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Could two galaxies (one big and one small)intersect at a velocity to allow the smaller black hole to escape but not the galaxy around it?
Yes, and in fact a mechanism somewhat like this has probably dumped a large number of BHs into intergalactic space.
Black holes tend to settle towards the center of galaxies (an effect of dynamical friction). As they settle, they "cool" by evaporation. The chaos of BHs orbiting the center of mass all interact, especially when two of them approach closely. Depending on the geometry of the near-miss, one BH can gain energy at the expense of the other. One swings into a larger orbit and the other goes into a smaller orbit.
Sometime the larger orbit is hyperbolic and the BH is thrown right out of the galaxy. This removes orbital energy from the assemblage of BHs and the whole thing shrinks a bit and encounters become a bit more common. In the end, many of the original set of BHs are thrown out into intergalactic space.
How many? No one knows yet. We have good evidence of a single very large BH (>106 solar masses) at the center of the Milky Way, but recent results have suggested there may be as many as 10,000 smaller BHs (~10 solar masses each) in orbit around it.
If the latter is right, there may be a lot of BHs wandering intergalactic space!
I assume you're asking about central supermassive black holes (SMBHs, one per galaxy), not stellar-mass black holes.
The answer is yes, but what actually happens is the two SMBHs have to merge first, and then the resulting combined SMBH can sometimes be ejected from the combined (merged) galaxy.
[Edited to add: Since you've updated the question with a series of diagrams, I should state explicitly that the scenario suggested by the diagrams -- stars in smaller galaxy merge into big galaxy, but SMBH continues on almost unaffected -- is not physically possible. Most of the stars from the smaller galaxy will not end up in the center of the big galaxy, but because of dynamical friction, the SMBH will.]
This NASA press release from 2017 describes the discovery of a quasar apparently ejected from a recently merged galaxy. I'll go ahead and quote their description of the suggested mechanism (this possibility has been suggested by theoretical studies going back at least ten or fifteen years):
According to their theory, two galaxies merge, and their black holes settle into the center of the newly formed elliptical galaxy. As the black holes whirl around each other, gravity waves are flung out like water from a lawn sprinkler. The hefty objects move closer to each other over time as they radiate away gravitational energy. If the two black holes do not have the same mass and rotation rate, they emit gravitational waves more strongly along one direction. When the two black holes collide, they stop producing gravitational waves. The newly merged black hole then recoils in the opposite direction of the strongest gravitational waves and shoots off like a rocket.
Since most massive galaxies -- including those that have undergone major mergers in the past -- have a SMBH in their center, the gravitational recoil usually isn't strong enough to eject the SMBH; instead, the SMBH loses energy to the stars in the inner part of the merged galaxy via dynamical friction, and settles back into the center. But it appears that sometimes there's enough of a kick to allow the SMBH to escape.
Another possibility is that if two galaxies merge and their SMBHs form a binary, and then another galaxy (with its own SMBH) merges before the previous two SMBHs have actually merged, then you can have a three-body interaction between the late-arrival SMBH and the binary SMBH, which could result in one of the SMBHs being ejected. But this requires the right timing, and probably doesn't happen very often.
Astronomers publish map showing 25,000 supermassive black holes
Sky map showing 25,000 supermassive black holes. Each white dot is a supermassive black hole in its own galaxy. Credit: LOFAR/LOL Survey
An international team of astronomers has published a map of the sky showing over 25,000 supermassive black holes. The map, to be published in the journal Astronomy & Astrophysics, is the most detailed celestial map in the field of so-called low radio frequencies. The astronomers, including Leiden astronomers, used 52 stations with LOFAR antennas spread across nine European countries.
To an untrained eye, the sky map appears to contain thousands of stars, but they are actually supermassive black holes. Each black hole is located in a different, distant galaxy. The radio emissions are emitted by matter that was ejected as it got close to the black hole.
Research leader Francesco de Gasperin (formerly Leiden University, now Universität Hamburg, Germany) says about the study: "This is the result of many years of work on incredibly difficult data. We had to invent new methods to convert the radio signals into images of the sky."
From the bottom of the pool
Observations at long radio wavelenghts are complicated by the ionosphere that surrounds the Earth. This layer of free electrons acts like a cloudy lens that constantly moves across the radio telescope. Co-author Reinout van Weeren (Leiden Observatory) explains: "It's similar to when you try to see the world while immersed in a swimming pool. When you look up, the waves on the water of the pool deflect the light rays and distort the view."
Map of the entire sky
The new map was created by combining 256 hours of observations of the northern sky. The researchers deployed supercomputers with new algorithms that correct the effect of the ionosphere every four seconds. Scientific Director of the Leiden Observatory Huub Röttgering is the last author of the publication. He is delighted with the results: "After many years of software development, it is so wonderful to see that this has now really worked out."
The map now covers 4 percent of the northern half of the sky. The astronomers plan to continue until they have mapped the entire northern sky. In addition to supermassive black holes, the map also provides insight into the large-scale structure of the universe, among other things.
Breakthrough in deciphering birth of supermassive black holes
A research team led by Cardiff University scientists say they are closer to understanding how a supermassive black hole (SMBH) is born thanks to a new technique that has enabled them to zoom in on one of these enigmatic cosmic objects in unprecedented detail.
Scientists are unsure as to whether SMBHs were formed in the extreme conditions shortly after the big bang, in a process dubbed a 'direct collapse', or were grown much later from 'seed' black holes resulting from the death of massive stars.
If the former method were true, SMBHs would be born with extremely large masses -- hundreds of thousands to millions of times more massive than our Sun -- and would have a fixed minimum size.
If the latter were true then SMBHs would start out relatively small, around 100 times the mass of our Sun, and start to grow larger over time by feeding on the stars and gas clouds that live around them.
Astronomers have long been striving to find the lowest mass SMBHs, which are the missing links needed to decipher this problem.
In a study published today, the Cardiff-led team has pushed the boundaries, revealing one of the lowest-mass SMBHs ever observed at the centre of a nearby galaxy, weighing less than one million times the mass of our sun.
The SMBH lives in a galaxy that is familiarly known as "Mirach's Ghost," due to its close proximity to a very bright star called Mirach, giving it a ghostly shadow.
The findings were made using a new technique with the Atacama Large Millimeter/submillimeter Array (ALMA), a state-of-the-art telescope situated high on the Chajnantor plateau in the Chilean Andes that is used to study light from some of the coldest objects in the Universe.
