Something interesting is going on in the galaxy NGC 6240, some 400 million light years from the Sun in Ophiuchus. Rather than sporting a single supermassive black hole at its center, this galaxy appears to have two, located about 3000 light years from each other. A merger seems likely, or is it? Centauri Dreams regular Don Wilkins returns to his astronomical passion with a look at why multiple supermassive black holes are puzzling scientists and raising questions that may even involve new physics.
By Don Wilkins
Super massive black holes (SMBH), black holes with a mass exceeding 100,000 solar masses, don’t behave as expected. When these galaxies collide, gas and dust smash into each other forming new stars. Existing stars are too far apart to collide. The two SMBH of the galaxies converge. Intuition foresees the two massive bodies coalescing into a single giant, Figure 1. The Universe, as frequently happens, ignores our intuition.
The relevant force is dynamical friction. [1-4] As a result of it, the SMBH experience a deceleration in the direction of motion. Gas and stars between the two SMBH leach momenta from the black holes. The smaller masses pick up enormous speeds and are hurled away from the SMBH. Over time and as immense masses are thrown away, the SMBH inch closer together.
The effect is similar to the gravitational assist maneuver, the fly-by of a massive object, used to accelerate space probes.
The paradox occurs when the gasses and stars have all been expelled from the volume between the two SMBH, a distance of about 3 lightyears. There is no more mass to siphon off momenta. Modeling indicates the standoff would last longer than the life of the Universe.
According to Dr. Gonzalo Alonso Alvarez, a postdoc at the University of Toronto:
“Previous calculations have found that this process [the merger of SMBH] stalls when the black holes are around 1 parsec away from each other, a situation sometimes referred to as the final parsec problem.”
Figure 1. Super Massive Black Holes Orbit Each Other. Credit: NASA.
Two Laser Interferometer Gravitational-Wave Observatories (LIGO) employ laser interferometry to detect gravitational waves in the 10 to 1 kiloHertz regions. This band is suitable to sense black holes 5 to 100 times more massive than the sun. To detect collisions of SMBH, the detector must sense nanometer distortions in spacetime.
The size of gravity wave detectors is inversely proportional to the wavelength. A gravity wave detector sized to measure the cry of a SMBH collision would be immense. Scientists in The NANOGrav Collaboration have overcome the need for detectors with dimensions of light-years by employing pulsars.
In this approach, the timing of a number of pulsars is very accurately measured. A pulsar timing array used sixty-eight pulsars as timing sources. Gravitational waves, compressing and expanding spacetime in their passage, alter the timing of each pulsar in a small way. Timing changes were collected for fifteen years.
Evidence for gravitational waves with periods of years to decades was found. The data are under evaluation to determine the source of the distortions. One possibility is the collision of SMBH.
There are five possible solutions to the paradox. The first is that the NanoGravity detections are not SMBH collisions. There is no paradox. Two SMBH will not merge within the life of the Universe. Rather boringly, our “maths” are correct. Nothing new to learn. [5-12]
Researchers propose that more realistic, triaxial and rotating galaxy models resolve the paradox in ten billion year or less. [13]
Another solution involves three SMBH. The third member continues to remove momentum from the other SMBH until gravitational attraction pulls its two partners into a single black hole. [14]
Expelled gas and stars may return to the two SMBH. These can continue to siphon off momentum until a collision occurs. [15]
Dark matter could contribute to reducing momenta. In this case, particles of dark matter must be able to interact with each other. [16] From Dr. Alvarez whose team published the dark matter paper:
“What struck us the most when Pulsar Timing Array collaborations announced evidence for a gravitational wave spectrum is that there was room to test new particle physics scenarios, specifically dark matter self-interactions, even within the standard astrophysical explanation of supermassive black hole mergers.”
Figure 2. Image: The distorted appearance of NGC 6240 is a result of a galactic merger that occurred when two galaxies drifted too close to one another. When the two galaxies came together, their central black holes did so, too. There are two supermassive black holes within this jumble, spiraling closer and closer to one another. They are currently only some 3,000 light-years apart, incredibly close given that the galaxy itself spans 300,000 light-years. Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).
For the moment, several theories have been advanced to explain the merger of SHBM. Whether further evaluation of the nano wavelength data reveals SBHM coalescence is the primary question.
