Astronomers try to catch titanic black hole clashes in action | Science

It’s the ultimate cosmic face-off: a pair of supermassive black holes (SMBHs), each with a mass of millions of Suns, warily circling each other and spiraling toward a titanic clash. Such mergers are thought to culminate in the universe’s most energetic blasts of gravitational waves, and they must be common to explain how SMBHs, found at the hearts of most galaxies, grow so big. But despite decades of searching, not a single SMBH binary has been conclusively identified. “We’ve been in a long dry spell of stuckness,” says Jenny Greene of Princeton University.

At a meeting this month at the Royal Astronomical Society (RAS) in London, researchers reported on ongoing searches that have found tantalizing hints of SMBH binaries from across the electromagnetic spectrum. None have been confirmed, but growing data sets and new instruments could finally catch SMBHs in their lumbering dances. “I hope one of these things will break through,” Greene says.

The challenges are many. By definition, black holes emit no light of their own. The gravitational waves from SMBH collisions are at frequencies beyond the reach of current Earth-based detectors. And SMBH duos would emit other detectable signals only when they are close together, separated by a few light-years or less in orbits lasting at most a few decades. At that separation, even those black holes with bright “accretion disks” of matter being sucked into the hole would be too close to be distinguished by today’s sharpest eyed telescopes.

Astronomers look instead for odd, periodic behavior in light from SMBH accretion disks. One signature might originate in the cooler gases just beyond a disk’s edge. They emit light at specific wavelengths, which the gases’ swirling motion smears into “broad emission lines” through the Doppler effect.

If each SMBH in a duo had an accretion disk, however, they would produce two distinct sets of broad emission lines, displaced from each other by the motion of the black holes. Repeated observations might reveal variations in the position of the lines as the SMBHs circle each other. “They are only tiny fractions of an orbit, but they should be measurable,” Greene says.

More than a decade ago, Greene and her colleagues searched in data from the Sloan Digital Sky Survey, which has logged spectra from millions of galaxies since 2000. Although their trawl turned up seven galaxies with duplicated broad emission lines, none has showed clear signs of shifting since then. “Time scales are too short,” Greene told the RAS meeting. “If [Sloan] goes for another 10 years … we may see signals.”

Another tactic is to look for periodic flaring in the overall brightness of an accretion disk, which could be a sign of a disturbance from an SMBH companion. For example, an SMBH on a close but tilted orbit around a companion with an accretion disk might crash through the disk twice per orbit, causing it to flare.

Last year, in a preprint posted on arXiv, a team reported seeing just such periodic flaring in a galactic core spied by an optical survey telescope in California, and it was speeding up: from yearly to monthly. The team believed it was the final death spiral of an SMBH binary and predicted a merger within the year. “Unfortunately, it did not turn out to work that way,” says team member Huan Yang of the Perimeter Institute: The flaring rhythm became erratic.

Another prime candidate, known as OJ287, has flared every 11 or 12 years since the 1970s. But its latest flare failed to appear when expected in October 2022. “OJ287 could still be a binary, but we also cannot rule out that it is no binary at all,” says Stefanie Komossa of the Max Planck Institute for Radio Astronomy (MPIfR), whose team has been monitoring it since 2015.

Greene says she isn’t surprised that the hunt for periodic flares hasn’t paid off. Accretion disks are inherently noisy and can flare from other events, such as the SMBH swallowing stars or gas clouds. “There are many candidates, but nobody believes them,” she says.

Another way that SMBHs announce their presence is via jets, narrow beams of ionized gas fired out from the black hole’s poles at close to the speed of light. The ions gyrate around the SMBH magnetic field lines, producing synchrotron radiation at many wavelengths. If the SMBH producing the jet is orbiting in a binary, it may wobble like a top and blast out a helical jet, leaving ghostly corkscrew trails of glowing gas visible at radio wavelengths. Maya Horton of the University of Hertfordshire says the gas trails can persist for thousands or millions of years.

