Tag Archives: black hole mergers

Artist's view of a black hole-neutron star merger. (Carl Knox, OzGrav - Swinburne University)

Neutron Star and Black Hole’s Final Dance Observed for the First Time

Astrophysicists have finally observed the spiralling merger between a neutron star and a black hole. The cataclysmic event was witnessed in a gravitational wave signal by the LIGO/Virgo/KAGRA collaboration and is the first time that one of these elusive but titanic ‘mixed’ merger events has been spotted and had its nature confirmed. And just like buses, you wait for an age for one to come and then two arrive at once.

The researchers also detected a gravitational wave signal from another event of the same nature just ten days after the first, with the signals picked up by LIGO/Virgo on 5th January 2020 and the 15th January 2020 respectively.

The finding is significant because of the three types of mergers between stellar remnant binaries–neutron star/neutron star mergers, black hole/ black hole mergers, and neutron star/ black hole or mixed mergers–this latter category is the only one we hadn’t detected until now and has proved fairly elusive.

 “With this new discovery of neutron star- black hole mergers outside our galaxy, we have found the missing type of binary,” says Astrid Lamberts, a CNRS researcher at Observatoire de la Côte d’Azur, in Nice, France. “We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way.”

Artist's view of a black hole-neutron star merger. (Carl Knox, OzGrav - Swinburne University)
Artist’s view of a black hole-neutron star merger. (Carl Knox, OzGrav – Swinburne University)

These detections of signals from separate mixed merger events come just six years after the LIGO/Virgo collaboration first detected the gravitational waves confirming predictions regarding ripples in the fabric of spacetime by Einstein’s theory of general relativity a century previous.

Though further observations are needed, the results produced by the team could help astronomers and astrophysicists refine their knowledge of systems in which these elusive mergers occur determining both how these mixed binary pairings form and how frequently their components spiral together and merge.

“Gravitational waves have allowed us to detect collisions of pairs of black holes and pairs of neutron stars, but the mixed collision of a black hole with a neutron star has been the elusive missing piece of the family picture of compact object mergers,” says Chase Kimball, a Northwestern University graduate student. “Completing this picture is crucial to constraining the host of astrophysical models of compact object formation and binary evolution. Inherent to these models are their predictions of the rates that black holes and neutron stars merge amongst themselves.

“With these detections, we finally have measurements of the merger rates across all three categories of compact binary mergers.”

Chase Kimball, Northwestern University

Kimball is the co-author of a study published in the Astrophysical Journal Letters and part of a team that includes researchers from the LIGO Scientific Collaboration (LSC), the Virgo Collaboration and the Kamioka Gravitational Wave Detector (KAGRA) project.

A Gravitational-Wave Signal Signal One Billion Years in the Making

One of the most astounding things about the detection of gravitational waves is just how precise a piece of equipment has to be to detect these tiny ripples in the fabric of spacetime. Since that first key detection in 2015, the National Science Foundation’s (NSF) operators at the LIGO laser interferometer and their counterparts at the Virgo detector in Italy have detected over 50 gravitational wave signals from mergers between black hole pairs and neutron star binaries.

The first mixed neutron star/black hole merger spotted by the collaboration on January 5th is believed to be the result of a merger of a black hole six times the mass of the Sun and a neutron star with a mass 1.5 times that of our star. The event which has been designated GW200105 occurred 900 million light-years away from Earth and was picked up as a strong signal at the LIGO detector located in Livingstone, Louisiana.

The masses of neutron stars and black holes measured through gravitational waves (blue and orange) and electromagnetic observations (yellow and purple). GW 200105 and GW 200115 are highlighted as the merger of neutron stars with black holes. (LIGO-Virgo / Frank Elavsky, Aaron Geller / Northwestern)



LIGO Livingstone’s partner detector located in Hanford, Washington, missed the signal as it was offline at the time. Virgo on the other hand caught the signal but it was somewhat obscured by noise. “Even though we see a strong signal in only one detector, we conclude that it is real and not just detector noise,” says Harald Pfeiffer, group leader in the Astrophysical and Cosmological Relativity department at Max Planck Institute for Gravitational Physics (AEI) in Potsdam, Germany. “It passes all our stringent quality checks and sticks out from all noise events we see in the third observing run.”

