“Dancing: A series of movements involving two partners, where speed and rhythm match harmoniously with music.”
— The Computer, explaining the meaning of ‘Dancing’ to the Ship’s Captain – ‘Wall-E’ (2008).
Although music isn’t something to be found in the airless void of space, astronomers using data from NASA’s WISE infrared space telescope and other ground-based observatories have found evidence of two black holes caught in a gravitational dance around each other.
Black holes are probably the most fascinating and mysterious objects in the Universe, capturing the imagination of astronomers and the general public alike. For decades, they have been portrayed in cinematic and televised science fiction stories as shortcuts through space-time for intrepid interstellar explorers. The reality of their true nature, though, is vastly different.
They arose as a mathematical possibility out of Einstein’s general theory of relativity in 1915. What Einstein had previously shown with his special relativity theory was that the Universe’s geometry consisted of a flat, four-dimensional space-time continuum. With general relativity, he explained how gravity, instead of being a separate force, was actually a property of space-time itself. Every object in the Universe that had any energy or mass, like the Sun and the planets, would curve space-time. (A simplistic, though not entirely accurate, analogy is an iron ball that is left rolling on an outstretched, flat bed sheet.) The greater the mass of the object, the greater its curvature upon space-time would be. Thus, this curvature in essence defines the object’s gravitational field. Black holes are objects whose entire mass is concentrated in a single point in space, called the singularity. Mathematically this means that their mass has become infinite, making their curvature on space-time infinite as well, from where nothing, not even light, can escape.
Black holes fall into two main categories: stellar-mass ones, which are created after a massive star’s death in a supernova explosion, and supermassive black holes. The latter have masses hundreds of thousands to billions of times more than that of the Sun, and are thought to be primordial ones, linked with the creation of early galaxies in the Universe. Indeed, revolutionary observations from the Hubble Space Telescope have shown us that nearly all of the galaxies in the Universe harbor a supermassive black hole in their centers. Our own Milky Way galaxy hosts one at its center, called Sagittarius A*, located 26,000 light-years away in the Sagittarius constellation. One of the best evidences for their existence comes from observations showing matter falling toward a very compact and very high-mass area at the centers of galaxies. This matter caught by the black hole’s immense gravity creates an accretion disk around it prior to falling inward, while being subjected to very extreme temperatures and pressures in the process. As a result, the disk starts glowing very brightly in various parts of the electromagnetic spectrum, releasing immense amounts of energy.
One phenomenon that is not very well understood is the creation of polar jets of material, being emitted along the axis of rotation of the accretion disk, seen in many galactic centers. These jets are streams of matter that are ejected from the black hole, often at relativistic velocities, that reach the speed of light. Although these relativistic jets have been observed inside active galactic centers, they were thought to be the result of a single supermassive black hole. Now, astronomers using data from NASA’s Wide-field Infrared Survey Explorer, or WISE, recently published a study showing evidence for the existence of two such black holes at the center of a faraway galaxy, being caught in a gravitational dance, circling each other as if engaged in a cosmic tango.
WISE is a space telescope observing the sky in infrared light. It was launched in 2009, completing two high-resolution surveys of the entire sky at infrared wavelengths, before its hydrogen coolant ran out. It helped to discover thousands of previously unseen Near-Earth Asteroids in the Solar System and millions of supermassive black holes scattered across the sky. A team of astronomers studying those millions of newly discovered black holes found a very peculiar object named WISE J233237.05-505643.5. This oddly named object is an Active Galactic Nucleus, or AGN, located approximately 4.7 billion light-years away. AGNs are galactic centers that emit very brightly in many different wavelengths, from x-rays to radio waves. It is believed that these powerful releases of energy come from the accretion discs around the supermassive black holes located at the galaxy’s center. These disks are often accompanied by relativistic jets of material shooting along the black hole’s axis. In the case of WISE J233237.05-505643.5, what the astronomers found were evidence of multi-jets, which is the presence of more than two jets coming out of the galactic center.
Active galactic centers exhibiting multi-jet features are extremely rare. Only a few have been observed before, like in the case of 3C 75 and 3C 66B, which are radio galaxies located 320 million and 296 million light-years away, respectively. The new study comes to add WISE J233237.05-505643.5 to the list. In addition to WISE’s infrared observations, WISE J233237.05-505643.5 had also been observed before, in radio wavelengths by the radio telescopes of the Australia Telescope Compact Array, and in optical wavelengths by the Magellan and Gemini South Telescopes in Chile. What the past observations, combined with the new ones by NASA’s WISE telescope, show is that WISE J233237.05-505643.5 displays two primary jets, as is the case with many other active galactic centers that harbor supermassive black holes. Although one of the jets is typically linear, pointing in a straight line, the other shows a distinct “zig-zag,” “windy” pattern. What’s more, astronomers observed a third linear structure coming out of the galactic core that is perpendicular to the two primary jets and a fourth arc-like structure, also very near the core.
The astronomers involved in the study concluded that one of the best explanations to account for the observations is that WISE J233237.05-505643.5 hosts a pair of two supermassive black holes gravitationally locked very close together, while being in the process of merging into a single bigger one. Such supermassive black hole binaries had long been theoretically proposed, but had never been observed directly. As described in the study, astronomers note that “Supermassive black hole binaries, or ‘SMBHBs,’ are the inevitable result of galaxy formation. In a major galaxy merger event, the SMBHs hosted in two large merging galaxies dynamically interact as the parent galaxies coalesce and form a final bulge.” WISE J233237.05-505643.5 represents the newest candidate to the very small list of black hole binary candidates, including the aforementioned 3C 75 and 3C 66B. “At first we thought this galaxy’s unusual properties seen by WISE might mean it was forming new stars at a furious rate,” says Peter Eisenhardt, project scientist for NASA’s WISE mission. “But on closer inspection, it looks more like the death spiral of merging giant black holes.”
Yet the astronomers offer a word of caution in the study, noting that black hole mergers are not the only viable explanation. One alternative is the presence of non-uniformly distributed gas inside a highly inclined accretion disk. “We note some caution in interpreting this mysterious system,” says Daniel Stern, project scientist for the NUSTAR mission at NASA’s JPL and co-author of the recent study. “There are several extremely unusual properties to this system, from the multiple radio jets to the Gemini data, which indicate a highly perturbed disk of accreting material around the black hole, or holes. Two merging black holes, which should be a common event in the universe, would appear to be the simplest explanation to explain all the current observations.” Although more observations are needed to help settle the matter, the astronomers seem to lean toward the black-hole binary scenario. “We think the jet of one black hole is being wiggled by the other, like a dance with ribbons,” says Chao-Wei Tsai of JPL, lead-author of the study. “If so, it is likely the two black holes are fairly close and gravitationally entwined.”
Black hole mergers are considered to be one of the most violent events in the Universe, and the prime sources for the as yet unseen gravitational waves. These are ripples through space-time, analogous to the ripples on the surface of a pond, created when a stone is thrown in. Although predicted by Einstein’s general theory of relativity, gravitational waves haven’t been observed to date.
Although black holes and black hole mergers in reality aren’t shortcuts between different areas of the Universe for brave astronauts to pass through, they nevertheless represent gateways toward a better understanding, because studying them could yield some really important insights—not only about the energetic events inside active galaxies and the processes that drive galactic evolution, but also about the nature of the Universe and space-time itself.