"The SMBH in Mirach's Ghost appears to have a mass within the range predicted by 'direct collapse' models," said Dr Tim Davis from Cardiff University's School of Physics and Astronomy.
"We know it is currently active and swallowing gas, so some of the more extreme 'direct collapse' models that only make very massive SMBHs cannot be true.
"This on its own is not enough to definitively tell the difference between the 'seed' picture and 'direct collapse' -- we need to understand the statistics for that -- but this is a massive step in the right direction."
Black holes are objects that have collapsed under the weight of gravity, leaving behind small but incredibly dense regions of space from which nothing can escape, not even light.
An SMBH is the largest type of black hole that can be hundreds of thousands, if not billions, of times the mass of the Sun.
It is believed that nearly all large galaxies, such as our own Milky Way, contain an SMBH located at its centre.
"SMBHs have also been found in very distant galaxies as they appeared just a few hundred million years after the big bang," said Dr Marc Sarzi, a member of Dr. Davis' team from the Armagh Observatory & Planetarium.
"This suggest that at least some SMBHs could have grown very massive in a very short time, which is hard to explain according to models for the formation and evolution of galaxies."
"All black holes grow as they swallow gas clouds and disrupt stars that venture too close to them, but some have more active lives than others."
"Looking for the smallest SMBHs in nearby galaxies could therefore help us reveal how SMBHs start off," continued Dr. Sarzi.
In their study, the international team used brand new techniques to zoom further into the heart of a small nearby galaxy, called NGC404, than ever before, allowing them to observe the swirling gas clouds that surrounded the SMBH at its centre.
The ALMA telescope enabled the team to resolve the gas clouds in the heart of the galaxy, revealing details only 1.5 light years across, making this one of the highest resolution maps of gas ever made of another galaxy.
Being able to observe this galaxy with such high resolution enabled the team to overcome a decade's worth of conflicting results and reveal the true nature of the SMBH at the galaxy's centre.
"Our study demonstrates that with this new technique we can really begin to explore both the properties and origins of these mysterious objects," continued Dr Davis.
"If there is a minimum mass for a supermassive black hole, we haven't found it yet."
Astronomers Are Homing in on The Colossal Feeding Processes of Huge Black Holes
The more we study the Universe, the more it seems likely that each galaxy is orbiting a cosmic colossus - a supermassive black hole, powering the galactic nucleus.
There's a lot we don't know about these giant objects - including the glaring question of how they grow so huge - but new research could help us fill in some of the gaps. According to a new radio survey of all the galaxies in a region of the sky, every supermassive black hole in a galactic nucleus devours matter, although they go about it a little differently.
"We are getting more and more indications that all galaxies have enormously massive black holes in their centers. Of course, these must have grown to their current mass," said astronomer Peter Barthel of the University of Groningen in the Netherlands.
"It seems that, thanks to our observations, we now have these growth processes in view and are slowly but surely starting to understand them."
There is a funny gap in the mass range of black holes that mean we're missing an important piece of the puzzle of how supermassive black holes form and grow. Stellar mass black holes - ones that have formed from the collapsed core of a massive star - have only been detected up to 142 times the mass of the Sun, and even that one was weightier than usual, the product of a collision between two smaller black holes.
Supermassive black holes, on the other hand, are typically between a few millions to billions of solar masses. You'd think that if supermassive black holes grew from stellar mass ones, there would be lots of intermediate mass ones out there, but very few detections have been made.
One way we can try to figure it out is by studying the black holes we have detected, to see if their behavior can give us any clues that is what a team of astronomers led by Jack Radcliffe of the University of Pretoria in South Africa did.
Their focus was a region of space known as GOODS-North, located in the constellation of Ursa Major. This region, the subject of a Hubble deep sky survey, has been well studied, but primarily in optical, ultraviolet, and infrared wavelengths.
A section of GOODS North, with each dot a galaxy. (NASA/ESA/G. Illingworth/P. Oesch/R. Bouwens and I. Labbé, and the Science Team)
Radcliffe and his team performed analyses of the region using a range of wavelengths up to X-ray, adding radio observations using very long baseline interferometry to the mix. Thus, they identified active galactic nuclei - those containing an active supermassive black hole - that were bright in different wavelengths.
When supermassive black holes are actively accreting material - slurping down gas and dust from their surrounding space - the material heats up, glowing with bright enough electromagnetic radiation to be seen across vast cosmic distances.
Depending on how much dust is obscuring the galactic nucleus, some wavelengths of this light may be stronger, so no single wavelength range can be used to identify all active galactic nuclei in a patch of sky.
Equipped with this additional information, the team made a study of the AGN in GOODS-North, and made several observations.
The first was that not all active accretion is the same. That may seem like a no-brainer, and we've certainly observed different supermassive black holes accreting at different rates, but the data are still useful. The researchers found that some active supermassive black holes devour material at a much faster rate than others, and some don't devour much at all.
Next, they investigated the presence of starburst activity - that is, a region and period of intense star formation - coinciding with an active galactic nucleus.
It's thought that feedback from an active galactic nucleus can quench star formation by blowing away all the material stars are made of, but some studies have shown that the opposite can happen, too - that material shocked and compressed by feedback can collapse into baby stars.
They found that some galaxies have starburst activity, and some don't. Interestingly, ongoing starburst activity can make an active galactic nucleus harder to see, suggesting that more investigation will need to be done to better define the role of feedback in quenching.
Finally, they studied the relativistic jets that can shoot from the poles of a supermassive black hole during active accretion. It's thought that these jets consist of a small fraction of material that gets funneled along magnetic field lines from the inner region of the accretion disk to the black hole's poles where it is blasted into space in the form of jets of ionized plasma, at speeds a significant percentage of the speed of light.
We're not entirely sure how and why these jets form, and the team's research suggests that the accretion rate of material doesn't play a huge role. They found that jets form only sometimes, and that whether a black hole is eating fast or slowly matters not.
This information, the researchers said, can help to better understand the accretion behavior and growth of supermassive black holes. And, they said, it also shows that radio astronomy can play a more significant role in these studies going forward.
Which means, in the future, we will have a more powerful toolset for trying to unravel one of the most perplexing black hole mysteries - where the heck do supermassive chonkers even come from?
The team's research has been published and accepted in two papers in Astronomy & Astrophysics. They can be found here and here.
