In a single swoop, astronomers have significantly increased the number of known supermassive black hole pairs, adding five new couples to the nine already known.
The international team of researchers used a combination of optical, infrared, and X-ray telescopes to discover the objects. Their results are published in two papers – one in the Astrophysical Journal and the other in Monthly Notices of the Royal Astronomical Society.
"Astronomers find single supermassive black holes all over the universe," one of the lead-authors, Shobita Satyapal from George Mason University, said in a statement. "But even though we've predicted they grow rapidly when they are interacting, growing dual supermassive black holes have been difficult to find."
The starting point of this research was to identify where black hole pairs might be hiding – the most likely place is merger galaxies. The team managed to select the right targets using galaxies already labeled as interacting by citizen scientists at the Galaxy Zoo project.
The researchers then used NASA’s Wide-Field Infrared Survey Explorer (WISE) and found seven potential targets that appeared to have at least one black hole. They followed up the observation with NASA’s Chandra X-ray Observatory and were able to identify two X-ray signatures in five of these sources. Compelling evidence for supermassive black hole pairs.
"Our work shows that combining the infrared selection with X-ray follow-up is a very effective way to find these black hole pairs," the lead other of the second paper, Sara Ellison of the University of Victoria in Canada, explained. "X-rays and infrared radiation are able to penetrate the obscuring clouds of gas and dust surrounding these black hole pairs, and Chandra's sharp vision is needed to separate them."
Black hole pairs are of great interest to scientists. Their interactions and eventual mergers allow for the testing of theoretical physics and the black holes themselves play a role in the evolution of their host galaxies. They also emit a lot of gravitational waves so it is important to understand as much as we can about them.
"It is important to understand how common supermassive black hole pairs are, to help in predicting the signals for gravitational wave observatories," added Satyapal. "With experiments already in place and future ones coming online, this is an exciting time to be researching merging black holes. We are in the early stages of a new era in exploring the universe."
LIGO/VIRGO cannot detect the gravitational waves from supermassive black hole mergers but future instruments like LISA should be capable of doing so.