Quasars are 'cosmic signposts' pointing to rare supermassive black hole pairs

Quasars, the brightest objects in the cosmos, could act as cosmic signposts, directing astronomers to elusive pairs of supermassive black holes.

Though scientists are aware that supermassive black holes with masses of millions or even billions of times that of the sun lurk at the heart of most, if not all, large galaxies, binary pairings of these cosmic titans have been difficult to detect. That can't be because supermassive black hole binaries are incredibly rare. After all, these behemoths form through mergers that begin when galaxies collide. That means there must be a large population of supermassive black hole binaries out there that are on the cusp of colliding and creating an even more monstrous daughter supermassive black hole. But where are they?

New research suggests that quasars — the luminous hearts of active galaxies, which are powered by feeding supermassive black holes — could be the answer to that question. The team behind the research thinks that galaxies with quasars could be seven times more likely to host supermassive black hole binaries than other galaxies. 

The findings could aid the hunt for these monstrous duos using gravitational waves, tiny ripples in space and time (united as a 4-dimensional entity called space-time), which were first predicted in Einstein's theory of general relativity in 1915.

"These findings are useful for targeted searches for supermassive black hole binaries, in which we search specific galaxies and quasars for continuous gravitational waves from individual supermassive black hole binaries," research lead author Andrew Casey-Clyde, a doctoral candidate at the University of Connecticut and visiting researcher at Yale University, told Space.com. 

"Our results mean that these targeted searches will be up to seven times more likely to find gravitational waves from a supermassive black hole binary in a quasar than in a random massive galaxy," Casey-Clyde said.

Related: Weirdly wobbly jets may be evidence of elusive supermassive black hole pairings

Ironically, the team's discovery began with something of a disappointing finding. In 2015, the Catalina Real-time Transient Survey (CRTS), conducted by three telescopes covering a vast area of the sky, proposed that 111 quasars with periodic light curves could be supermassive black hole binary candidates.

However, using the recently measured hum of the universe called the "gravitational wave background," Casey-Clyde and colleagues determined that most of these binary quasar candidates were likely to be false detections.

"Even after correcting for the large number of false positives in the CRTS  binary candidate samples, this work shows that quasars may be more likely to host supermassive black hole binaries than random galaxies," Casey-Clyde said.

Supermassive black hole binaries hide behind the universe's brightest objects

Some supermassive black holes are surrounded by a vast amount of material, in the form of a flattened cloud of gas and dust called an accretion disk that gradually feeds them matter. The immense gravitational influence of these supermassive black holes generates powerful tidal forces in accretion disks, which cause friction that heats this material and causes it to glow brightly across the electromagnetic spectrum.

Additionally, material not fed to the black hole is channeled to its poles, where it is blasted out as highly collimated, high-energy jets. These jets also emit electromagnetic radiation. As a result of these phenomena, these central galactic regions, called "active galactic nuclei" (AGNs), seen as quasars, can be so bright that they outshine the combined light of every star in the galaxy that surrounds them. 

Often, the supermassive black hole feasts and is thus able to generate a quasar because it is within a galaxy that has merged with another similarly sized galaxy. This collision acts as a cosmic Grubhub, bringing the black hole a fresh supply of gas and dust. The galactic merger also brings two supermassive black holes into close proximity. 

Binary quasars are systems of supermassive black hole binaries with associated quasar activity from an accretion disk that surrounds both supermassive black holes in the binary.

"We know that quasars can be triggered by galaxy major mergers, where
two galaxies of similar mass merge. These mergers also lead to the
eventual formation of a supermassive black hole binary," Casey-Clyde said. "Since supermassive black hole binaries are formed by galaxy major mergers, and quasars can be triggered by those mergers, this suggests that some quasars might be associated with supermassive black hole binaries."

Supermassive black hole binaries don't like their quasars too bright

For this research, the team specifically looked at quasars with light outputs that repeat over a set period of time, emissions known as periodic light curves. Simulations have suggested that periodic light curves associated with quasars might be the signature of a supermassive black hole binary. Integral to their study was a collection of highly precisely rotating neutron star pulsars called the NANOGrav pulsar array. Spinning hundreds of times a second, pulsars can be used as a highly sensitive cosmic stopwatch when considered en masse. 

Last year, the NANOGrav pulsar array detected the faint signal of background gravitational waves from distant black hole mergers, and the team was able to use this detection to constrain the supermassive black hole binary population. The pulsars of NANOGrav then helped the team to place constraints on the population of quasars.

Because the team used a combination of electromagnetic observations of quasars and NANOGrav pulsar array gravitational wave detections, the research is an example of "multi-messenger astronomy" — investigations of the universe that use at least two entirely different signals in unison.

"Multi-messenger astronomy was crucial for constraining the binary quasar population in this work. Specifically, because binary quasars are a subset of both the quasar and supermassive black hole binary populations, constraints on each of these are also constraints on the binary quasar population," Casey-Clyde said. "We’ve suspected that quasars could signpost supermassive black hole binaries for a long time because of the connections both have to major mergers of galaxies. Now we’ve shown that association is still plausible, even after considering contamination in the CRTS sample."

The results also surprised Casey-Clyde and the team, as they found that brighter quasars are less likely to host a supermassive black hole binary than fainter quasars. 

"The fact that the brightest binary quasar candidates are the least likely to be genuine was surprising. However, it makes sense when considering the rarity of high-mass supermassive black hole binaries," Casey-Clyde said. "This is because the brightest binary quasars must be associated with the most massive supermassive black hole binaries. However, the most massive supermassive black hole binaries are rare, because they merge relatively quickly."

That means that lower mass supermassive black hole binaries spend longer in the range of such objects that pulsar timing arrays can detect and are thus much more likely to be detected.

Related: What are pulsars?

Casey-Clyde added that targeted gravitational wave searches are one of the most important next steps for this research, adding that the team also intends to hunt for widely separated black hole pairs that represent the stage before a close supermassive black hole binary forms.

"In particular, detecting gravitational waves from a galaxy hosting a quasar will allow us to test how the orbital motion of a supermassive black hole binary imprints on a
quasar’s light curve," he said. "Searches for dual AGN will be important for constraining supermassive black hole pairings, which are wide-separation precursors to supermassive black hole binaries resulting from recent galaxy mergers."

This will allow the team to better constrain the number of supermassive black hole binaries they expect to see in the cosmos and thus better understand the relationship between quasars and galaxy mergers.

"The Legacy Survey of Space and Time (LSST) soon to be conducted by the Vera C. Rubin Observatory will be crucial for improving constraints on the binary quasar population," Casey-Clyde concluded.  "We’ll need to wait for about a decade
of observations to do so, though, since binary quasar light curves are thought to have periods on the scale of years."

The team's research is posted as a pre-peer-reviewed paper on the repository site arXiv.