By combing through archives of radio images, Horton and her colleagues have compiled a list of 20 candidates with oddly shaped jets. They hope to find more when the Low Frequency Array (LOFAR), a set of radio antennas stretching across Northern Europe, releases a new data set in the coming months. LOFAR has asked citizen scientists in the Radio Galaxy Zoo project to look for curves in the galaxy images.

But a solitary SMBH can also mimic that signature if its accretion disk is tilted compared with the spin of the black hole. Through a process known as frame-dragging, the black hole causes the disk’s axis of rotation to swing round, or “precess.” And because jets are thought to align with the axis of the disk, a precessing disk should also produce a corkscrew jet.

In addition, theorists don’t yet fully understand how jets operate, let alone how interacting SMBHs might affect them. “They can hardly be modeled at the moment,” says MPIfR’s Silke Britzen. So she and other observers can’t be sure a curvy jet signals a black hole pair. “We’re more or less guessing.”

In search of a more definitive signal, Britzen’s team zoomed in with high-resolution radio observatories to the base of the jet, to see whether it varies over time. They targeted the flaring galaxy OJ287, whose jet is thought to be aimed almost directly at Earth. In 2018 they published an analysis of 120 images made over more than 2 decades with the Very Long Baseline Array, a set of 10 radio dishes stretching across the United States whose data are combined to achieve very high resolution. They found that OJ287’s jet changed shape in a way that seems to repeat every 22 years. Its brightness followed the same pattern. Recent, unpublished results show OJ287’s distribution of energy across frequencies also pulses over a 22-year cycle.

The three synchronous phenomena are evidence for a wobbling jet, Britzen argues. “The jet is working like a clock,” she says. Although she and her colleagues can’t rule out a precessing disk around a single SMBH, they favor a binary explanation, and have identified another 11 galactic cores showing similar patterns.

Britzen hopes that someday astronomers will be able to zoom in even further and see the binary SMBHs themselves with an upgraded version of the Event Horizon Telescope—an array of radio dishes spanning the globe that in 2019 produced the first image of an SMBH. The array might need to be expanded with radio dishes in space to get the resolution needed to discern an SMBH pair at galactic distances, but the payoff would be worth it, she says. “It would be fantastic to really see the two cores rotating.”

It’s the ultimate cosmic face-off: a pair of supermassive black holes (SMBHs), each with a mass of millions of Suns, warily circling each other and spiraling toward a titanic clash. Such mergers are thought to culminate in the universe’s most energetic blasts of gravitational waves, and they must be common to explain how SMBHs, found at the hearts of most galaxies, grow so big. But despite decades of searching, not a single SMBH binary has been conclusively identified. “We’ve been in a long dry spell of stuckness,” says Jenny Greene of Princeton University.

At a meeting this month at the Royal Astronomical Society (RAS) in London, researchers reported on ongoing searches that have found tantalizing hints of SMBH binaries from across the electromagnetic spectrum. None have been confirmed, but growing data sets and new instruments could finally catch SMBHs in their lumbering dances. “I hope one of these things will break through,” Greene says.

The challenges are many. By definition, black holes emit no light of their own. The gravitational waves from SMBH collisions are at frequencies beyond the reach of current Earth-based detectors. And SMBH duos would emit other detectable signals only when they are close together, separated by a few light-years or less in orbits lasting at most a few decades. At that separation, even those black holes with bright “accretion disks” of matter being sucked into the hole would be too close to be distinguished by today’s sharpest eyed telescopes.

Astronomers look instead for odd, periodic behavior in light from SMBH accretion disks. One signature might originate in the cooler gases just beyond a disk’s edge. They emit light at specific wavelengths, which the gases’ swirling motion smears into “broad emission lines” through the Doppler effect.

If each SMBH in a duo had an accretion disk, however, they would produce two distinct sets of broad emission lines, displaced from each other by the motion of the black holes. Repeated observations might reveal variations in the position of the lines as the SMBHs circle each other. “They are only tiny fractions of an orbit, but they should be measurable,” Greene says.

More than a decade ago, Greene and her colleagues searched in data from the Sloan Digital Sky Survey, which has logged spectra from millions of galaxies since 2000. Although their trawl turned up seven galaxies with duplicated broad emission lines, none has showed clear signs of shifting since then. “Time scales are too short,” Greene told the RAS meeting. “If [Sloan] goes for another 10 years … we may see signals.”

