Einstein’s Theory of General Relativity predicted that black holes would form and eventually collide. It also predicted the creation of gravitational waves from the collision. But how often does this happen, and can we calculate how many stars this will happen to?
A new study from a physicist at Vanderbilt University sought to answer these questions.
Previously, seven such events had been confirmed, six of which were caused by the mergers of binary black holes (BBH) and one by the merger of a binary neutron star. But on Saturday, Dec. 1st, a team of scientists the LIGO Scientific Collaboration (LSC) and Virgo Collaboration presented new results that indicated the discovery of four more gravitational wave events. This brings the total number of GW events detected in the last three years to eleven.
And now, an international team led by MIT astrophysicist Carl Rodriguez has produced a study that suggests that black holes may merge multiple times. According to their study, these “second-generation mergers” likely occur within globular clusters, the large and compact star clusters that typically orbit at the edges of galaxies – and which are densely-packed with hundreds of thousands to millions of stars.
“We think these clusters formed with hundreds to thousands of black holes that rapidly sank down in the center. These kinds of clusters are essentially factories for black hole binaries, where you’ve got so many black holes hanging out in a small region of space that two black holes could merge and produce a more massive black hole. Then that new black hole can find another companion and merge again.”
Globular clusters have been a source of fascination ever since astronomers first observed them in the 17th century. These spherical collections of stars are among the oldest known stars in the Universe, and can be found in most galaxies. Depending on the size and type of galaxy they orbit, the number of clusters varies, with elliptical galaxies hosting tens of thousands while galaxies like the Milky Way have over 150.
For years, Rodriguez has been investigating the behavior of black holes within globular clusters to see if they interact with their stars differently from black holes that occupy less densely-populated regions in space. To test this hypothesis, Rodriguez and his colleagues used the Quest supercomputer at Northwestern University to conduct simulations on 24 stellar clusters.
These clusters ranged in size from 200,000 to 2 million stars and covered a range of different densities and metallic compositions. The simulations modeled the evolution of individual stars within these clusters over the course of 12 billion years. This span of time was enough to follow these stars as they interacted with each other, and eventually formed black holes.
The simulations also modeled the evolution and trajectories of black holes once they formed. As Rodriguez explained:
“The neat thing is, because black holes are the most massive objects in these clusters, they sink to the center, where you get a high enough density of black holes to form binaries. Binary black holes are basically like giant targets hanging out in the cluster, and as you throw other black holes or stars at them, they undergo these crazy chaotic encounters.”
Whereas previous simulations were based on Newton’s physics, the team decided to add Einstein’s relativistic effects into their simulations of globular clusters. This was due to the fact that gravitational waves were not predicted by Newton’s theories, but by Einstein’s Theory of General Relativity. As Rodriguez indicated, this allowed for them to see how gravitational waves played a role:
“What people had done in the past was to treat this as a purely Newtonian problem. Newton’s theory of gravity works in 99.9 percent of all cases. The few cases in which it doesn’t work might be when you have two black holes whizzing by each other very closely, which normally doesn’t happen in most galaxies… In Einstein’s theory of general relativity, where I can emit gravitational waves, then when one black hole passes near another, it can actually emit a tiny pulse of gravitational waves. This can subtract enough energy from the system that the two black holes actually become bound, and then they will rapidly merge.”
What they observed was that inside the stellar clusters, black holes merge with each other to create new black holes. In previous simulations, Newtonian gravity predicted that most binary black holes would be kicked out of the cluster before they could merge. But by taking relativistic effects into account, Rodriguez and his team found that nearly half of the binary black holes merged to form more massive ones.
As Rodriguez explained, the difference between those that merged and those that were kicked out came down to spin:
“If the two black holes are spinning when they merge, the black hole they create will emit gravitational waves in a single preferred direction, like a rocket, creating a new black hole that can shoot out as fast as 5,000 kilometers per second — so, insanely fast. It only takes a kick of maybe a few tens to a hundred kilometers per second to escape one of these clusters.”