BASIC EXTRAGALACTIC ASTRONOMY Part 8: Central Supermassive Black Holes - Discovery and Properties
Several lines of observational evidence have led to the conclusion that virtually all major galaxies contain a central supermassive black hole (SMBH) ranging from millions to tens of billions of solar masses.
In 1909, Edward Fath reported emission lines in the spectra of M 81 and NGC 1068 galactic nuclei, suggesting powerful sources of high energy ionizing radiation. Over the next several decades, Vesto Slipher, Milton Humason, Carl Seyfert, and others reported spectroscopic evidence of emission lines, some narrow and some broad, in the central regions of those nearby galaxies which had exceptionally bright nuclei.
With the advent of radio astronomy in the mid 1930s, some of these galaxies, including the center of the Milky Way, were also found to be sources of radio emissions. Further, numerous quasars were discovered which were strong radio wave emitters, but presented in the optical band as high redshift star-like objects of great luminosity. Early on it became evident that such extreme energy outputs could not be generated by starburst activity alone. No reasonable mechanism was proposed until 1958, when Soviet Armenian astrophysicist Viktor Ambartsumian, who had introduced the term Active Galactic Nucleus, suggested that these galactic nuclei must contain bodies of immense mass, small size, and unknown nature. Accretion, or gravitational compression and heating of infalling matter around such bodies would be able to account for extreme energy outputs. Although the idea was initially met with skepticism, a path was opened toward the possibility that black holes, hypothesized by Karl Schwarzschild in 1915 (see Part 5, Section 22), are not merely a mathematical curiosity, but actually exist in physical reality.
The advent of X-ray astronomy in the 1960s revealed that some active galactic nuclei and quasars are also strong X-ray sources. The discovery opened a new universe of energetic phenomena which presented persuasive evidence for the existence of black holes. Temperatures of tens and hundreds of millions K are necessary for the generation of X-rays, up to a thousand times higher than temperatures of even the hottest stars. In theory, the most efficient process for generating these temperatures would be the release of potential (gravitational) energy of infalling matter onto the accretion disk of a black hole. Resulting mass to energy conversion by nuclear fission could be orders of magnitude higher than the efficiency in stellar nuclear fusion. However, at the time, this compelling theory lacked proof.
The first stellar mass black hole, confidently identified in 1971, was Cygnus X-1, an X-ray binary star system in which an invisible, small object is pulling mass from a large, visible companion. In 1974, a much stronger X-ray source, Sagittarius A, was discovered in the center of the Milky Way galaxy, and was eventually attributed to the presence of a supermassive black hole (SMBH). The launch of fully imaging space X-ray observatories, starting with Einstein in 1978, and followed in 1999 by ROSAT, XMM-Newton, and Chandra, led to the detection of numerous Ultra-Luminous X-ray Sources, or ULX, associated with all types of galaxies. While the least luminous of these objects (10^32 to 10^33 watts) can be explained by beamed emissions from ordinary X-ray binary stars, energy levels released by most of these sources greatly exceed what is possible from stellar processes, including supernovae, neutron stars (pulsars), and stellar-mass black holes. It is presently thought that ULX emissions are caused by accretion around SMBH or intermediate-mass black holes, IMBH, with masses of hundreds to thousands solar.
Extreme energy levels and a wide range of emitted radiation, from radio waves to gamma rays, were one line of evidence for the existence of central SMBH.
Another line of evidence comes from astrophysical jets, or relativistic jets, which are massive outflows of superheated plasma ejected at relativistic speeds from the cores of some active galaxies and quasars (See Part 5, Section 22). Such jets, thin, long, and often beaded, some stretching over three million light years, were first detected in 1918 by Heber Curtis, who reported an inexplicable jet radiating from the nucleus on optical images of the nearby galaxy Messier 87. Similar features have since been observed in many other objects, and in all bands, from radio waves to X-rays. Once again, temperatures and energy levels required for such phenomena are many orders of magnitude greater than the levels that can be produced by nuclear reactions in ordinary stars, and are only found within accretion disks of black holes.
Since the 1990s, a number of astronomers - among them recent Nobel Prize recipients Andrea Ghez and Reinhard Genzel - have been observing Sagittarius A, a strong radio and X-ray source in the center of our Galaxy. Observations were made in the infrared band because the region is obscured in the visible band by thick layers of gas and dust in the galactic plane. Following tight elliptical orbits of stars around the source, they identified the location of a dark "supermassive compact object" of approximately 4.1 million solar masses that is generally recognized to be consistent with a minimally accreting supermassive black hole.
Fig. 8-1: Plots of star orbits around a "supermassive compact object" in the center of our Galaxy
In 2019, the Event Horizon Telescope Collaboration team released the first ever image of the accretion disk around the central supermassive black hole in neighboring galaxy Messier 87. The image, created with long-base interferometry techniques using radio telescopes across the entire planet, bears a remarkable resemblance to theoretical predictions based on computer simulations.
While direct visualization is the most convincing evidence for the existence of central supermassive black holes, for technical and economic reasons the method is not applicable to large scale surveys of remote galaxies. Plotting star orbits around central SMBHs in other galaxies is also not practicable for the same reasons. Fortunately, spectroscopic analysis of the motions of stars and globular clusters (in the near-visible bands), and of interstellar gas clouds (in the radio bands) yields good results in detecting central SMBHs in remote galaxies, and in estimating their mass.
(35) SPECTROSCOPIC ANALYSIS, VELOCITY DISPERSION, AND DERIVED SYSTEM PROPERTIES
In gravitationally bound systems, the motion of particles is dependent to the total mass of the system . In general terms, higher mass leads to more rapid motion of particles around the center of gravity.This motion may be rotationally stabilized, as in the disks of spiral galaxies, or random, as in globular clusters, elliptical galaxies, some irregular galaxies, and central bulges of spiral galaxies. Higher total mass also leads to greater velocity dispersion, which is the variability, or spread of individual particle velocities around their mean velocity. In a random system near a galactic center, approximately half of the light sources (stars, clusters, and gas clouds) will be moving away from the observer, and will manifest a relative Doppler redshift, while the other half moving toward the observer manifest a blueshift relative to the mean. As a result, spectroscopic analysis of the system will show broadening of spectral lines. In reverse logic, wider spectral lines imply higher velocity dispersion, more rapid random motion of light sources, and higher total mass of a gravitationally bound system. The presence of a central supermassive black hole will significantly increase stellar velocity dispersion in the nucleus of a galaxy from what is anticipated based on the mass of luminous matter alone.