References
1. S. Chandrasekhar, Dynamical Friction I. General Considerations: the Coefficient of Dynamical Friction, https://articles.adsabs.harvard.edu/pdf/1943ApJ….97..255C
2. S. Chandrasekhar, The Rate of Escape of Stars from Clusters and the Evidence for the Operation of Dynamical Friction, https://articles.adsabs.harvard.edu/pdf/1943ApJ….97..263C
3. S. Chandrasekhar, Dynamical Friction III. A More Exact Theory of the Rate of Escape of Stars from Clusters, https://articles.adsabs.harvard.edu/pdf/1943ApJ….98…54C
4. John Kormendy and Luis C. Ho, Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies, https://arxiv.org/pdf/1304.7762
5. The NANOGrav Collaboration, Focus on NANOGrav’s 15 yr Data Set and the Gravitational Wave Background, The Astrophysical Journal Letters, Volume 951, Number 1, 29 June 2023, https://iopscience.iop.org/collections/apjl-230623-245-Focus-on-NANOGrav-15-year
6. Gabriella Agazie, et al, The nanograv 15 yr data set: evidence for a gravitational-wave background, The Astrophysical Journal Letters, Volume 951, Number 1, 29 June 2023, https://iopscience.iop.org/article/10.3847/2041-8213/acdac6/meta
7. Gabriella Agazie, et al, The NANOGrav 15 yr Data Set: Observations and Timing of 68 Millisecond Pulsars, The Astrophysical Journal Letters, Volume 951, Number 1, 29 June 2023, https://iopscience.iop.org/article/10.3847/2041-8213/acda9a/meta
8. Gabriella Agazie, et al, The NANOGrav 15 yr Data Set: Detector Characterization and Noise Budget, The Astrophysical Journal Letters, Volume 951, Number 1, 29 June 2023, https://iopscience.iop.org/article/10.3847/2041-8213/acda88/meta
9. Gabriella Agazie, et al, The NANOGrav 15 yr Data Set: Search for Signals from New Physics, The Astrophysical Journal Letters, Volume 951, Number 1, 29 June 2023, https://iopscience.iop.org/article/10.3847/2041-8213/acdc91/meta
10. Gabriella Agazie, et al, 15 yr Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries, The Astrophysical Journal Letters, Volume 951, Number 1, 29 June 2023. https://iopscience.iop.org/article/10.3847/2041-8213/ace18a/meta
11. Gabriella Agazie, et al, The NANOGrav 15 yr Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational-wave Background, The Astrophysical Journal Leteters, Volume 951, Number 1, 29 June 2023
https://iopscience.iop.org/article/10.3847/2041-8213/ace18b
12. Gabriella Agazie, et al, The NANOGrav 15 yr Data Set: Search for Anisotropy in the Gravitational-wave Background, The Astrophysical Journal Letters, Volume 951, Number 1, 29 June 2023, https://iopscience.iop.org/article/10.3847/2041-8213/acf4fd/meta
13. Peter Berczik, David Merritt, Rainer Spurzem, Hans-Peter Bischof, Efficient Merger of Binary Supermassive Black Holes in Non-Axisymmetric Galaxies, https://arxiv.org/pdf/astro-ph/0601698
14. Masaki Iwasawa, Yoko Funato and Junichiro Makino, Evolution of Massive Blackhole Triples I — Equal-mass binary-single systems, https://arxiv.org/pdf/astro-ph/0511391
15. Milos Milosavljevic and David Merritt, Long Term Evolution of Massive Black Hole Binaries, https://arxiv.org/pdf/astro-ph/0212459
16. Gonzalo Alonso-Álvarez et al, Self-Interacting Dark Matter Solves the Final Parsec Problem of Supermassive Black Hole Mergers, Physical Review Letters (2024). https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.021401
Two super massive black holes in close orbit around each other loose their orbital momentum in accordance with general relativity, the radiation of gravity waves. The same is true about two neutron stars or white dwarfs. The loss of orbital angular momentum was first detected by radio telescope in the 1970’s. The radio waves came from an orbiting pair of pulsars. They eventually merge and that is what LIGO detects. I didn’t know that LIGO detects gravitational waves in the 10 to 1 kiloHertz regions. Interesting.
From the work on pulsars it would seem that the Universe (multiverse?) is chock-full of information as a consequence ofthe interconnectedness of all things; it is not so recognized because of shortcomings in perceiving, registering and interpreting the deluge of data.
These instances of progress are to be welcomed.
Interesting. I wondered the same thing, but never had enough interest to look into research on the question.
It may be a timing issue. That is, how much material from the colliding and orbiting merging galaxies is left when the SMBH get close but not yet close enough? New material will continue to flow through the area for quite some time, no matter that the SMBH continuously clear their respective and joint volumes. That is difficult to model, but there may have to be enough for the merger to run to completion in most or many cases unless another mechanism comes into play.
But then I haven’t read any of the references.
The relevant force is dynamical friction. [1-4] As a result of it, the SMBH experience a deceleration in the direction of motion. Gas and stars between the two SMBH leach momenta from the black holes. The smaller masses pick up enormous speeds and are hurled away from the SMBH. Over time and as immense masses are thrown away, the SMBH inch closer together.
