Quasars, the brightest objects in the universe, can serve as cosmic signposts that lead astronomers to pairs of supermassive black holes.
While scientists are aware that supermassive black holes with masses of millions or even billions of times the sun lurk at the center of all large galaxies, detecting binary pairings of these cosmic titans has been difficult. It 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 lot of supermassive black hole binaries out there, on the verge of colliding and creating an even more monstrous daughter supermassive black hole. So where are they?
Quasars, the luminous hearts of active galaxies powered by the feeding of supermassive black holes, could be the answer, new research suggests. 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 help hunt these terrible twos using gravitational waves, tiny ripples in space and time (which coalesce into a four-dimensional entity called space-time) that were first predicted in Einstein’s 1915 theory of general relativity.
“These findings are useful for targeted searches for supermassive black hole binaries, where we search specific galaxies and quasars for persistent gravitational waves from individual supermassive black hole binaries,” lead author Andrew Casey-Clyde, a doctoral candidate at the University of Connecticut and a visiting scholar at Yale University, told Space.com.
“Our results mean that these targeted searches would be seven times more likely to find gravitational waves from a supermassive black hole binary in a quasar than they would be to find gravitational waves in a randomly large galaxy,” Casey-Clyde said.
Relating to: Strangely flickering jets could be evidence of elusive supermassive black hole pairs
Ironically, the team’s exploration began with a disappointing finding. In 2015, the Catalina Real-Time Transient Survey (CRTS), conducted by three telescopes covering a wide area of the sky, suggested that 111 quasars with periodic light curves could be candidates for supermassive black hole binaries.
But 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 sample, this study shows that quasars are 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 an enormous amount of matter in the form of an accretion disk, a flattened cloud of gas and dust that gradually feeds matter into them. The immense gravitational pull of these supermassive black holes produces strong tidal forces in the accretion disks, which in turn cause friction that heats this matter and causes it to glow brightly in the electromagnetic spectrum.
In addition, material that does not feed into the black hole is directed to its poles, where it is ejected 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 become so bright that they are brighter than the combined light of every star in the surrounding galaxy.
Usually, a supermassive black hole will feast and thus produce a quasar because it is inside a galaxy that has merged with another galaxy of similar size. This collision acts like a cosmic Grubhub, bringing a fresh source of gas and dust into the black hole. The galactic merger also brings two supermassive black holes into close proximity.
Binary quasars are supermassive black hole binary systems with associated quasar activity originating from an accretion disk surrounding both supermassive black holes in the binary.
“We know that quasars can be triggered by major mergers of galaxies.
two galaxies of similar mass merge. These mergers also occur
“Eventually, the formation of a supermassive black hole binary,” Casey-Clyde said. “Since supermassive black hole binaries form in galaxy major mergers, and quasars can be triggered by these mergers, this suggests that some quasars may be associated with supermassive black hole binaries.”
Supermassive black hole binaries don’t like their quasars to be too bright
For this research, the team looked at quasars with light bursts that repeat over a specific time period, known as periodic light curves. The simulations suggested that the periodic light curves associated with the quasars could be the signature of a supermassive black hole binary. An integral part of their study was a collection of extremely precisely spinning neutron star pulsars called the NANOGrav pulsar array. Spinning hundreds of times per second, the pulsars can collectively be used as an extremely precise cosmic chronometer.
Last year, the NANOGrav pulsar array detected a faint signal of background gravitational waves from distant black hole mergers, and the team was able to use this detection to constrain the population of supermassive black hole binaries. NANOGrav’s pulsars then helped the team place limits on the quasar population.
Because the team used a combination of electromagnetic observations of quasars and detections of gravitational waves from the NANOGrav pulsar array, the research is an example of “multi-messenger astronomy,” in which the universe is studied using at least two completely different signals in combination.
“Multiple-messenger astronomy was crucial to constraining the binary quasar population in this study. In particular, since 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 have long suspected that quasars could lead to supermassive black hole binaries because they both have connections to major mergers of galaxies. We have now shown that the relationship is still reasonable, even after accounting for contamination in the CRTS sample.”
The results also surprised Casey-Clyde and her team, as they found that brighter quasars were less likely to host a supermassive black hole binary than fainter quasars.
“It was surprising that the brightest binary quasar candidates were the least likely to be true. However, it makes sense given the rarity of high-mass supermassive black hole binaries,” Casey-Clyde says. “This is because the brightest binary quasars should 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.”
This means that lower-mass supermassive black hole binaries remain within range of such objects that pulsar timing arrays can detect for longer periods of time, and are therefore much more likely to be detected.
Relating to: What are pulsars?
RELATED STORIES:
— What is the black hole event horizon (and what happens there)?
— Where do black holes lead to?
— Astronomers see black hole jet change direction in cosmic first glance — and it’s headed straight for us
Targeted gravitational wave searches are one of the most important next steps for this research, Casey-Clyde added, and the team also aims to hunt for very distant pairs of black holes, which represent the stage before the formation of a close supermassive black hole binary.
“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 impacts on a quasar.
“The search for binary AGNs will be important for constraining supermassive black hole pairings, which are the widely spaced precursors of 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 universe, and thus better understand the relationship between quasars and galaxy mergers.
“The upcoming Space and Time Legacy Survey (LSST) conducted by the Vera C. Rubin Observatory will be crucial to improving the constraints on the binary quasar population,” Casey-Clyde concluded. “We will have to wait about a decade
“The number of observations is too small to do this, because binary quasar light curves are thought to have periods on the order of years.”
The team’s research is published as a preliminary peer-reviewed paper on the arXiv repository.