Another tactic is to look for periodic flaring in the overall brightness of an accretion disk, which could be a sign of a disturbance from an SMBH companion. For example, an SMBH on a close but tilted orbit around a companion with an accretion disk might crash through the disk twice per orbit, causing it to flare.

Last year, in a preprint posted on arXiv, a team reported seeing just such periodic flaring in a galactic core spied by an optical survey telescope in California, and it was speeding up: from yearly to monthly. The team believed it was the final death spiral of an SMBH binary and predicted a merger within the year. “Unfortunately, it did not turn out to work that way,” says team member Huan Yang of the Perimeter Institute: The flaring rhythm became erratic.

Another prime candidate, known as OJ287, has flared every 11 or 12 years since the 1970s. But its latest flare failed to appear when expected in October 2022. “OJ287 could still be a binary, but we also cannot rule out that it is no binary at all,” says Stefanie Komossa of the Max Planck Institute for Radio Astronomy (MPIfR), whose team has been monitoring it since 2015.

Greene says she isn’t surprised that the hunt for periodic flares hasn’t paid off. Accretion disks are inherently noisy and can flare from other events, such as the SMBH swallowing stars or gas clouds. “There are many candidates, but nobody believes them,” she says.

Another way that SMBHs announce their presence is via jets, narrow beams of ionized gas fired out from the black hole’s poles at close to the speed of light. The ions gyrate around the SMBH magnetic field lines, producing synchrotron radiation at many wavelengths. If the SMBH producing the jet is orbiting in a binary, it may wobble like a top and blast out a helical jet, leaving ghostly corkscrew trails of glowing gas visible at radio wavelengths. Maya Horton of the University of Hertfordshire says the gas trails can persist for thousands or millions of years.

By combing through archives of radio images, Horton and her colleagues have compiled a list of 20 candidates with oddly shaped jets. They hope to find more when the Low Frequency Array (LOFAR), a set of radio antennas stretching across Northern Europe, releases a new data set in the coming months. LOFAR has asked citizen scientists in the Radio Galaxy Zoo project to look for curves in the galaxy images.

But a solitary SMBH can also mimic that signature if its accretion disk is tilted compared with the spin of the black hole. Through a process known as frame-dragging, the black hole causes the disk’s axis of rotation to swing round, or “precess.” And because jets are thought to align with the axis of the disk, a precessing disk should also produce a corkscrew jet.

In addition, theorists don’t yet fully understand how jets operate, let alone how interacting SMBHs might affect them. “They can hardly be modeled at the moment,” says MPIfR’s Silke Britzen. So she and other observers can’t be sure a curvy jet signals a black hole pair. “We’re more or less guessing.”

In search of a more definitive signal, Britzen’s team zoomed in with high-resolution radio observatories to the base of the jet, to see whether it varies over time. They targeted the flaring galaxy OJ287, whose jet is thought to be aimed almost directly at Earth. In 2018 they published an analysis of 120 images made over more than 2 decades with the Very Long Baseline Array, a set of 10 radio dishes stretching across the United States whose data are combined to achieve very high resolution. They found that OJ287’s jet changed shape in a way that seems to repeat every 22 years. Its brightness followed the same pattern. Recent, unpublished results show OJ287’s distribution of energy across frequencies also pulses over a 22-year cycle.

The three synchronous phenomena are evidence for a wobbling jet, Britzen argues. “The jet is working like a clock,” she says. Although she and her colleagues can’t rule out a precessing disk around a single SMBH, they favor a binary explanation, and have identified another 11 galactic cores showing similar patterns.

Britzen hopes that someday astronomers will be able to zoom in even further and see the binary SMBHs themselves with an upgraded version of the Event Horizon Telescope—an array of radio dishes spanning the globe that in 2019 produced the first image of an SMBH. The array might need to be expanded with radio dishes in space to get the resolution needed to discern an SMBH pair at galactic distances, but the payoff would be worth it, she says. “It would be fantastic to really see the two cores rotating.”