The fact that GW200105 was only strongly picked up by one detector makes it difficult to pinpoint in the sky with the international team only able to ascertain that it came from a region about 34 thousand times the size of the Moon.

“While the gravitational waves alone don’t reveal the structure of the lighter object, we can infer its maximum mass,” says Bhooshan Gadre, a postdoctoral researcher at the AEI. “By combining this information with theoretical predictions of expected neutron star masses in such a binary system, we conclude that a neutron star is the most likely explanation.”

Despite the fact that the second mixed merger occurred farther away–1 billion light-years distant from Earth– its signal was spotted by both LIGO detectors and the Virgo detector. This means that the team have been able to localise the merger–named GW200115– more precisely, to a region of the sky that is around three thousand times the size of Earth’s moon. This second merger is believed to have occurred between a black hole nine times the mass of our Sun and a neutron star almost twice the size of the Sun.

These Black Holes Weren’t Messy Eaters


Because of the extraordinary distances involved, astronomers have yet to confirm either merger in the electromagnetic spectrum upon which traditional astronomy is based. Despite being informed of the event almost immediately astronomers could not find telltale flashes of light indicating the mergers.

This is unsurprising as any light from such distant events would be incredibly dim after one billion years of journeying to Earth no matter what wavelength it is observed in, or how powerful the telescope is that is used to attempt the follow-up observation.

There also remains another possibility why no light could be seen from these events. The lack of a signal in electromagnetic radiation could be because the neutron star elements of these mergers were swallowed whole by their black hole partners.

“These were not events where the black holes munched on the neutron stars like the cookie monster and flung bits and pieces about,” explains Patrick Brady, a professor at the University of Wisconsin-Milwaukee and Spokesperson of the LIGO Scientific Collaboration, colourfully. “That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases.”

From a MAYA collaboration numerical relativity simulation of a neutron star-black hole binary merger. Focused on the merging objects showing the disruption of the neutron star. (Deborah Ferguson (UT Austin), Bhavesh Khamesra (Georgia Tech), Karan Jani (Vanderbilt))

Whilst these are the first two confirmed examples of such mixed mergers, there have been suspects spotted by their gravitational-wave signals in the past. In August 2019 a signal designated GW190814 was detected which researchers say involved a collision of a 23-solar-mass black hole with an object of about 2.6 solar masses. This second object could have been either the heaviest known neutron star or the lightest known black hole ever found. That ambiguity left this signal unconfirmed as the product of a mixed merger event and other similar finds have been plagued with similar ambiguities.

Now that two confirmed detections of mixed mergers have been made, astrophysicists can set about discovering if current estimates that say such collisions should occur at a frequency of around one per month within a distance of 1 billion light-years of Earth are correct.

They can also set about discovering the origins of such binaries, possibly eliminating one or two of the proposed locations in which such events are believed to occur: stellar binary systems, dense stellar environments including young star clusters, and the centers of galaxies.

Key to these investigations will be the fourth observation run of the laser interferometers that act as our gravitational wave detectors, set to begin in summer 2022.

“The detector groups at LIGO, Virgo, and KAGRA are improving their detectors in preparation for the next observing run scheduled to begin in summer 2022,” concludes Brady. “With the improved sensitivity, we hope to detect merger waves up to once per day and to better measure the properties of black holes and super-dense matter that makes up neutron stars.”

Artist’s impression of binary black holes about to collide. Image credit: Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

Gravitational waves reveal largest black hole merger and the first intermediate-mass black hole

Gravitational waves have been detected from what appears to be the largest black hole merger ever observed. The powerful and previously unobserved hierarchical merger resulted in  an intermediate-mass black hole, an object never before detected.

A massive burst of gravitational waves equivalent to the energy output of eight Suns has been detected by the LIGO laser interferometer. Researchers at LIGO and its sister project VIRGO believe that the waves originate from a merger between two black holes. But, this isn’t your average black hole merger (if there is such a thing). The merger — identified as gravitational wave event GW190521 — is not only the largest ever detected in gravitational waves — but it is also the first recorded example of what astrophysicists term a ‘hierarchical merger’ occurring between two black holes of different sizes, one of which was born from a previous merger.

