Long ago and far away, a duo of dancing supermassive black holes appear to be spiraling in towards one another, eventually doomed to collide in a stupendous, almost unimaginable, cosmic smash-up. Dancing in the dark, the strange pair will merge a mere million years from now, liberating energy equivalent to 100 million supernova blasts, in which massive stars perish. The dark-hearts of most, if not all, large galaxies in the Universe—including our own Milky Way—contain supermassive black holes with masses equivalent to millions, or even billions, of Suns, and these objects of incredible darkness and their host galaxies appear to evolve together, or «co-evolve». Theory predicts that as galaxies collide and eventually merge, growing more and more massive as a result, so too do their hearts of darkness. In the January 7, 2015 issue of the journal Nature, a team of astronomers report on a weird repeating light signal from a distant quasar that they say is most likely the result of a duo of dancing supermassive black holes in the last act of a merger—something that is predicted from theory but which has never been seen before! A quasar is an extremely brilliant, luminous object that out-dazzles all of the stars in its host galaxy combined, and is visible from across the entire Universe!
Black holes by themselves are impossible to observe, cloaked as they are in impenetrable darkness. However, their gravity can hoist in ambient gas to create a whirling, swirling band of material that is termed an accretion disk. The jitter-bugging particles of the disk are accelerated to enormous speeds and liberate stupendous quantities of energy in the form of heat and powerful X-rays and gamma rays. When this strange process occurs in the case of a supermassive black hole, the result is a quasar.
«Quasars are valuable probes of the evolution of galaxies and their central black holes,» noted Dr. George Djorgovski in a January 7, 2015 California Institute of Technology (Caltech) Press Release. Dr. Djorgovski is a professor of astronomy and director of the Center for Data-Driven Discovery at Caltech, which is in Pasadena, California.
The discovery of this dancing duo of supermassive black holes could help shed new light on a long-standing mystery in astrophysics termed the final parsec problem. The final parsec problem refers to the failure of theoretical models to predict what the end stages of a black hole merger look like or even how long this incredible process might take.
«The end stages of the merger of these supermassive black hole systems are very poorly understood. The discovery of a system that seems to be at this late stage of its evolution means we now have an observational handle on what is going on,» explained the study’s lead author, Dr. Matthew Graham, in the January 7, 2015 Caltech Press Release. Dr. Graham is a senior computational scientist at Caltech.
Dancing In The Dark
Supermassive black holes lurking in the secretive, hidden hearts of galaxies, grow by devouring their surroundings, feasting hungrily on gas and the stuff of doomed stars with unimaginable greed. They are also very sloppy, and attempt to swallow more than they actually can, violently hurling some of the tattered remains of their terrible feast into the surrounding space.
Black holes are anything but empty space—despite their name. Actually, they represent a great quantity of matter packed into a very small space—and they come in at least two sizes, supermassive and stellar mass. There may also be intermediate mass black holes that are much heavier than those of «only» stellar mass, but considerably lighter than their supermassive kin.
Black holes of stellar mass form when a very heavy star collapses in the fiery tantrum of a supernova explosion that blasts the dying, massive star into oblivion—thus heralding the end of its beautiful, active life as a main-sequence (hydrogen-burning) star. After a black hole has emerged from the stellar mess, it can go on to gain more and more weight by devouring whatever it can snare with its gravitational claws that snatch. Many astronomers think that by eating doomed stars, blobs of gas, and by merging with others of its own strange kind, supermassive beasts form from the smaller variety.
Astronomers have known for years that it is probable every large galaxy in the Universe harbors a hungry, greedy, supermassive beast in its mysterious heart. There the strange object resides, secreted in its host galaxy’s core, waiting for its lunch to tumble down into its ravenous maw.
Clouds of gas and doomed stars swirl around in the violent maelstrom surrounding supermassive black holes, thus forming the immense, brilliant accretion disk. This ill-starred material grows ever hotter and hotter, and emits a tremendous amount of radiation, especially as it approaches the dreadful point of no return called the event horizon, which is the innermost region of the accretion disk.
The deeper we peer into space, the further we look back in time. In astronomy long ago is the same as far away, because the more distant a shining object is in space, the longer it takes for its traveling light to reach us. No known signal in Spacetime can travel faster than light, and the light that wanders to us from the most distant objects in the Universe can travel to us no faster than this universal speed limit. It is not possible to determine the position of an object in space without also locating it in time.
Astronomers use what is termed the redshift or z to describe how long ago and far away an object is. The measurable quantity of 1 + z is the factor by which the Cosmos has expanded between the time when a luminous source first sent forth its brilliant light and today. In addition, it is the factor by which the wavelength of light currently reaching Earth has been stretched as a result of the expansion of space. In astronomy, both time and distance, as well as the wavelength of light at which the observations are made, are all inextricably connected.
Albert Einstein’s Theory of General Relativity (1915) predicts the existence of black holes, which are described as objects possessing such strong gravity that absolutely nothing, not even light, can break free and escape from their merciless gravitational embrace—anything unfortunate enough to travel in too close to one of these gravitational beasts is doomed to be eaten by it. Nevertheless, the real existence of black holes in Nature seemed so far fetched at the time, that Einstein doubted his own predictions. However, eventually he went on to characterize them by saying that «Black holes are where God divided by zero.»
