Supermassive black holes are thought to lurk hungrily in the hearts of perhaps every large Galaxy in the Universe. These voracious gravitational beasts can weigh millions to billions of times more than our Star, the Sun, and they dine greedily on hearty portions of hapless stars and/or gas that whirl around too close to their clutches. In December 2013, a team of astronomers announced that they had seen what appears to be a dancing duo of supermassive black holes, circling around each other, in the dark heart of a remote galaxy. The incredibly rare discovery of this Cosmic Danse Macabre was made with the invaluable aid of NASA’s Wide-field Infrared Survey Explorer (WISE). Follow up observations were conducted using the Australian Telescope Compact Array near Narrabri, Australia, and the Gemini South Telescope in Chile. These observations revealed bizarre behavior in the distant galaxy, which included a clumpy jet that the astronomers believe to have been caused by one black hole influencing the jet of the other to wiggle like a ribbon in the wind.
The WISE satellite surveyed the entire sky twice at infrared wavelengths before being placed into hibernation in 2011. NASA gave the highly productive spacecraft a second chance at life, when it was awakened from its deep slumber to scan through our Solar System, looking for asteroids, in a mission called NEOWISE.
“We think the jet of one black hole is being wiggled by the other, like a dance with ribbons,” Dr. Chao-Wei Tsai explained in a December 3, 2013 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Tsai, of the JPL in Pasadena, California, is lead author of a study describing the findings appearing in the December 10, 2013 issue of The Astrophysical Journal. “If so, it is likely the two black holes are fairly close and gravitationally entwined,” he added.
The dance of this loving couple could teach astronomers more about how supermassive black holes merge, and thereby attain their extraordinarily heavy masses.
“Weird Things Might Be True Things”
Astronomers have known for over a decade that perhaps every large galaxy in the Cosmos has a heart of darkness, where a supermassive black hole dwells, lurking in sinister secret, awaiting its next meal. Anything that travels too close to the sinister gravitational embrace of these objects is doomed, and cannot escape from the clutches of their ferocious gravity–not even light.
Back in the 18th century, John Michell and Pierre-Simon Laplace suggested that such weird objects might exist in the Universe, and Albert Einstein’s Theory of General Relativity later predicted the existence of entities possessing such strong gravitational fields that anything unlucky enough to travel too close to them would be consumed. However, the prospect of the reality of such objects seemed so unlikely that Einstein actually rejected the concept, even though his own calculations indicated otherwise. However, scientists came to realize that these gravitational beasts really do exist in nature. As Dr. Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics (CfA) once said: “Weird things might be true things.”
In 1916, Karl Schwarzschild calculated the first modern solution of Einstein’s General Relativity that could describe black holes–although its interpretation as a region of Space from which absolutely nothing could escape to freedom was not understood for another 40 years. In fact, black holes were considered to be mere mathematical oddities for decades. It was not until the 1960s that theoretical work showed that black holes are a generic prediction of General Relativity.
Supermassive black holes are both mysterious and puzzling. This is primarily because they were apparently already around when the Universe was very ancient. It is believed that the stars and gas that swirl around, and then down, down, down, never to return from the churning maelstrom surrounding these dark-hearted beasts, create an in-falling whirling storm of material–an enormous disk, termed an accretion disk. This material grows ever hotter and hotter, and emits a flood of radiation, especially as it approaches the point of no return–the innermost region of the disk–called the event horizon.
The further we look into Space, the further back we look in Time. This is because the more distant a luminous object is, the longer it has taken for its wonderful light to reach Earth. No signal in the Universe can travel faster than light in a vacuum, and the light from remote, shining objects in the Cosmos can wander to us no faster than this universal speed limit. A large population of supermassive black holes already existed in the very ancient Universe and, lurking in the hearts of the most distant and ancient of galaxies, they reveal their hungry presence in the form of fiercely shining Quasi-stellar objects, or Quasars, which are especially voracious, youthful Active Galactic Nuclei (AGN). Quasars are powered by brilliant accretion disks, and many astronomers think that they caught fire when our Universe was still in its babyhood.
Supermassive black holes can grow to be at least as large as our Solar System. These incredible objects are noteworthy for their hefty weight, messy table manners, and greed. When its outside supply of energy is depleted, a quasar dims and then switches off. Most galaxies probably went through a quasar stage in their youth, and now these galaxies contain a dormant beast in their quieter, aging hearts.
The December 2013 study was based on previously released information derived from all-sky WISE data. The astronomers carefully studied images of literally millions of hungry, active supermassive black holes dwelling throughout our sky.
Then they spotted the “oddball”! It leaped right out at them from the WISE data. They named the strange beast WISE J233237.05-505643.6.
“At first we thought this galaxy’s unusual properties seen by WISE might mean it was forming new stars at a furious rate. But on closer inspection, it looks more like the death spiral of merging giant black holes,” noted Dr. Peter Eisenhardt in the December 3, 2013 JPL Press Release. Dr. Eisenhardt is WISE project manager at the JPL.
It has been a longstanding mystery how supermassive black holes grow to be so large–with masses up to billions of Suns. One way they might become so obese is by devouring surrounding stellar and gaseous materials. A second way might result from “galactic cannibalism”, whereby a larger galaxy consumes a smaller galaxy that has wandered too close to it–black hole and all. When galaxies collide, their supermassive beasts sink down to the heart of the new galactic structure, and then dance together in a sort of gravitational tango. Ultimately, they merge into a single even-more-hefty black hole.
This Cosmic Danse Macabre starts out slowly, with the two dancers circling each other at a cautious distance of approximately a few thousand light-years. At this point, only a small number of supermassive black holes have been confirmed to be prancing around in this early stage of inevitable merging. As the black holes continue to spiral in towards one another–as their flirtatious, bizarre dance continues–they get ever closer and closer and closer, separated by only a few light-years.
Closely dancing black holes, termed black hole binaries, have proven to be the most difficult of their kind for astronomers to spot. These strange objects are normally too petite to be resolved even by the most technologically advanced telescopes currently available. Only a few strong candidates have been observed to date–and they are all relatively close to us. However, WISE J233237.05-505643.5 is a new, strong candidate! It also dwells much farther away than others of its sort, at the truly impressive distance of 3.8 billion light-years from our planet.
Radio images derived from the Australian Telescope Compact Array proved crucial in discovering the bizarre dual nature of this mysterious, distant object. Supermassive beasts, inhabiting the hearts of galaxies, normally emit pencil-straight jets. However, in the case of WISE J233237.05-505643.5, the jet zigzags. The astronomers think that a second black hole could be altering the shape of the jet.
Visible-light spectra data obtained from the Gemini South Telescope revealed similar deformities that might have resulted from one black hole causing disk material surrounding the other to clump. If taken together, these observations indicate what is likely a closely dancing pair of circling black holes. Although, at this point, the astronomers cannot pinpoint the exact distance that separates the two dance partners.
In the end, merging supermassive black holes are predicted to dispatch gravitational waves rippling through the fabric of Spacetime. Astronomers are dedicatedly looking for signs of these waves using arrays of stellar corpses termed pulsars (spinning neutron stars) in their quest to learn more about how black holes dance.
“We note some caution in interpreting this mysterious system. 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,” Dr. Daniel Stern noted in the December 3, 2013 JPL Press Release. Dr. Stern is a study co-author.