“This doesn’t look much like a chirp, which is what we typically detect,” says Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO’s first detection of gravitational waves in 2015. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”

GW190521 facts courtesy of LIGO-VIRGO (R. Ewing, R. Huxford, D. Singh
The Pennsylvania State University)
GW190521 facts courtesy of LIGO-VIRGO (R. Ewing, R. Huxford, D. Singh
The Pennsylvania State University)

Even more excitingly, it seems that black hole birthed in the event has a mass of between 100–1000 times that of the Sun, putting it in the mass range of an intermediate-mass black hole (IMBH). Something that researchers have theorised about for decades, but up until now, have failed to detect.

The gravitational wave signal–spotted by LIGO on 21st May 2019–appears to the untrained eye as little more than four short squiggles that lasted little more than one-tenth of a second, but for Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration, whose group focuses on the development of highly-accurate waveform models, it holds a wealth of information. “It’s amazing, but from about four gravitational-wave cycles, we could extract unique information about the astrophysical source,” she tells ZME Science.

“The waves are fingerprints of the source that has produced them.”

Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.

As well as containing vital information about black holes and a staggering merger event, as the signal originated 17 billion light-years from Earth and at a time when the Universe was half its current age, it is also one of the most distant gravitational wave sources ever observed. The incredible distance the signal has travelled may initially seem at odds with the fact that the Universe is only 14.8 billion years old, but the disparity arises from the fact our universe is not static but is expanding.

A still from a numerical relativity simulation for GW190521 showing the gravitational waves emitted just before merger, overlaid with the signal as observed by the detectors. This is the largest binary system yet detected as shown by the horizons of this event compared to several previous events. (EPO)
A still from a numerical relativity simulation for GW190521 showing the gravitational waves emitted just before merger, overlaid with the signal as observed by the detectors. This is the largest binary system yet detected as shown by the horizons of this event compared to several previous events. (Deborah Ferguson, Karan Jani, Deirdre Shoemaker, Pablo Laguna, Georgia Tech, MAYA Collaboration)

Details of the international team’s important findings are featured in a series of papers publishing in journals such as Physical Review Letters, and The Astrophysical Journal Letters, today.

Missing Intermediete-Mass Black Holes

Thus far, the black holes discovered by astronomers have either been those with a mass inline with that of larger stars–so-called stellar-mass black holes, or supermassive black holes, with masses far exceeding this. Black holes that exist between these masses have remained, frustratingly hidden. Until now.

“The LIGO and Virgo collaborations detected a gravitational wave corresponding to a very interesting black hole merger. This was named GW190521 and corresponds to two large black holes during the final orbit and merger,” Pedro Marronetti, program director for gravitational physics at the National Science Foundation (NSF) responsible for the oversight of LIGO, tells ZME Science.

“What makes GW190521 extraordinary in comparison to other gravitational wave events is the mass of the black holes involved, the product of the merger is a 142 solar mass black hole and the first object of its kind with mass above 100 solar masses but below a million solar masses to be discovered.”

Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)

Thus, the resultant black hole of 142 solar masses  exists in that crucial, thus far undetected, mass range indicating an intermediate-mass black hole (IMBH).

“These black holes, heavier than 100 solar masses but much lighter than the supermassive black holes at the centre of galaxies — which can be millions and billions of solar masses — have eluded detection until now,” Marronetti says. “Additionally, the heavier of the original black holes with 85 solar masses also presents an enigma.”

Pair Instability and Hierarchical Black Hole Mergers

The enigma that Marronetti refers to is the fact that heavier of the two black holes that entered the merger, is of a size that suggests it too must have been created by a merger event between two, even smaller, black holes. “The most common channel of formation of black holes involves heavy stars that end their lives in supernova explosions,” the NSF program director points out. “However, this formation channel prevents the creation of black holes heavier than 65 solar masses but lighter than 130 solar masses due to a phenomenon called ‘pair-instability’.”