This raised another interesting fact about previous simulations, where astronomers believed that the product of any black hole merger would be kicked out of the cluster since most black holes are assumed to be rapidly spinning. However, the gravity wave measurements recently obtained from LIGO appear to contradict this, which has only detected the mergers of binary black holes with low spins.
This assumption, however, seems to contradict the measurements from LIGO, which has so far only detected binary black holes with low spins. To test the implications of this, Rodriguez and his colleagues reduced the spin rates of the black holes in their simulations. What they found was that nearly 20% of the binary black holes from clusters had at least one black hole that ranged from being 50 to 130 solar masses.
Essentially, this indicated that these were “second generation” black holes, since scientists believe that this mass cannot be achieved by a black hole that formed from a single star. Looking ahead, Rodriguez and his team anticipate that if LIGO detects an object with a mass within this range, it is likely the result of black holes merging within dense stellar cluster, rather than from a single star.
“If we wait long enough, then eventually LIGO will see something that could only have come from these star clusters, because it would be bigger than anything you could get from a single star,” Rodriguez says. “My co-authors and I have a bet against a couple people studying binary star formation that within the first 100 LIGO detections, LIGO will detect something within this upper mass gap. I get a nice bottle of wine if that happens to be true.”
The detection of gravitational waves was a historic accomplishment, and one that has enabled astronomers to conduct new and exciting research. Already, scientists are gaining new insight into black holes by studying the byproduct of their mergers. In the coming years, we can expect to learn a great deal more thanks to improve methods and increased cooperation between observatories.
For decades, astronomers have known that Supermassive Black Holes (SMBHs) reside at the center of most massive galaxies. These black holes, which range from being hundreds of thousands to billions of Solar masses, exert a powerful influence on surrounding matter and are believed to be the cause of Active Galactic Nuclei (AGN). For as long as astronomers have known about them, they have sought to understand how SMBHs form and evolve.
In two recently published studies, two international teams of researchers report on the discovery of five newly-discovered black hole pairs at the centers of distant galaxies. This discovery could help astronomers shed new light on how SMBHs form and grow over time, not to mention how black hole mergers produce the strongest gravitational waves in the Universe.
The second study, which reported the fifth dual black hole candidate, was led by Sarah Ellison – an astrophysics professor at the University of Victoria. It was recently published in the Monthly Notices of the Royal Astronomical Society under the title “Discovery of a Dual Active Galactic Nucleus with ~8 kpc Separation“. The discovery of these five black hole pairs was very fortuitous, given that pairs are a very rare find.
“Astronomers find single supermassive black holes all over the universe. But even though we’ve predicted they grow rapidly when they are interacting, growing dual supermassive black holes have been difficult to find.“
For the sake of their studies, Satyapal, Ellison, and their respective teams sought to detect dual AGNs, which are believed to be a consequence of galactic mergers. They began by consulting optical data from the SDSS to identify galaxies that appeared to be in the process of merging. Data from the all-sky WISE survey was then used to identify those galaxies that displayed the most powerful AGNs.
From the combined data, they found that five out of the seven merging galaxies hosted possible dual AGNs, which were separated by less than 10 kiloparsecs (over 30,000 light years). This was evidenced by the infrared data provided by WISE, which was consistent with what is predicated of rapidly growing supermassive black holes.
In addition, the Chandra data showed closely-separated pairs of x-ray sources, which is also consistent with black holes that have matter slowly being accreted onto them. This infrared and x-ray data also suggested that the supermassive black holes are buried in large amounts of dust and gas. As Ellison indicated, these findings were the result of painstaking work that consisted of sorting through multiple wavelengths of data:
“Our work shows that combining the infrared selection with X-ray follow-up is a very effective way to find these black hole pairs. X-rays and infrared radiation are able to penetrate the obscuring clouds of gas and dust surrounding these black hole pairs, and Chandra’s sharp vision is needed to separate them”.