Fig. 8-2: The relation between velocity dispersion and spectral line width. Red and blue colors represent Doppler shifted regions of the central wavelength, not actual colors.
The spectral region most commonly used for estimating velocity dispersion is the infrared singly ionized Calcium (CaII) triplet at the wavelengths of 8498 A, 8542 A, and 8662 A
Velocity dispersion is a very useful measure which has been empirically related to a number of physical properties of gravitationally bound systems from star clusters, to galaxies, to entire galaxy clusters, and to the properties of galactic central SMBHs.
For example, equation (43) approximates the total mass of a galaxy, M, based on its stellar velocity dispersion, S, where R is its radius, and G is the gravitational constant:
The M - sigma relation empirically correlates the mass of a central supermassive black hole, Mbh, in solar masses, Ms, to the stellar velocity dispersion, S, measured from the spectrum of the galactic nucleus. The relation was originally presented in 1999 as the Faber-Jackson Law for Black Holes, but has since undergone a number of revisions based on a growing number of published central SMBH masses in nearby galaxies. According to McConnell et al. (2011) the relation is:
Log( Mbh / Ms ) = 8.29 + 5.12 Log( S / 200km/s ) (44a)
The most recent study by Marsden et al. (2020) suggests that the best statistical fit is given by:
Log( Mbh / Ms ) = 8.21 + 3.83 Log( S / 200km/s ) (44b)
An earlier work by American astronomers Sandra Faber and Robert Jackson in 1976 established the Faber-Jackson Relation which empirically correlated stellar velocity dispersion to the absolute magnitude for a sample of elliptical galaxies. They concluded that galactic luminosity is proportional to its stellar velocity dispersion raised to the 4th power. Based on much subsequent data, this relation has also undergone significant revisions, with the exponent now dependent on the size and type of the host galaxy. The relation essentially suggests that larger galaxies with higher absolute magnitude statistically have larger central SMBHs.
(36) PHOTOMETRIC ANALYSIS AND DERIVED CENTRAL SMBH PROPERTIES
Another method for detecting and estimating central SMBH properties is based on photometric measurements. The subject is treated at some length in Part 5 of this article series which discusses black holes and quasars. A revised and abbreviated section is given here for the sake of convenience.
Numerous galaxies manifest variability in their central regions throughout the electromagnetic spectrum. As previously mentioned, energy requirements (temperatures) for producing such high levels of radiation, from radio waves to gamma rays, exceed by many orders of magnitude anything which is possible in stellar nuclear fusion reactions. This implies the presence of supermassive gravitational whirlpools which generate energy by the accretion of matter. At present, black holes are the only known objects of such small size and extreme mass.
Black hole emissions can be highly variable. Generally speaking, variability is caused by changes in the availability of matter flowing into its accretion disk. Numerous black holes manifest optical long period variability of several magnitudes over an observation period of years.
Fig. 8-3: Long period variability in accreting super-massive black holes of quasars
Black holes also manifest short period variability on the level of minutes to weeks. For example, in 2002 infrared flux density of Sagittarius A, the SMBH in the center of the Milky Way, was measured to change by a factor of 4 in one week, and by a factor of 2 in merely 40 minutes.
Short period fluctuations are particularly useful in estimating black hole properties.
On large scales, the shortest possible period of variability is determined by the diameter of the emitting object. To demonstrate, refer to Fig. 8-4, and consider an object 1,000 light seconds in diameter which emits aninstantaneous flash of light from its entire volume. While travelling toward a distant observer, the leading edge wavefront, W1, will be separated from the trailing edge wavefront, W3, by 1,000 light seconds. The observed light curve will show an initial rise upon the arrival of W1, at time T1', a maximum on the arrival of W2 from the widest part of the emitting object, and a decline to baseline on the arrival of W3, at time T3'.
In theory, the shortest measured period of variability in seconds, Tp = T3' - T1', is equal to the time interval between travelling wavefronts , Tdw = Tw1 - Tw3, and indicates the largest possible diameter of the light-emitting object in light seconds, D = C x Tp = C x (T3' - T1' ), where C is the speed of light.
Fig. 8-4: The diameter of a light source in light seconds can not be larger than the width of the light curve in seconds.During their long journey, travelling wavefronts are subject to cosmological magnification.
In reality, large objects do not emit truly instantaneous light flashes. The duration of a light-generating event, Te, including its gradual propagation throughout the volume of the source, is added to the time interval between travelling wavefronts, Tdw, to widen the light curve period: Tp = Tdw + Te.
Furthermore, in the case of distant, high redshift objects, the space between the first and the last travelling wavefront is subject to cosmological magnification caused by the expansion of the universe (see Article 4, section 25). This has the effect of increasing the measured period, Tp, by one factor of (Z + 1).
A general relation between the light-emitting object's diameter (in light seconds), D, the measured period, Tp, the duration of the light-generating event (in seconds), Te, and redshift, Z, is then described by the following equation:
D = C x (Tp - Te) / (Z+1) (45a)
Although redshift can be measured very precisely, the duration of the light-generating event is in practice virtually never accurately known, The only possible interpretation of equation (45a) then becomes:
In other words, the diameter of the object in light seconds must be smaller than the light curve period in seconds, corrected for cosmological magnification. Or, the variability period in seconds is always greater than the diameter of the source in light seconds due to the duration of light emission and the effect of cosmological magnification.
For nearby objects with negligible redshifts, which are not subject to significant cosmological magnification, the equation is reduced to simply:
As previously mentioned, infrared flux density of Sagittarius A, the SMBH in the center of our Galaxy, was measured to increase by a factor of 2 in merely 40 minutes. Since change in luminosity involves photons reaching the observer from the near to the far edge of the light source, the maximum occurs when photons from the widest, middle cross-section arrive to the observer. The time period, Tm = T2' - T1', between the minimum and the maximum on the light curve is an indicator of the radius, R, of the accretion disk. This can then be used in equation (45c) to estimate the mass and the size of an accreting, non-rotating black hole.
