New method developed for detecting colliding supermassive black holes

Scientists have developed an innovative method for detecting colliding supermassive black holes. The study is published in the Astrophysical Journal and conducted by Xinjiang Zhu PhD student at the ARC Center of Excellence for the Discovery of Gravitational Waves (OzGrav) at Monash University.

At the center of every galaxy in the universe is a supermassive black hole that is millions or billions of times the mass of the sun. Large galaxies gather from smaller galaxies merging with each other. This suggests that collisions of supermassive black holes will be commonplace in space. But this process remains elusive: no convincing evidence of its existence has yet been found.

One way to see such mergers is by emitting gravitational waves – ripples in the fabric of space and time. Distant merging pairs of supermassive black holes emit gravitational waves as they spiral around each other. Since black holes are large, each such wave travels past the Earth for many years. Astronomers have used a technique known as the pulsar time matrix to observe gravitational waves from supermassive binary black holes. But, alas, so far to no avail.

In parallel, astronomers were looking for a collision of supermassive black holes using light. A number of candidates for light sources have been identified by searching for quasars. Quasars are very bright and are thought to be powered by clusters of gas clouds on supermassive black holes.

If there are two black holes in the center of a quasar, orbiting each other (instead of one black hole), the orbital motion can change the cluster of the gas cloud and lead to a periodic change in its brightness. Hundreds of candidates have been identified through this type of search.

“If we can find a pair of merging supermassive black holes, it will not only tell us how galaxies evolved, but also show the expected signal strength of gravitational waves for observers of pulsars,” the scientists explain.

OzGrav’s research aims to resolve the controversy by determining whether any of the identified quasars could be powered by colliding black holes. Verdict? Probably no.

“We have developed a new method that allows us to simultaneously search for a periodic signal and measure the noise properties of a quasar,” explains Xinjiang Zhu of OzGrav. “Therefore, it must provide a reliable estimate of the statistical significance of the detected signal.”

By applying this method to one of the best-known candidate sources, named PG1302-102, the researchers found compelling evidence for periodic variation; however, they argued that the signal is likely to be more complex than existing models.

“The conventional model for quasar noise is wrong,” adds Zhu. “The data show additional features in the random fluctuations of gas accumulation in supermassive black holes.”

“Our results show that quasars are complex,” explains co-author and principal investigator of OzGrav, Eric Train. “We will need to improve our models if we are going to use them to identify supermassive binary black holes.”