This graphic shows the masses of black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange). GW190521 is highlighted in the middle of the graphic as the merger of two black holes that produced a remnant that is the most massive black hole observed yet in gravitational waves. [Image credit: LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]
This graphic shows the masses of black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange). GW190521 is highlighted in the middle of the graphic as the merger of two black holes that produced a remnant that is the most massive black hole observed yet in gravitational waves. [Image credit: LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

As nuclear fusion ceases, there is no longer enough outward radiation pressure to prevent gravitational collapse. “The star suddenly starts producing photons that are energetic enough to create electron-positron pairs,” Marronetti explains. “These photons, in turn, create an outward pressure that is not strong enough to stop the star from collapsing violently due to its self-gravitational pull.”

This results in a difference in gravitational pressure between the star’s core and its outer layers. As a massive shock travels through these ‘puffed out’ outer layers they are blown away in a massive explosion. With smaller stars, this leaves behind an exposed core that becomes a stellar remnant such as a white dwarf, neutron star or black hole. But if the star is a range above 130 solar masses, but below 200 solar masses, the result is more disastrous.

“The resulting supernova explosion completely obliterates the star, leaving nothing behind– no black hole or neutron star is produced,” Marronetti says. “It will take stars heavier that 200 solar masses to collapse into black hole fast enough to avoid this complete disintegration.”

LIGO and Virgo have observed their largest black hole merger to date, an event called GW190521, in which a final black hole of 142 solar masses was produced. This chart compares the event to others witnessed by LIGO and Virgo and indicates that the remnant of the GW190521 merger falls into a category known as an intermediate-mass black hole – and is the first clear detection of a black hole of this type. Intermediate-mass black holes, which have previously been predicted theoretically, would have masses between those of stellar-mass black holes and the supermassive ones at the hearts of galaxies. Image credit: LIGO/Caltech/MIT/R. Hurt (IPAC)
LIGO and Virgo have observed their largest black hole merger to date, an event called GW190521, in which a final black hole of 142 solar masses was produced. This chart compares the event to others witnessed by LIGO and Virgo and indicates that the remnant of the GW190521 merger falls into a category known as an intermediate-mass black hole – and is the first clear detection of a black hole of this type. Intermediate-mass black holes, which have previously been predicted theoretically, would have masses between those of stellar-mass black holes and the supermassive ones at the hearts of galaxies. Image credit: LIGO/Caltech/MIT/R. Hurt (IPAC)

As Marronetti points out; this means that the 85-solar mass back hole could only be formed by the merger of two smaller black holes, as at these masses, collapsing stars can’t form black holes. “This is a quite unusual event that can only occur in regions of dense black hole population such as globular clusters,” the researcher adds. “GW190521 is the first detection that is likely to be due to this ‘hierarchical merger’ of black holes.”

Marronetti continues by explaining that a hierarchical merger consists of one or more black holes that were produced by a previous black hole merger. This hierarchy of mergers allows for the formation of progressively heavier and heavier black holes from an original population of small ones.

“We don’t really know how common these hierarchical mergers are since this is the first time we have direct evidence of one. We can only say that they are not very common.”

Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)

LIGO Delivering Discoveries and Surprises

The team uncovered the unusual nature of this particular merger by assessing the gravitational wave signal with a powerful state of the art computational models. Not only did this reveal that GW190251 originates from the most massive black hole merger ever observed and that this was no ordinary merger but a hierarchical merger, but also crucial information about the black holes involved in the event.

“The signal carries information about the masses and spins of the original back holes as well as their final product,” Marronetti adds, alluding to the fact that the LIGO -VIRGO team were able to measure that spin and determine that as the black holes circled together, they were also spinning around their own axes. The angles of these axes appeared to have been out of alignment with the axes of their orbit. This misaligned spin caused the black holes to ‘wobble’ as they moved together.

Artistic interpretation of the binary black hole merger responsible for GW190521. The space-time, figured by a fabric on which a view of the cosmos is printed, is distorted by the GW190521 signal. The turquoise and orange mini-grids represent the dragging effects due to the individually rotating black holes. The estimated spin axes, or self-rotations, of the black holes are indicated with the corresponding colored arrows. The background suggests a star cluster, one of the possible environments where GW190521 could have occurred. Credits: Raúl Rubio / Virgo Valencia Group / The Virgo Collaboration.
An artistic interpretation of the binary black hole merger responsible for GW190521. The space-time, figured by a fabric on which a view of the cosmos is printed, is distorted by the GW190521 signal. The turquoise and orange mini-grids represent the dragging effects due to the individually rotating black holes. The estimated spin axes, or self-rotations, of the black holes, are indicated with the corresponding coloured arrows. The background suggests a star cluster, one of the possible environments where GW190521 could have occurred. Credits: Raúl Rubio / Virgo Valencia Group / The Virgo Collaboration.)