Tm = 40 min = 2400 sec. Method (1)
1 Astronomical Unit = 149.6x10^6 Km
The radius of Sagittarius A accretion disk is smaller than 4.8 AU, or somewhat less than the orbit of Jupiter
The validity of this approach is shown by the following study by Morgan et al. based on the variability of eleven quasars
The study derives an empirical relationship between the accretion disk radius in cm, R, the black hole mass, M, and the solar mass, Ms:
log R = 15.8 + 0.8 log ( M / 10^9 Ms) Method (2) (46a)
Solving this equation for Sagittarius A with 4 x 10^6 solar masses yields an estimated accretion disk radius of 5.1 AU, fairly consistent with Method (1).
If Method (1) is used to estimate the accretion disk radius, equation (46a) in Method (2) can be solved for the black hole mass:
log ( M / 10^9 Ms ) = ( log R - 15.8 ) / 0.8
log M - log ( 10^9 Ms ) = ( log R - 15.8 ) / 0.8
log M = log ( 10^9 Ms ) + [ ( log R - 15.8 ) / 0.8 ] (46b)
Entering the black hole mass, M, into the Schwarzschild's equation (43) (see Part 5, section 22) will then give the radius of the event horizon, Rs, for a non-rotating black hole.
In the meter-kilogram-second (MKS) system of units, the gravitational constant G = 6.674×10^-11, and the speed of light C = 3x10^8 m/s. For radius, Rs, in meters, and the black hole mass, M, in kilograms, Schwarzschild's equation (43) becomes:
Rs = [ 2 (6.674×10^-11) M ] / (3x10^8)^2 (43a)
(37) RELATIONSHIPS BETWEEN THE CENTRAL SMBH MASS AND THE HOST GALAXY
We have shown how stellar velocity dispersion and photometric studies of a galactic bulge are used to estimate the mass and the dimension of the central SMBH. These values have in turn been empirically related to the morphological properties of host galaxies.
In 2011, McConnell et al. related central SMBH mass in solar units, Mbh/Ms, and host galaxy luminosity in solar units, L/Ls, with the following empirical equation:
Mbh - L relationship: Log( Mbh / Ms ) = 9.16 + 1.16 Log( L / Ls 10^11 ) (47)
Another study by Gutelkin et al. (2009) related central SMBH mass to the mass and the luminosity of the host galaxy. The following two diagrams are based on their data.
Fig. 8-5: The observed relationship between the central SMBH mass and host galaxy mass
Fig. 8-6: The observed relationship between the central SMBH mass and host galaxy luminosity
Substantial variance from the mean (on a logarithmic scale) indicates that the relationship between the central SMBH mass and host galaxy mass and luminosity is quite approximate. In general terms, larger galaxies contain larger central SMBHs, while most dwarf galaxies have no detectable ones. However, there are numerous inconsistencies and exceptions.
For example, the Milky Way SMBH has a mass of 4.1 million solar, while the Andromeda Galaxy SMBH has a much greater mass of 110-230 million solar, although the two galaxies are of approximately equal size. The moderately sized Triangulum Galaxy, M33, contains no SMBH at all. XMM-Newton studies of its central region detect an ULX compatible with an intermediate-mass black hole (IMBH) of only 1,500 solar. But the dwarf galaxy NGC 404 contains an IMBH nearly 35 times more massive. The supergiant elliptical galaxy M87, with baryonic mass about twice that of the Milky Way [ https://arxiv.org/abs/astro-ph/0508463 ], has a central SMBH that is nearly 1,600 times greater, at 6.5 billion solar [ https://arxiv.org/abs/1906.11243 ]. An even larger elliptical galaxy, A2261-BCG, 10 times the diameter, and 1,000 times the baryonic mass of the Milky Way, contains no detectable central black hole of any size. Yet, an elliptical radio galaxy 4C +37.11 was found to have two central SMBHs with a combined mass around 15 billion solar, orbiting each other at a distance of 24 light years with a period of 30,000 years [ https://iopscience.iop.org/article/10.3847/1538-4357/aa74e1 ]. Another SMBH binary system (SMBHB), with masses of 18.35 billion and 150 million solar, was detected in the remote quasar OJ 287.
While a majority of dwarf galaxies does not have a central SMBH, there are exceptions in this area as well. A local early stage dwarf starburst galaxy without a central bulge, He 2-10, was discovered in 2011 to contain a 3 million solar mass SMBH. In 2014, a 20 million solar mass SMBH was detected in a dense ultracompact dwarf galaxy, M60-UCD1, constituting more than 10% of the total mass of the host galaxy. It was extraordinary to find a black hole five times larger than the Milky Way's in a galaxy which is more than 5,000 times smaller. In 2012, a 5 billion solar mass SMBH was reported in the compact lenticular galaxy NGC 1277, which constitutes about 5% of the total baryonic mass of the galaxy, or 20% of the stellar mass of the central bulge.
Can We Detect Binary Supermassive Black Holes?
At the center of most galaxies are black holes so massive—up to several billion times the mass of our sun—that they have earned the superlative descriptor supermassive. Compare this to your run-of-the-mill black holes, known as stellar-mass black holes, which are a measly 10 to 100 times our sun’s mass. Understanding these supermassive black holes will help astronomers understand the origin and evolution of galaxies. One open question is whether they can form binaries.
Stellar-mass black holes form binary systems, two black holes orbiting each other, if they form from the collapse of a binary star system, or possibly when two black holes capture each other in their gravitational pull. They spiral in, eventually merging in an event so powerful that it sends a ripple through space and time known as a gravitational wave. A few years ago, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from such an event for the first time.
The merging of two galaxies could result in a binary supermassive black hole, but so far astronomers have not unequivocally detected one of these events. Penn State Professor of Astronomy and Astrophysics Michael Eracleous is on the forefront of the hunt.
“About ten years ago, several papers were published claiming to have detected binary supermassive black holes,” he said. “I had done some work on binary supermassive black holes as a graduate student, so I felt compelled to embark on a project to gather a lot of data to be able to make a counterpoint to the claims of those papers. Once I got into it, I saw how connected it was to galaxy evolution.”
“When I came to Penn State, I knew that the department was a perfect fit for the type of research that I do,” he said. “I’ve made some great connections with my colleagues here, and now I know that if I’m ever stuck, all it takes is a cup of coffee and a conversation to clear things up.”
So how do you look for something you’ve never seen?
“In much of astronomy, observation comes first—we see something and that informs our theory,” said Eracleous. “For binary supermassive black holes, theory is driving the observations. Until we find one, the questions are ‘Should they exist?’ and ‘Should we look for them?’ And the answer to both questions is definitely ‘Yes.’”
