beyond this range the emission lines were redshifted out of the wavelength range the survey could detect. Also, only non star-forming red galaxies were chosen because blue galaxies produce emission lines from star formation that could have been confused with the black holes.

One more black hole pair was discovered thanks to the visual inspection of a galaxy merger imaged by the Advanced Camera for Surveys onboard the Hubble Space Telescope. The galaxy displays all the classic signs of a recent merger – tidal tails of stars and gas, a distorted shape and two bright regions near the centre. To confirm these bright regions were two black holes orbiting one another, Comerford and her team looked at this four billion light year distant galaxy with the DEIMOS Spectrograph on the Keck II telescope to confirm that they were indeed ‘dancing’ around one another, 8,000 light years apart.

Their findings have helped bolster the ranks of known black hole pairs, which astronomers have been expecting to find because of the frequency of galactic mergers, but until now had only identified a handful.

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