“Our waveform models were used to detect GW190521 and also to interpret its nature, extracting the properties of the source, such as masses, spins, sky location, and distance from Earth. For the first time, the waveform models included new physical effects, notably the precession of the spins of the black holes and higher harmonics,” Buonanno says. “What we mean when we say higher harmonics is like the difference in sound between a musical duet with musicians playing the same instrument versus different instruments.

“The more substructure and complexity the binary  has — for example, black holes with different masses or spins—the richer is the spectrum of the radiation emitted.”

Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.

Unanswered Questions and Future Investigations

Even with the staggering amount of information the team has been able to collect about the merger that gave rise to the signal GW190251, there are still some unanswered questions and details that must be confirmed.

The LIGO-VIRGO detectors use two very distinct methods to search the Universe for gravitational waves, an algorithm to pick out a specific wave pattern most commonly produced by compact binary mergers, and more general ‘burst’ searches. The latter searches for any signal ‘out of the ordinary’ and it’s the mechanism via which the researchers found GW190215.

Morronetti expresses some surprise that the methods used by the team were able to unlock these secrets, believing that this result demonstrates the versatility of LIGO. “My main surprise was that this event was detected using a search algorithm that was not specifically created to find merger signals,” says the NSF director. “This is the first detection of its kind and shows the capability of LIGO to detect phenomena beyond compact mergers.”

 “This is of tremendous importance since it showcases the instrument’s ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected.”

Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)


This leaves open the small chance that the signal was created by something other than a hierarchical merger. Perhaps something entirely new. The authors hint at the tantalising prospect of some new phenomena, hitherto unknown, in their paper, but Marronetti is cautious: “By far, the most likely cause is the merger of two black holes, as explained above. However, this is not as certain as with past LIGO/Virgo detections.

“There is still the small chance that the signal was caused by a different phenomenon such as a supernova explosion or an event during the Big Bang. These scenarios are possible but highly unlikely.”

Confirming the nature of the event that gave rise to the GW190251 signal is something that the LIGO team will be focusing on in the future as the interferometer also searches for similar events via the gravitational waves they emit. “

With GW190521, we have seen the tip of the iceberg of a new population of black holes,” Buonanno says, adding that LIGO’s next operating run (O4) will explore a volume of space 3 times larger than the current run (O3). “Having access to a larger number of events, which were too weak to be observed during O3, will allow us to shed light on the formation scenario of binary black holes like GW150921.”

Do Black Holes Merge? (NASA/Public Domain)

Double Trouble! Hunting for Supermassive Black Hole Mergers

Supermassive black holes sat at the centre of active galaxies could have company. Binary pairs of these titanic cosmic objects could merge to form an even more monstrous black hole. Observational methods are finally becoming sensitive enough to spot such an event. 

The image of a supermassive black hole sat monolithic and alone at the centre of its galaxy, mercilessly swallowing any matter unfortunately enough to cross its path could be seriously challenged over the coming years. Theories of how galaxies grow and evolve and the role supermassive black holes play in these processes have long suggested that these objects may not dwell alone. In fact, not only may such spacetime events live in pairs, but after being brought together, they may merge in what could be the most powerful single event in the Universe, profoundly affecting its evolution.

Do Black Holes Merge? (NASA/Public Domain)
Artist’s impression of a violent merger between two supermassive black holes (NASA/ Pubic Domain)

“Astrophysical black holes are among the most fascinating objects in the Universe: they are ideal laboratories to study the fundamental laws of physics and one of the main drivers of the evolution of the Universe,” explains Alessandra De Rosa, a research astrophysicist at the National Institute of Astrophysics, Roma, Italy. “Understanding how they work and interact with their close environment, and unveil the physical conditions of the medium around them is one of the major challenges of 21st-century Astrophysics.”