Our spiral, starry Milky Way Galaxy’s resident supermassive black hole is named Sagittarius A*. It is relatively light, as supermassive black holes go, weighing «only» about 4 million Suns as opposed to billions of solar-masses. It is also elderly, quiet, and lazy now, gaining weight at a comparatively sluggish pace.
Duo Of Dancing Black Holes
Dr. Djorgovski discussed the new research at the annual winter meeting of the American Astronomical Society (AAS) held in Seattle, Washington in January 2015. He and his team spotted the weird light signal emanating from a distant quasar dubbed PG 1302-102 after analyzing results obtained from the Catalina Real-Time Transient Survey (CRTS), which uses a trio of ground telescopes located in the United States and Australia to keep a constant eye on 500 million celestial sources of light, strewn across approximately 80 percent of the night sky.
«There has never been a data set on quasar variability that approaches this scope before. In the past, scientists who study the variability of quasars might only be able to follow some tens, or at most hundreds of objects with a limited number of measurements. In each case, we looked at a quarter million quasars and were able to gather a few hundred data points for each one,» Dr. Djorgovski, who directs CRTS, noted in the January 7, 2015 Caltech Press Release.
Study co-author Dr. Daniel Stern also explained to the press that «Until now, the only known examples of supermassive black holes on their way to a merger have been separated by hundreds of thousands of light years. 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.» Dr. Stern is a scientist at the JPL.
Dr. Djorgovski and his team did not originally set out to discover a black hole merger. Instead, they started out by embarking on a systematic study of quasar brightness variability with the goal of discovering new clues about their mysterious physics. However, after sifting through the data using a pattern-seeking algorithm that Dr. Graham developed, the astronomers discovered 20 quasars that appeared to be emitting periodic optical signals. This was a surprise, because the light curves of most quasars are extremely chaotic as a result of the random nature by which material from the accretion disk spirals into a black hole. Dr. Graham explained to the press on January 7, 2015 that «You just don’t expect to see a periodic signal from a quasar. When you do, it stands out.»
PG 1302-102 proved to be the best example out of the collected sample of 20 periodic quasars that CRTS detected. This is because it had a strong, clean signal that apparently repeated approximately every five years. «It has a really nice smooth up-and-down signal, similar to a sine wave, and that just hasn’t been seen before in a quasar,» Dr. Graham continued to explain.
The team was at first cautious about jumping to conclusions. However, they became increasingly more confident after co-author Dr. Eilat Glikman analyzed the quasar’s light spectrum. Dr. Glikman is an assistant professor of physics at Middlebury College in Vermont. The black holes that the scientists believe are powering the quasars do not emit light themselves, but the gases swirling around them in the accretion disks are zipping around so speedily that they become heated into a glaring, searing-hot plasma.
«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. 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,» Dr. Glikman explained to the press on January 7, 2015.
Dr. Avi Loeb agrees with the team’s interpretation of their findings that a «tight» supermassive black hole binary is the most probable explanation for the weird periodic signal they had observed. «The evidence suggests that the emission originates from a very compact region around the black hole and that the speed of the emitting material in that region is at least a tenth of the speed of light. A secondary black hole would be the simplest way to induce a periodic variation in the emission from that region, because a less dense object, such as a star cluster, would be disrupted by the strong gravity of the primary black hole,» Dr. Loeb, who chairs the astronomy department at Harvard University in Cambridge, Massachusetts, explained in the January 7, 2015 JPL Press Release.
The team’s discovery is also a testament to the power of «big data» science, where the challenge resides not only in the gathering of high-quality data, but also formulating methods to mine it for important information. «We’re basically moving from having a few pictures of the whole sky or repeated observations of tiny patches of the sky to having a movie of the entire sky all the time. Many objects in the movie will not be doing anything very exciting, but there will also be a lot of interesting ones that we missed before,» explained Dr. Sterl Phinney to the press on January 7, 2015. Dr. Phinney, a professor of theoretical physics at Caltech, was not involved in the study.
However, it remains unclear what physical mechanism is responsible for the quasar’s repeating light signal. One possible explanation, according to Dr. Graham, is that the quasar is funneling material from the accretion disk into a luminous twin duo of plasma jets that are rotating in a way similar to the beams emanating from a lighthouse. «If the glowing jets are sweeping around in a regular fashion, then we would only see them when they’re pointed directly at us. The end result is a regularly repeating signal,» Dr. Graham explained to the press.
Another possible explanation is that the accretion disk that surrounds both black holes is distorted. «If one region is thicker than the rest, then as the warped section travels around the accretion disk, it could be blocking light from the quasar at regular intervals. This would explain the periodicity of the signal that we’re seeing,» Dr. Graham continued to explain.
There is yet another possibility that something is occurring in the accretion disk that is forcing it to dump material onto the black hole at regular intervals, resulting in the observed periodic bursts of energy.
«Even though there are a number of visible 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,» Dr. Graham commented.