Before this study, less than ten pairs of growing black holes had been confirmed based on X-ray studies, and these were mostly by chance. This latest work, which detected five black hole pairs using combined data, was therefore both fortunate and significant. Aside from bolstering the hypothesis that supermassive black holes form from the merger of smaller black holes, these studies also have serious implications for gravitational wave research.
“It is important to understand how common supermassive black hole pairs are, to help in predicting the signals for gravitational wave observatories,” said Satyapa. “With experiments already in place and future ones coming online, this is an exciting time to be researching merging black holes. We are in the early stages of a new era in exploring the universe.”
Since 2016, a total of four instances of gravitational waves have been detected by instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO Observatory. However, these detections were the result of black hole mergers where the black holes were all smaller and less massive – between eight and 36 Solar masses.
Supermassive Black Holes, on the other hand, are much more massive and will likely produce a much larger gravitational wave signature as they continue to draw closer together. And in a few hundred million years, when these pairs eventually do merge, the resulting energy produced by mass being converted into gravitational waves will be incredible.
At present, detectors like LIGO and Virgo are not able to detect the gravitational waves created by Supermassive Black Hole pairs. This work is being done by arrays like the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which relies on high-precision millisecond pulsars to measure the influence of gravitational waves on space-time.
The proposed Laser Interferometer Space Antenna (LISA), which will be the first dedicated space-based gravitational wave detector, is also expected to help in the search. In the meantime, gravitational wave research has already benefited immensely from collaborative efforts like the one that exists between Advanced LIGO and Advanced Virgo.
In the future, scientists also anticipate that they will be able to study the interiors of supernovae through gravitational wave research. This is likely to reveal a great deal about the mechanisms behind black hole formation. Between all of these ongoing efforts and future developments, we can expect to “hear” a great deal more of the Universe and the most powerful forces at work within it.
Be sure to check out this animation that shows what the eventual merger of two of these black hole pairs will look like, courtesy of the Chandra X-ray Observatory:
Most large galaxies harbor central supermassive black holes with masses equivalent to millions, or even billions, of Suns. Some, like the one in the center of the Milky Way Galaxy, lie quiet. Others, known as quasars, chow down on so much gas they outshine their host galaxies and are even visible across the Universe.
Although their brilliant light varies across all wavelengths, it does so randomly — there’s no regularity in the peaks and dips of brightness. Now Matthew Graham from Caltech and his colleagues have found an exception to the rule.
Quasar PG 1302-102 shows an unusual repeating light signature that looks like a sinusoidal curve. Astronomers think hidden behind the light are two supermassive black holes in the final phases of a merger — something theoretically predicted but never before seen. If the theory holds, astronomers might be able to witness two black holes en route to a collision of incredible scale.
Graham and his colleagues discovered the unusual quasar on a whim. They were aiming to study quasar variability using the Catalina Real-Time Transient Survey (CRTS), which uses three ground-based telescopes to monitor some 500 million objects strewn across 80 percent of the sky, when 20 or so periodic sources popped up.
Of those 20 periodic quasars, PG 1302-102 was the most promising. It had a strong signal that appeared to repeat every five years or so. But what causes the repeating signal?
The black holes that power quasars do not emit light. Instead the light originates from the hot accretion disk that feeds the black hole. Orbiting clouds of gas, which are heated and ionized by the disk, also contribute in the form of visible emission lines.
“When you look at the emission lines in a spectrum from an object, what you’re really seeing is information about speed — whether something is moving toward you or away from you and how fast. It’s the Doppler effect,” said study coauthor Eilat Glikman from Middlebury College in Vermont, in a news release. “With quasars, you typically have one emission line, and that line is a symmetric curve. But with this quasar, it was necessary to add a second emission line with a slightly different speed than the first one in order to fit the data. That suggests something else, such as a second black hole, is perturbing this system.”
So a tight supermassive black hole binary is the most likely explanation for this oddly periodic quasar.