A major difference between binary supermassive black holes and stellar-mass black holes is gas. When stellar-mass black holes form after a star explodes in a supernova, most of the gas is driven away. But supermassive black holes are thought to carry gases with them. These gases emit light signals that can be detected by large telescopes equipped with spectrographs here on Earth, such as the 11-meter Hobby-Eberly Telescope (HET).
Eracleous explained that the gases are detected by the spectrograph as emission lines of a particular wavelength and they could hold the key to identifying a supermassive binary. As the black holes orbit one another, the emission lines from these gases shift due to the Doppler effect. The emission lines from one black hole are shifted to longer wavelengths, and those from the other are shifted to shorter wavelengths. So we expect two separate emission lines, one from each black hole.
“If we could follow the emission lines over the course of an orbit, we would see them crossing back and forth as the signals from each black hole shifted one way and then the other,” said Eracleous.
Of course, the actual search is not that straightforward. Practicalities like limited availability of time on the large telescopes necessary to make these observations mean astronomers can’t just watch and wait to see the telltale signs of a supermassive binary. But they don’t need to. Instead, they identify candidates from an initial survey and make regular check-ins to see if the spectra from these candidates have changed as would be expected based on theoretical models.
“Using the Hobby-Eberly Telescope to make these observations makes our life easier because we don’t even need to go to the observatory to collect the data,” said Eracleous. “The HET is operated by resident astronomers who make the observations and send us the data.”
The process is slow, but Eracleous explained that once they find one binary supermassive black hole, the search should accelerate.
“The first confirmed binary supermassive black hole will be like the Rosetta Stone,” he said. “It will tell us which of our models were right and which were wrong. It will allow us to refine our next searches and we should be able to find more.”
Astronomers are already developing the technology for those next searches. Eracleous is involved in the planning for the Laser Interferometer Space Antenna (LISA). LISA is to LIGO what a supermassive black hole is to a stellar-mass black hole. Where LIGO consists of two four-kilometer-long lasers at right angles to each other, LISA’s three spacecrafts will be connected by lasers that travel 2.5 million kilometers forming an equilateral triangle. LISA’s scale and the fact that it is space based means that it can detect low-wavelength gravitational waves away from noise sources here on Earth.
“LISA will be tuned to find gravitational waves like those that would result from a supermassive black hole merger,” said Eracleous.
For Eracleous, Penn State’s Department of Astronomy and Astrophysics has provided the supportive environment necessary for his search.
Astronomers Unexpected Discovery Could Explain Supermassive Black Hole Growth in Early Universe
Artist impression of the heart of galaxy NGC 1068, which harbors an actively feeding supermassive black hole, hidden within a thick doughnut-shaped cloud of dust and gas. ALMA discovered two counter-rotating flows of gas around the black hole. The colors in this image represent the motion of the gas: blue is material moving toward us, red is moving away. Credit: NRAO/AUI/NSF, S. Dagnello
At the center of a galaxy called NGC 1068, a supermassive black hole hides within a thick doughnut-shaped cloud of dust and gas. When astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to study this cloud in more detail, they made an unexpected discovery that could explain why supermassive black holes grew so rapidly in the early Universe.
“Thanks to the spectacular resolution of ALMA, we measured the movement of gas in the inner orbits around the black hole,” explains Violette Impellizzeri of the National Radio Astronomy Observatory (NRAO), working at ALMA in Chile and lead author on a paper published in the Astrophysical Journal Letters. “Surprisingly, we found two disks of gas rotating in opposite directions.”
ALMA image showing two disks of gas moving in opposite directions around the black hole in galaxy NGC 1068. The colors in this image represent the motion of the gas: blue is material moving toward us, red is moving away. The white triangles are added to show the accelerated gas that is expelled from the inner disk – forming a thick, obscuring cloud around the black hole. Credit: ALMA (ESO/NAOJ/NRAO), V. Impellizzeri NRAO/AUI/NSF, S. Dagnello
Supermassive black holes already existed when the Universe was young – just a billion years after the Big Bang. But how these extreme objects, whose masses are up to billions of times the mass of the Sun, had time to grow in such a relatively short timespan, is an outstanding question among astronomers. This new ALMA discovery could provide a clue. “Counter-rotating gas streams are unstable, which means that clouds fall into the black hole faster than they do in a disk with a single rotation direction,” said Impellizzeri. “This could be a way in which a black hole can grow rapidly.”
NGC 1068 (also known as Messier 77) is a spiral galaxy approximately 47 million light-years from Earth in the direction of the constellation Cetus. At its center is an active galactic nucleus, a supermassive black hole that is actively feeding itself from a thin, rotating disk of gas and dust, also known as an accretion disk.
“We did not expect to see this, because gas falling into a black hole would normally spin around it in only one direction.Something must have disturbed the flow, because it is impossible for a part of the disk to start rotating backward all on its own.” – Violette Impellizzeri
Previous ALMA observations revealed that the black hole is not only gulping down material, but also spewing out gas at incredibly high speeds – up to 500 kilometers per second (more than one million miles per hour). This gas that gets expelled from the accretion disk likely contributes to hiding the region around the black hole from optical telescopes.
Star chart showing the location of NGC 1068 (also known as Messier 77), a spiral galaxy approximately 47 million light-years from Earth in the direction of the constellation Cetus. Credit: IAU Sky & Telescope magazine NRAO/AUI/NSF, S. Dagnello
Impellizzeri and her team used ALMA’s superior zoom lens ability to observe the molecular gas around the black hole. Unexpectedly, they found two counter-rotating disks of gas. The inner disk spans 2-4 light-years and follows the rotation of the galaxy, whereas the outer disk (also known as the torus) spans 4-22 light-years and is rotating the opposite way.
“We did not expect to see this, because gas falling into a black hole would normally spin around it in only one direction,” said Impellizzeri. “Something must have disturbed the flow, because it is impossible for a part of the disk to start rotating backward all on its own.”
Counter-rotation is not an unusual phenomenon in space. “We see it in galaxies, usually thousands of light-years away from their galactic centers,” explained co-author Jack Gallimore from Bucknell University in Lewisburg, Pennsylvania. “The counter-rotation always results from the collision or interaction between two galaxies. What makes this result remarkable is that we see it on a much smaller scale, tens of light-years instead of thousands from the central black hole.”