The effect is similar to the gravitational assist maneuver, the fly-by of a massive object, used to accelerate space probes.
This is confusing two different processes (both of which are operating on different scales). Dynamical friction involves a massive object (e.g. a supermassive black hole) moving through a background population of much less massive objects (e.g. stars, dark-matter particles). The gravity of the massive object attracts the less massive objects, which move towards the massive object. But because the massive object is moving, the attracted objects accumulate in a higher density behind the massive object (a “gravitational wake”). The resulting situtation — a higher density of less massive objects behind the more massive object than the (undisturbed) density in front of it — means a net backwards pull on the more massive object, robbing it of energy.
In the case of a merger of two galaxies with supermassive black holes, the two black holes will lose energy as they move through the back ground of stars and dark matter and move towards the center of the merged galaxy. This is thought to continue until the two black holes form a binary with a separation of a hundred parsecs or so.
Once the binary has formed, a more efficient process takes over (this is probably in part because the black holes are moving too fast for dynamical friction to be strong), which is three-body interactions between individual stars near the galaxy center and the black-hole binary. This tends to result in stars being ejected to larger radii and the black-hole binary shrinking, a process which can proceed until the binary separation is less than ten parsecs or so.
The problem then is that there are thought to be too few stars within the central few parsecs to keep the process going (rapidly). This is because there are fewer stars within the central few parsecs than within the central hundred parsecs to begin with, and because those that are there tend to get ejected to larger radii by the process, and can then no longer interact with the black-hole binary. (This is thought to be the main reason why massive elliptical galaxies have cores with relatively low stellar densities: lots of the stars that ought to be there have been ejected to larger radii.)
(The paper being discussed is proposing an additional stage of dynamical friction operating at small binary separations due to dark-matter particles concentrated near the individual black holes.)
Could it be the rotating BH’s themselves, they will drag spacetime around themselves in a huge way and these spacetime distortions could interact with each other. Each BH’s has plenty of rotational energy to bring them together. In effect the blackholes rotations slow down at the expense of them getting closer together until gravity wave energy disapation becomes dominate and they collide.
As Don says in the article, they are too far apart. Even for their great size the in-spiral time is effectively infinite. Other material (third body/gas/dust) is required, as Peter noted above.
Ron, spacetime is sort of elastic it has a tension and the spacetime path is not straight between BH’s if they are rotating, so in way the BH’s are lopsided allowing a torque to be setup between them. A way to check this would to look at near merging BH’s and check their spin velocities.
Michael, I don’t think you’ve grasped the issue. Until the SMBH are within a fraction of a parsec they will not merge within cosmological time because they are too far apart for gravitational wave radiation to complete the inspiral process more rapidly.
However, there are additional parameters of the binary system that will effect the inspiral time. Obviously, as already discussed, a third party enters the picture. That can be another SMBH or new material/stars from some source. A subsequent galactic merger is the usual scenario for these to occur.
Initial conditions matter; that is, how the SMBH transition from hyperbolic trajectories to formation of a binary system. Depending on those conditions the orbital eccentricities can be extreme which would periodically bring them closer together. The literature I briefly skimmed today says this scenario is low probability.
A few additional references for your reading pleasure:
https://en.wikipedia.org/wiki/Binary_black_hole
https://arxiv.org/html/2403.10804v2
https://academic.oup.com/mnras/article/501/2/2531/6032193?login=false
Ron, forget the gravity wave radiation, we know it’s too small at these distances. My thoughts are along the lines of the distortions of space time around the BH’s due to their spin. Frame dragging extends to great distance from the these massive BH’s and I was wondering if they interact in such a way as to dump their spin energy into reducing the distance between these massive objects.
Indeed, while the gravitational force is immense close to any kind of BH, when they’re a parsec away from each other. Gravity act as for any object made from regular matter, and the accretion disk and other features will not affect them and they could very well end up orbiting for aeons.
It would seem that binaries are common in the universe across size scales. Stellar binaries, black hole binaries, asteroid binaries, and even rogue planet binaries. SMBH binaries will last a long time even with Hawking radiation taking so much longer to dismantle them.
The appearance of binaries is a natural consequence of the accretion of planetesimals from a rotating nebula. They also follow the “rule of twos”, that is two objects orbit a common center of mass. However, one or both of those objects may be a binary itself! Although it is theoretically possible for several objects to orbit their common center of mass, such an arrangement is unstable and doesn’t seem to appear in nature, although it does not violate Newton’s Laws. Naturally occurring binaries seem to have near circular orbits and their rotations and revolution are in the same plane and in the same direction, although perturbations from later encounters with other bodies can change this. In our solar system, all planetary and satellite rotations and revolutions follow the Right-Hand Rule. There are exceptions, like Uranus, but this is probably due to a collision with another body in the distant past.
