by Quasars, the brightest objects in the cosmos, could act as similar beacons, pointing astronomers to undiscovered pairs of supermassive black holes.
Although scientists understand that supermassive black holes containing millions or even billions of times the sun lie at the core of most, or all, large galaxies, binary pairings of these cosmic titans have been difficult to detect. That can’t be because supermassive black hole binaries are extremely 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 about to collide and create an even more monstrous daughter supermassive black hole. But where are they?
New research suggests that quasars — the luminous cores of active galaxies, powered by feeding supermassive black holes — may be the answer to that question. The team behind the research think that galaxies with quasars could be seven times more likely to host elite quasars than other galaxies.
The results could help search for these honest duos using gravitational waves, tiny ripples in space and time (united as a 4-dimensional entity called space-time), predicted for the first time in theory Einstein on general relativity in 1915.
“These results are useful for targeted searches of supermassive black hole binaries, in which we search individual galaxies and quasars for persistent gravitational waves from supermassive black hole binaries,” research author Andrew Casey-Clyde, a doctoral candidate at the University of Connecticut . and a visiting researcher at Yale University, Space.com said.
“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 supermassive galaxy,” said Casey-Clyde.
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Ironically, the team’s discovery began with a disappointing result. In 2015, the Catalina Real-Time Survey (CRTS), conducted by three telescopes that covered a wide area of the sky, suggested that 111 quasars with periodic light curves could be supermassive black hole binary candidates.
However, using a recently measured hum of the universe called the “gravitational wave background,” Casey-Clyde and her colleagues determined that most of these quasar binary candidates were likely 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,” said Casey-Clyde.
Binaries hide a massive black hole behind the brightest objects in the universe
A large amount of matter is surrounded by several supermassive black holes, in the form of a flat cloud of gas and dust called an accretion disk that gradually feeds on matter. The massive gravitational influence of these supermassive black holes generates powerful tidal forces in accretion disks, causing friction that causes this material to glow brightly across the electromagnetic spectrum.
In addition, material that does not enter the black hole is sent to its poles, where it is blasted out as high-energy, high-collision jets. These jets also emit electromagnetic radiation. As a result of these phenomena, these galactic central regions, known as “active galactic nuclei” (AGNs), seen as quasars, can be so bright that they block out the combined light of all the stars in the surrounding galaxy. on them.
Often, the black hole is supermassive and therefore able to generate a quasar because it is inside a galaxy that has merged with another galaxy of the same size. This collision acts as a Cosmic Grubhub, bringing a fresh supply of gas and dust to the black hole. The galactic merger also produces two supermassive black holes in close proximity.
Quasar binaries are systems of supermassive black hole binaries with associated quasar activity from an accretion disk surrounding the two supermassive black holes in the binary.
“We know that quasars can be triggered by large galaxy mergers, where
merger of two galaxies of the same mass. These compounds are also the result of the
supermassive black hole binaries eventually form,” Casey-Clyde said. “Since supermassive black hole binaries are created by massive galaxy mergers, and quasars can be fueled by those mergers, this suggests that some quasars may be associated with supermassive black hole binaries. .”
Supermassive black hole binaries do not like their very bright quasars
For this research, the team looked specifically at quasars with light emissions that repeat themselves over a set period of time, emissions known as periodic light curves. Simulations have suggested that periodic light curves associated with a quasar could be a signature of a supermassive black hole binary. Central to his study was a collection of highly accurate rotating neutron star pulsars known as the NANOGRAV pulsar array. Spinning hundreds of times per second, pulses can be used as a very 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. NANOGRAv pulsars then helped the team put constraints on the quasar population.
Because the team used a combination of electromagnetic observations of quasars and the NANOgrav pulsar array gravitational wave detectors, the research is an example of “multi-message astronomy” — investigations of the universe that use at least two different signals together.
“Multi-message astronomy was critical to constraining the binary quasar population in this work. Specifically, because binary quasars are a subset of both the binary quasar and supermassive black hole populations, each of these has constraints are also constraints on the quasar binary population,” said Casey-Clyde. “We suspect that quasars could signal supermassive black hole binaries for a long time because of the connection between the two with major galaxy mergers. Now we have shown that an association is still plausible, even after consideration of 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 supermassive black hole binaries than fainter quasars.
“It was surprising that the brightest quasar binary candidates are the least likely to be true. However, it makes sense when considering the rarity of high-mass black hole binaries,” Casey-Clyde said. . “This is because the brightest quasar binaries must be associated with the largest supermassive black hole binaries. However, the largest supermassive black hole binaries are rare, as they merge relatively quickly.”
This means that lower-mass supermassive black hole binaries are further into the range of such material that pulsed timing arrays can detect and are therefore more likely to be detected.
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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 dense supermassive black hole binary form.
“In particular, by detecting gravitational waves from a quasar host galaxy, we will be able to test how the orbital motion of a supermassive black hole makes binary imprints on its
quasar light curve,” he said. “Searches for binary AGN will be important to constrain supermassive black hole pairings, which are widely separated 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 cosmos and thus better understand the relationship between quasars and galaxy mergers.
“The upcoming Legacy Space and Time Survey (LSST) at the Vera C. Rubin Observatory will be critical to improving constraints on the quasar binary population,” Casey-Clyde said. “We will have to wait about twenty years
of observations to do so, however, since the light curves of binary quasars are thought to have periods on the scale of years.”
The team’s research has been posted as a pre-peer-reviewed paper on the arXiv repository site.