Understanding the relationship between black holes and the galaxies that host them is key to building a model that satisfactorily describes the evolution of both. But, thus far evidence of this process is sparse. So, why are supermassive black hole mergers so hard to spot?

Hidden in Plain Sight. How Supermassive Black Hole Binaries and Mergers evade Observation

Despite the potential power of such a merger event, we haven’t as of yet managed to distinguish individual binary supermassive black holes or much evidence that such collisions occur. This is because these pairings and the mergers that may eventually arise from them lurk in what is known as the Active Galactic Nuclei (AGN) — compact regions at the centre of galaxies where the electromagnetic emissions dwarf that of the entire galaxy which surrounds it.

Because this emission — which occurs from the radio wave to the gamma-ray regions of the electromagnetic spectrum — is so powerful, astronomers believe that it does not arise as a result of stellar activity. Rather, they theorise that the powerful electromagnetic radiation emitted by the AGN is the result of at least one supermassive black hole accreting matter — a violent process in which dust, gas, and even stars are ripped apart in a violent and tremendously hot accretion disc surrounding a central supermassive black hole before falling onto what can roughly be described as its ‘surface.’

Here’s the problem; that electromagnetic emission is so overwhelmingly powerful and the AGN is so small in comparion to its host galaxy that there is no way that traditional astronomy — which relies on electromagnetic signals — alone, can distinguish the finer detail of this region. Finer detail that could reveal occupation by two, rather than just one, supermassive black holes.

“Currently, observational evidence for these pairs is almost non-existent,” De Rosa laments. “This can be explained if they quickly shrink to small separations and become impossible to be resolved with telescopes as pairs. So,  we must rely on indirect signatures.”

Fortunately, supermassive black hole mergers, if they occur, would not just be prodigious producers of electromagnetic radiation. They should also produce intense gravitational wave signals.

De Rosa is the lead author of a review paper published in the journal New Astronomy Reviews that looks both the history of our search for supermassive black hole binaries and puts forward a road map for future discovery of such events. The researcher emphasises the importance of ‘multimessenger astronomy’ — which combines traditional electromagnetic observations with the detection of gravitational waves, allowing astronomers to view the Universe in an entirely new way, thus making events and objects are previously hidden to them — events like black hole mergers — accessible.

But before examing mergers, it’s worth considering the truly epic processes that bring supermassive black hole pairings together in the first place.

Cosmic Matchmaking: Bringing Together Supermassive Black Holes

It may not be too surprising to find supermassive black holes hanging-out together in pairs, as our observations of the Universe thus far, show that stellar objects seem to prefer to hang out in pairs. These binary systems are far more common than single star systems such as our solar system, and three-star systems — the latter of which prove to be far too unstable.

“A binary supermassive black hole is made up of two supermassive black holes that are orbiting around each other,” says Julie Comerford, an Associate Professor in the Department of Astrophysical and Planetary Sciences at the University of Colorado, Boulder who specializes in the study of AGNs. “Such binary systems are common in the universe — around half of all stars are in binary systems, where two stars are orbiting around each other.”

As black holes evolve from such stellar objects, and these objects enjoy the company, it would seem intuitive to believe that black hole binaries should be fairly common. There’s a problem with that thinking though.

Only the most massive stars end their lives as black holes, and supermassive black holes are even rarer. Couple that with the fact that most binary systems contain a massive star coupled with a much smaller counterpart. Thus, It’s quite unlikely that two stars in the same binary system would both end up as supermassive black holes. In fact, after the transformation of the first star, it’s likely its partner will be stripped of material and left as a neutron star, a much smaller white dwarf, or destroyed entirely– possibly consumed by its counterpart.

So, if supermassive binaries aren’t likely to grow together, this means that some event must create this union– the merger of two galaxies.

“Each massive galaxy has a supermassive black hole at its centre, so the way you make a supermassive black hole binary is by merging two galaxies together,” Comerford tells ZME Science. “Each galaxy brings its own supermassive black hole to the merger, and as the galaxies combine the supermassive black holes begin their own dance of orbiting around each other.”