Funded by the U.S. National Science Foundation and its international partners (NRAO/ESO/NAOJ), ALMA is among the most complex and powerful astronomical observatories on Earth or in space. The telescope is an array of 66 high-precision dish antennas in northern Chile. Credit: ALMA (ESO/NAOJ/NRAO)
The astronomers think that the backward flow in NGC 1068 might be caused by gas clouds that fell out of the host galaxy, or by a small passing galaxy on a counter-rotating orbit captured in the disk.
At the moment, the outer disk appears to be in a stable orbit around the inner disk. “That will change when the outer disk begins to fall onto the inner disk, which may happen after a few orbits or a few hundred thousand years. The rotating streams of gas will collide and become unstable, and the disks will likely collapse in a luminous event as the molecular gas falls into the black hole. Unfortunately, we will not be there to witness the fireworks,” said Gallimore.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Reference: “Counter-Rotation and High Velocity Outflow in the Parsec-Scale Molecular Torus of NGC 1068” by C. M. Violette Impellizzeri, Jack F. Gallimore, Stefi A. Baum, Moshe Elitzur, Richard Davies, Dieter Lutz, Roberto Maiolino, Alessandro Marconi, Robert Nikutta, Christopher P. O’Dea and Eleonora Sani, 14 October 2019, The Astrophysical Journal Letters.
How Spiral Graph Works
To investigate further, astronomers launched Spiral Graph. The project aims to measure how spiral arms wind in thousands of distant galaxies.
First, users confirm that each galaxy they’re shown is indeed a spiral. Then, they draw lines to sketch out its shape. These lines measure how tight or open the spiral galaxy’s arms are.
Tight spiral arms suggest a large supermassive black hole. Open spiral arms indicate a more modest black hole.
As citizen scientists find interesting candidates, it creates a list of target galaxies that astronomers can study in more detail with their telescopes.
Energy Production around a Black Hole
By now, you may be willing to entertain the idea that huge black holes lurk at the centers of active galaxies. But we still need to answer the question of how such a black hole can account for one of the most powerful sources of energy in the universe. As we saw in Black Holes and Curved Spacetime, a black hole itself can radiate no energy. Any energy we detect from it must come from material very close to the black hole, but not inside its event horizon.
In a galaxy, a central black hole (with its strong gravity) attracts matter—stars, dust, and gas—orbiting in the dense nuclear regions. This matter spirals in toward the spinning black hole and forms an accretion disk of material around it. As the material spirals ever closer to the black hole, it accelerates and becomes compressed, heating up to temperatures of millions of degrees. Such hot matter can radiate prodigious amounts of energy as it falls in toward the black hole.
To convince yourself that falling into a region with strong gravity can release a great deal of energy, imagine dropping a printed version of your astronomy textbook out the window of the ground floor of the library. It will land with a thud, and maybe give a surprised pigeon a nasty bump, but the energy released by its fall will not be very great. Now take the same book up to the fifteenth floor of a tall building and drop it from there. For anyone below, astronomy could suddenly become a deadly subject when the book hits, it does so with a great deal of energy.
Dropping things from far away into the much stronger gravity of a black hole is much more effective in turning the energy released by infall into other forms of energy. Just as the falling book can heat up the air, shake the ground, or produce sound energy that can be heard some distance away, so the energy of material falling toward a black hole can be converted to significant amounts of electromagnetic radiation.
What a black hole has to work with is not textbooks but streams of infalling gas. If a dense blob of gas moves through a thin gas at high speed, it heats up as it slows by friction. As it slows down, kinetic (motion) energy is turned into heat energy. Just like a spaceship reentering the atmosphere (Figure 3) gas approaching a black hole heats up and glows where it meets other gas. But this gas, as it approaches the event horizon, reaches speeds of 10% the speed of light and more. It therefore gets far, far hotter than a spaceship, which reaches no more than about 1500 K. Indeed, gas near a supermassive black hole reaches a temperature of about 150,000 K, about 100 times hotter than a spaceship returning to Earth. It can even get so hot—millions of degrees—that it radiates X-rays.
Figure 3. Friction in Earth’s Atmosphere: In this artist’s impression, the rapid motion of a spacecraft (the Apollo mission reentry capsule) through the atmosphere compresses and heats the air ahead of it, which heats the spacecraft in turn until it glows red hot. Pushing on the air slows down the spacecraft, turning the kinetic energy of the spacecraft into heat. Fast-moving gas falling into a quasar heats up in a similar way. (credit: modification of work by NASA)
The amount of energy that can be liberated this way is enormous. Einstein showed that mass and energy are interchangeable with his famous formula E = mc 2 (see The Sun: A Nuclear Powerhouse). A hydrogen bomb releases just 1% of that energy, as does a star. Quasars are much more efficient than that. The energy released falling to the event horizon of a black hole can easily reach 10% or, in the extreme theoretical limit, 32%, of that energy. (Unlike the hydrogen atoms in a bomb or a star, the gas falling into the black hole is not actually losing mass from its atoms to free up the energy the energy is produced just because the gas is falling closer and closer to the black hole.) This huge energy release explains how a tiny volume like the region around a black hole can release as much power as a whole galaxy. But to radiate all that energy, instead of just falling inside the event horizon with barely a peep, the hot gas must take the time to swirl around the star in the accretion disk and emit some of its energy.
Most black holes don’t show any signs of quasar emission. We call them “quiescent.” But, like sleeping dragons, they can be woken up by being roused with a fresh supply of gas. Our own Milky Way black hole is currently quiescent, but it may have been a quasar just a few million years ago (Figure 4). Two giant bubbles that extend 25,000 light-years above and below the galactic center are emitting gamma rays. Were these produced a few million years ago when a significant amount of matter fell into the black hole at the center of the galaxy? Astronomers are still working to understand what remarkable event might have formed these enormous bubbles.
Figure 4. Fermi Bubbles in the Galaxy: Giant bubbles shining in gamma-ray light lie above and below the center of the Milky Way Galaxy, as seen by the Fermi satellite. (The gamma-ray and X-ray image is superimposed on a visible-light image of the inner parts of our Galaxy.) The bubbles may be evidence that the supermassive black hole at the center of our Galaxy was a quasar a few million years ago. (credit: modification of work by NASA’s Goddard Space Flight Center)
The physics required to account for the exact way in which the energy of infalling material is converted to radiation near a black hole is far more complicated than our simple discussion suggests. To understand what happens in the “rough and tumble” region around a massive black hole, astronomers and physicists must resort to computer simulations (and they require supercomputers, fast machines capable of awesome numbers of calculations per second). The details of these models are beyond the scope of our book, but they support the basic description presented here.