“Until now, the only known examples of supermassive black holes on their way to a merger have been separated by tens or hundreds of thousands of light years,” said study coauthor Daniel Stern from NASA’s Jet Propulsion Laboratory. “At such vast distances, it would take many millions, or even billions, of years for a collision and merger to occur. In contrast, the black holes in PG 1302-102 are, at most, a few hundredths of a light year apart and could merge in about a million years or less.”
But astronomers remain unsure about what physical mechanism is responsible for the quasar’s repeating light signal. It’s possible that one quasar is funneling material from its accretion disk into jets, which are rotating like beams from a lighthouse. Or perhaps a portion of the accretion disk itself is thicker than the rest, causing light to be blocked at certain spots in its orbit. Or maybe the accretion disk is dumping material onto the black hole in a regular fashion, causing periodic bursts of energy.
“Even though there are a number of viable physical mechanisms behind the periodicity we’re seeing — either the precessing jet, warped accretion disk or periodic dumping — these are all still fundamentally caused by a close binary system,” said Graham.
Astronomers still don’t have a good handle on what happens in the final few light-years of a black hole merger. And of course these two black holes still won’t collide for thousands to millions of years. Even watching for the period to shorten as they spiral inward would dwarf human timescales. But the discovery of a system so late in the game proves promising for future work.
Why does this galaxy appear to be smiling? The answer might be because it has been holding a secret that astrophysicists have only now just uncovered: there are two — count ‘em – two gigantic black holes inside this nearby galaxy, named Markarian 739 (or NGC 3758), and both are very active. While massive black holes are common, only about one percent of them are considered as active and powerful – called active galactic nuclei (AGN). Binary AGN are rarer still: Markarian 739 is only the second identified within half a billion light-years from Earth.
Markarian 739 is actually a pair of merging galaxies. For decades, astronomers have known that the eastern nucleus of Markarian 739 contains a black hole that is actively accreting matter and generating an exceptional amount of energy. Now, data from the Swift satellite along with the Chandra X-ray Observatory Swift has revealed an AGN in the western half as well. This makes the galaxy one of the nearest and clearest cases of a binary AGN.
The galaxy is 425 million light-years away from Earth.
How did the second AGN remain hidden for so long? “Markarian 739 West shows no evidence of being an AGN in visible, ultraviolet and radio observations,” said Sylvain Veilleux, a professor of astronomy at University of Maryland in College Park , and a coauthor of a new paper published in Astrophysical Journal Letters. “This highlights the critical importance of high-resolution observations at high X-ray energies in locating binary AGN.”
Since 2004, the Burst Alert Telescope (BAT) aboard Swift has been mapping high-energy X-ray sources all around the sky. The survey is sensitive to AGN up to 650 million light-years away and has uncovered dozens of previously unrecognized systems.
Michael Koss, the lead author of this study, from NASA’s Goddard Space Flight Center and UMCP, did follow-up studies of the BAT mapping and he and his colleagues published a paper in 2010 that revealed that about a quarter of the Swift BAT AGN were either interacting or in close pairs, with perhaps 60 percent of them poised to merge in another billion years.
“If two galaxies collide and each possesses a supermassive black hole, there should be times when both black holes switch on as AGN,” said coauthor Richard Mushotzky, professor of astronomy at UMCP. “We weren’t seeing many double AGN, so we turned to Chandra for help.”
Swift’s BAT instrument is scanning one-tenth of the sky at any given moment, its X-ray survey growing more sensitive every year as its exposure increases. Where Swift’s BAT provided a wide-angle view, the X-ray telescope aboard the Chandra X-ray Observatory acted like a zoom lens and resolved details a hundred times smaller.
The distance separating the two black holes is about 11,000 light-years , or about a third of the distance separating the solar system from the center of our own galaxy. The dual AGN of Markarian 739 is the second-closest known, both in terms of distance from one another and distance from Earth. However, another galaxy known as NGC 6240 holds both records.