Quasar shows black holes on track to collide
By Amanda Jermyn



Long ago, in a galaxy 3.5 billion light years away, two massive black holes circled around each other, spiraling ever closer until they very likely merged in a cataclysmic collision, scattering stars and obliterating the galaxy. In the process, about as much energy would have been released as in 100 million violent supernova explosions, mostly in the form of ripples in space-time known as gravitational waves. Because this happened so far away, the light from the build up to the event is only reaching us now, and it appears to us as if the collision will still take place about 100,000 years in the future.

Earlier this year I reported on the predicted merger, first detected by Matthew Graham and his team of Caltech scientists when they discovered an unusual repeating light signal from a quasar known as PG1302-102. A quasar is a compact region of dust and gas surrounding a galaxy’s supermassive black hole that emits powerful radiation in the form of rotating beams of energy, some of which we view as light. The signal from the quasar varied by about 14% roughly every five years, and the team concluded that the only thing powerful enough to affect a massive black hole in this way would be another massive black hole nearby. The combined mass of the two black holes was predicted to be more than that of a billion suns.

A new analysis of this data by graduate student Daniel D’Orazio and his team from Columbia University helps bolster this conclusion. The scenario, according to the Columbia team, involves a black hole orbiting a supermassive black hole about 10 times its size. Because one black hole is much smaller and they orbit so closely, less than a tenth of a light year apart, the smaller one circles the larger one extremely fast, at about 7% the speed of light. D’Orazio and his team suggest that most of the light emitted by the quasar comes from the disk of gas surrounding the smaller of the two black holes. As the black holes circle each other, the light from the disk coming towards us receives what is known as a Doppler boost, in which matter moving at high speed towards us appears brighter, just as a siren sounds louder and more high-pitched as it approaches us. This phenomenon explains why the quasar’s signal varies by about 14% every five years.

Because the black holes are orbiting each other so closely, their gravitational attraction causes them to spiral towards each other very rapidly, with the final cataclysm as little as 100,000 years away. In reality though, given that the signal is coming from a quasar 3.5 billion light years away, the scenario described here must already have taken place, with the light from the closely spiraling black holes only reaching us now, and the final cataclysm itself still about 100,000 years away.

Though mergers of black holes have been detected before, this is the first time the very end stage of a merger has been observed, providing an unprecedented opportunity to understand what is actually going on.
Until now, existing equipment has not been sensitive enough to detect the weak gravitational waves the black holes should emit as they spiral towards a collision. However, the Advanced LIGO Project, a major upgrade of the Laser Interferometer Gravitational-Wave Observatory, operated jointly by MIT and Caltech, is now up and running, and should see its first observations within the next five years. Detecting gravity waves from the spiraling black holes would provide the ultimate confirmation.

Join the Springfield Stars Club on Tuesday, October 27th at 7pm at the Springfield Science Museum for a talk by astronomy educator Ed Faits on “Twenty-five Years of the Hubble Space Telescope.” Faits is a founding director and current president of the Arunah Hill Natural Science Center in Cummington, MA and is a past president of the Springfield Stars Club. He has given up on his dream of playing Left Field for the Red Sox but still hopes to become an astronaut. Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.

On Friday, November 6th at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Astronomer Paul Cardone will talk on the New Horizons fly-by of Pluto. A fee of $3 for adults and $2 for children under 18 will be charged.