Caltech scientists track cosmic collision
By Amanda Jermyn



I love the serendipity of science. Sometimes your search in one area results in a totally different discovery, perhaps more interesting than the one you had originally set out to find. Something like this happened when George Djorgovski of Caltech and his research team embarked on a study of the variability in brightness of quasars, the most luminous, powerful and energetic objects we know of in the universe.

A quasar is a compact region near the center of a galaxy surrounding the galaxy’s supermassive black hole. The black hole’s powerful gravity attracts nearby dust and gas which form an accretion disk that swirls around it and spirals inward towards the center of the black hole. This process generates tremendous friction, heat and gravitational stresses as the material is compressed by gravity, resulting in the emission of powerful radiation in the form of rotating beams of energy, some of which we view as light. While you can’t see a black hole itself, the radiation generated by its quasar can be detected because it comes from the accretion disk which is outside the event horizon, the spherical boundary of the black hole beyond which nothing can escape.

Earlier this year, Djorgovski and his research team reported in the journal Nature that, while looking for patterns in the brightness of quasars, they discovered an unusual repeating light signal from a quasar known as PG1302-102, about 3.5 billion light-years away, in the constellation Virgo. They concluded that this was most likely the result of two supermassive black holes spiraling towards each other in the final stages of a merger, something that had been predicted in theory but never observed before. The discovery was made after analyzing the results from the Catalina Real-Time Transient Survey which uses three ground telescopes in the United States and Australia to continuously monitor about 500 million light sources scattered across 80 percent of the night sky.

The study’s first author, Matthew Graham of Caltech, found that the signal from the quasar varied by about 14 percent roughly every five years. 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 exact mechanism involved is still unclear. One possibility is that the accretion disk surrounding both black holes is distorted. Another is that the quasar is funneling material from its accretion disk into twin jets rotating like beams from a lighthouse. A third option is that the accretion disk is dumping material onto the black holes at regular intervals, causing periodic bursts of energy. According to Graham, any of these mechanisms would still be the result of having two black holes in close proximity.

The cataclysmic collision is expected to occur within a million years, which, in cosmic terms, is very soon. The two black holes are already very close, circling each other at a distance of only a few hundredths of a light year apart, which in cosmic terms is very close. Their collision will be spectacular, releasing about as much energy as 100 million of the violent supernova explosions in which massive stars end their lives. According to Djorgovski, this could scatter nearby stars and wreak havoc with the quasar’s galaxy. In reality, given that the signal is coming from a quasar 3.5 billion light years away, the scenario described here must already have taken place in the distant past, with the light from the quasar only reaching us now.

Mergers of black holes should be common in the universe because galaxies are frequently colliding, and indeed, such mergers have been detected before. However, this is the first time the very end stage of a merger has been observed. According to Matthew Graham, “The end stages of the merger of these supermassive black hole systems are very poorly understood.” So the discovery of a system at this stage provides an opportunity to observe what is actually going on. It could also provide a preview of the fate of our own Milky Way galaxy, which, in a few billion years, will collide with the nearby Andromeda galaxy, resulting in the merger of the black holes at the center of each.

Join the Springfield Stars Club on Tuesday, March 24th at 7pm at the Springfield Science Museum for a panel discussion on Great Unanswered Questions in Astronomy. 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. For more information please visit the Stars Club – Massachusetts page on Facebook at https://www.facebook.com/groups/929125583765113/ or the Stars website, reflector.org.

Also, on Friday, April 3rd at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars Over Springfield.” Alan Rifkin will speak on A Tour of the Universe with Binoculars. A fee of $3 for adults and $2 for children under 18 will be charged.