A star may be a binary, and that binary may be orbiting another star, or another binary. There appears to be no fundamental limit to the number of levels that this pairing can be nested. Objects like open clusters have many members orbiting only one common center of mass but these objects are unstable, and gradually evaporate or disperse themselves due to galactic tidal stresses. In a planetary system environment, planets orbit the star (which may itself be a member of a binary system) and satellites orbit the planets. Although our own system does not seem to exhibit satellites of satellites (except for temporary ones like the Apollo orbiter), there is no reason why they may not exist in other, less cluttered, systems.
Although it is possible for binary systems to form from two objects that meet in space, this is highly unusual for dynamical reasons (a third body must be nearby to absorb excess gravitational energy). The combined velocity of two independent objects attracted by mutual gravitation always exceeds the escape velocity. They will either collide or swing past each other as they head in opposite hyperbolic paths. However, when planets, stars, galaxies etc form from the interstellar medium, binary pairs seem to be a common, if not preferred, result.
This must be a consequence of Newton’s Laws of Gravitation, although I don’t know if anyone has mathematically derived why this should be so. Like Orbital Resonances, it appears to be one of the unanticipated consequences of Classical Celestial Mechanics.
Who was it that said; “That which is not forbidden is mandatory.”?
“binary pairs seem to be a common”
What makes two objects in a multitude (for example during an accretion or rotation phase of a nebula) come together as binary? Why these two black holes or these two stars and not the third one next to it; I suppose that’s a matter of mass?
In short: why 2 and not 3 As explained, could we consider the phenomenon as a kind of galactic turbo?
“why two and not three?”
Masses do not orbit each other. They orbit their respective centers of mass. Two objects in orbit about each other both actually orbit their common center of mass (which may or may not be inside either object). A third object will orbit the center of mass (C.O.M.) of the entire system. For example, Alpha Centauri A and B orbit the center of mass of the two stars. But this system consists of three stars, A, B and C (also known as Proxima). Proxima orbits the center of mass of A and B AND C, while the C.O.M. of A and B ALSO orbits the C.O.M. of A, B and C. And I am ignoring any planets that may reside in this system. Incidentally, the C.O.M. of an orbiting pair is closer to the more massive member of the pair. For example, the C.O.M. of the earth-moon pair is actually beneath the earth’s surface!
In any orbiting pair, the gravitational force is thought of as emanating from the C.O.M. of each member, and each member may itself consist of another orbiting pair. The C.O.M. of the earth-moon system orbits the C.O.M. of the (earth-moon)-sun system, and it gets more complex than that when one considers all the other objects in the solar system.
Now consider three identical masses distributed equidistant on the circumference of a perfectly circular orbit, that is, 120 degrees apart. The C.O.M. of this system is located at the perfect center of this circle, so theoretically, this system could consist of three orbiting masses, each revolving about the center of mass of the system. But in practice, this system is so unstable that the tiniest perturbation from even a tiny distant mass could cause minute accelerations that would cause the system to fall apart. These hypothetical systems do not violate Newton’s Laws of orbital motion, but they do not exist in nature. Instead, orbiting systems arise in a hierarchical nested cascade of binaries, pairs orbiting pairs, or pairs of pairs, and so on.
Still, there is still room for breathtaking beauty: Consider the double-double Epsilon Lyrae, a favorite of amateur astronomers.
https://en.wikipedia.org/wiki/Epsilon_Lyrae#/media/File:Epsilon_Lyrae_the_double-double.jpg
To the naked eye, a double star. But under magnification, each member resolves further to two doubles. And larger telescopes reveal still more members! I remember reading somewhere that there is one multiple star that consists of at least eleven individual components. And if any one of those has planets…
An excellent discussion of stellar multiplicity can be found here:
https://en.wikipedia.org/wiki/Star_system
Exciting! I had never thought about it. That’s what I like with Centauri Dreams every article brings questions to which we don’t always think. This is because example the case with the idea mentioned above of the space-time frame that would deform into spirals around black holes in rotations: a conception of time which we do not think often! Thank you Henry
fred,
Supermassive black holes become binaries when galaxies collide. Binary and multiple star systems start that way. Indeed it is believed that most stars including the Sun start as binaries. Triple and quadruple systems do occur but binary systems are more common. There are many articles and videos explaining the ways they can form.
I am not sure what you mean by galactic turbo.
Perhaps some SMBH’s will never collide in the current state of the universe. There may have just been more matter and dark matter available in the past to allow it to happen but now not enough. They will be forever orbiting each other until negative energy eventually throws them apart.
https://www.caltech.edu/about/news/colossal-black-holes-locked-in-dance-at-heart-of-galaxy