This means that spotting such a supermassive black hole binary would provide good evidence that the galaxy it occupies is the result of a merger, or even, that such a merger is still ongoing. It would also give us a hint at what is to come for our own galaxy. “This will one day happen to our Milky Way Galaxy — when it merges with the Andromeda Galaxy in about 4 billion years,” Comerford continues. “Our supermassive black hole and Andromeda’s supermassive black hole will form a binary!”

Mathematical modelling of these galaxy mergers seems to show that the process causes major gas inflow towards the central supermassive black hole — or black holes, as the case may be — this powers accretion and various nuclear processes activating the galactic nucleus. This inflow of gas, dust and other material could also result in the growth of the supermassive black hole.

“Astronomers believe that galaxies merge one or more times during their cosmological life,” says Alessandra De Rosa, who is is a research astrophysicist at the National Institute of Astrophysics, Roma, Italy. “These gigantic collisions are likely to be the primary process by which supermassive black holes are activated.”

Thus, galactic mergers aren’t just responsible for bringing supermassive black holes together, they also could kick start the feeding frenzy that makes an AGN the source of incredibly powerful radiation.

But what happens when these binary pairs of supermassive black holes form? Do they remain in a binary, or do they combine to form an even larger supermassive black hole? The merger of supermassive black holes to form larger objects would explain certainly one lingering cosmological question; how did these objects grow to such tremendous sizes in such a short period of time?

Despite the convenience of this phenomenon to tie up some loose cosmic-ends, we still don’t really know if it’s happening or not.

Using Gravitational Waves to Shed Light on Black Hole Binaries

After being brought together by a galaxy merger, when the supermassive black holes are very small separations, the gravitational waves that they emit carry away energy and enable the black holes to merge.

Thus, supermassive black holes at the centre of each galaxy are dragged close to each other, and eventually, form what is known as a dual active galactic nucleus. Theoretically, the final stage of this coming together — particularly if the black holes are gravitationally bound — will be the coalescence of these monsters in a merger that results in an even larger supermassive black hole. This merger would be accompanied by the emission of a gravitational-wave signal. Signals that thanks to the Laser Interferometer Gravitational-Wave Observatory LIGO, and its upcoming space-based counterpart Laser Interferometer Space Antenna (LISA), we can now theoretically detect.

“We think that binary supermassive black holes ultimately merge with each other and produce very energetic gravitational waves. In fact, supermassive black hole mergers are second only to the Big Bang as the most energetic phenomena in the Universe,” Comerford explains. The problem is, that even LIGO — responsible for the first detection of gravitational waves from colliding stellar-mass black holes — isn’t yet capable of detecting gravitational waves from merging supermassive black hole.

“These gravitational waves are too high frequency to be detected by LIGO, so they have not yet been detected,” Comerford adds. “But, we expect that pulsar timing arrays will detect gravitational waves from supermassive black hole mergers for the first time in just a few years.”

De Rosa concurs with the possible breakthrough in detecting gravitational waves from supermassive black hole mergers, highlighting not just the future contribution of pulsar timing arrays, but also, that of LISA — a space-based laser interferometer set to launch in 2034. “In the next decades, space-borne gravitational wave observatories, such as the next large mission of the European Space Agency, LISA, and experiments such as the Pulsar Timing Arrays, will provide first direct evidence of binary and merging SMBHs in the Universe,” she explains. 

For Comerford, the breakthrough new gravitational wave detection methods and multi-messenger astronomy stand poised to answer fundamental questions that have influenced her entire career. “When I was a graduate student, my group found some intriguing galaxy spectra that we thought might be produced by supermassive black hole pairs. I wondered if these unusual spectra could be the key to finding supermassive black hole binaries. I’ve been working on new and better ways to find supermassive black hole pairs ever since,” the researcher concludes. 

“I think the shocking thing is that we don’t actually know if supermassive black hole binaries merge! It could be that they just circle around each other and are not able to get close enough to each other where the gravitational waves can take over and make them merge. 

“When we detect gravitational waves from supermassive black holes, that will be the first time that we actually know that supermassive black hole binaries do merge.”


Sources and Further Reading 

De Rosa. A, Vignali. C, Bogdanovic. T, et al, ‘The Quest for Dual and Binary Supermassive Black Holes: A Multi-messenger View,” New Astronomy Reviews, [2020].