Astronomers Discover Supermassive Black Holes on a Collision Course
Titanic Twosome: A galaxy roughly 2.5 billion light-years away has a pair of supermassive black holes (inset). The locations of the black holes are lit up by warm gas and bright stars that surround the objects. The finding improves estimates of when astronomers will first detect gravitational wave background generated by supermassive black holes. A.D. Goulding et al./Astrophysical Journal Letters 2019
Astronomers have spotted a distant pair of titanic black holes headed for a collision.
Each black hole’s mass is more than 800 million times that of our sun. As the two gradually draw closer together in a death spiral, they will begin sending gravitational waves rippling through space-time. Those cosmic ripples will join the as-yet-undetected background noise of gravitational waves from other supermassive black holes.
Even before the destined collision, the gravitational waves emanating from the supermassive black hole pair will dwarf those previously detected from the mergers of much smaller black holes and neutron stars.
“Supermassive black hole binaries produce the loudest gravitational waves in the universe,” says co-discoverer Chiara Mingarelli, an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City. Gravitational waves from supermassive black hole pairs “are a million times louder than those detected by LIGO.”
The study was led by Andy Goulding, an associate research scholar at Princeton University. Goulding, Mingarelli and collaborators from Princeton and the U.S. Naval Research Laboratory in Washington, D.C., report the discovery July 10 in The Astrophysical Journal Letters.
The two supermassive black holes are especially interesting because they are around 2.5 billion light-years away from Earth. Since looking at distant objects in astronomy is like looking back in time, the pair belong to a universe 2.5 billion years younger than our own. Coincidentally, that’s roughly the same amount of time the astronomers estimate the black holes will take to begin producing powerful gravitational waves.
In the present-day universe, the black holes are already emitting these gravitational waves, but even at light speed the waves won’t reach us for billions of years. The duo is still useful, though. Their discovery can help scientists estimate how many nearby supermassive black holes are emitting gravitational waves that we could detect right now.
Detecting the gravitational wave background will help resolve some of the biggest unknowns in astronomy, such as how often galaxies merge and whether supermassive black hole pairs merge at all or become stuck in a near-endless waltz around each other.
“It’s a major embarrassment for astronomy that we don’t know if supermassive black holes merge,” says study co-author Jenny Greene, a professor of astrophysical sciences at Princeton. “For everyone in black hole physics, observationally this is a long-standing puzzle that we need to solve.”
Supermassive black holes contain millions or even billions of suns’ worth of mass. Nearly all galaxies, including the Milky Way, contain at least one of the behemoths at their core. When galaxies merge, their supermassive black holes meet up and begin orbiting one another. Over time, this orbit tightens as gas and stars pass between the black holes and steal energy.
Once the supermassive black holes get close enough, though, this energy theft all but stops. Some theoretical studies suggest that black holes then stall at around 1 parsec (roughly 3.2 light-years) apart. This slowdown lasts nearly indefinitely and is known as the final parsec problem. In this scenario, only very rare groups of three or more supermassive black holes result in mergers.
Astronomers can’t just look for stalled pairs because long before the black holes are 1 parsec apart, they’re too close to distinguish as two separate objects. Moreover, they don’t produce strong gravitational waves until they overcome the final-parsec hurdle and get closer together. (Observed as they were 2.5 billion years ago, the newfound supermassive black holes appear about 430 parsecs apart.)
If the final parsec problem doesn’t exist, then astronomers expect that the universe is filled with the clamor of gravitational waves from supermassive black hole pairs. “This noise is called the gravitational wave background, and it’s a bit like a chaotic chorus of crickets chirping in the night,” says Goulding. “You can’t discern one cricket from another, but the volume of the noise helps you estimate how many crickets are out there.” (When two supermassive black holes finally collide and combine, they send out a thundering chirp that dwarfs all others. Such an event is brief and extraordinarily rare, though, so scientists don’t expect to detect one any time soon.)
The gravitational waves generated by supermassive black hole pairs are outside the frequencies currently observable by experiments such as LIGO and Virgo. Instead, gravitational wave hunters rely on arrays of special stars called pulsars that act like metronomes. The rapidly spinning stars send out radio waves in a steady rhythm. If a passing gravitational wave stretches or compresses the space between Earth and the pulsar, the rhythm is slightly thrown off.
Detecting the gravitational wave background using one of these pulsar timing arrays takes patience and plenty of monitored stars. A single pulsar’s rhythm might be disrupted by only a few hundred nanoseconds over a decade. The louder the background noise, the bigger the timing disruption and the sooner the first detection will be made.
Goulding, Greene and the other observational astronomers on the team detected the two titans with the Hubble Space Telescope. Although supermassive black holes aren’t directly visible through an optical telescope, they are surrounded by bright clumps of luminous stars and warm gas drawn in by the powerful gravitational tug. For its time in history, the galaxy harboring the newfound supermassive black hole pair “is basically the most luminous galaxy in the universe,” Goulding says. What’s more, the galaxy’s core is shooting out two unusually colossal plumes of gas. After the researchers pointed the Hubble Space Telescope at the galaxy to uncover the origins of its spectacular gas clouds, they discovered that the system contained not one but two massive black holes.
The observationalists then teamed up with gravitational wave physicists Mingarelli and Princeton graduate student Kris Pardo to interpret the finding in the context of the gravitational wave background. The discovery provides an anchor point for estimating how many supermassive black hole pairs are within detection distance of Earth. Previous estimates relied on computer models of how often galaxies merge, rather than actual observations of supermassive black hole pairs.
Based on the findings, Pardo and Mingarelli predict that in an optimistic scenario there are about 112 nearby supermassive black holes emitting gravitational waves. The first detection of the gravitational wave background from supermassive black holes should therefore come within the next five years or so. If such a detection isn’t made, that would be evidence that the final parsec problem may be insurmountable. The team is currently looking at other galaxies similar to the one harboring the newfound supermassive black hole pair. Finding additional pairs will help them further hone their predictions.
Publication: Andy D. Goulding, et al., “Discovery of a Close-separation Binary Quasar at the Heart of a z
0.2 Merging Galaxy and Its Implications for Low-frequency Gravitational Waves,” ApJL, 879, L21, 2019 doi:10.3847/2041-8213/ab2a14