A billion light-years from Earth, a rare and exotic “dance of death” is being performed by a trio of supermassive black holes, doomed to merge along with their host galaxies that are on a collision course. Supermassive black holes, that weigh-in at an incredible millions to billions of times solar-mass, are thought to lurk hungrily in the hearts of perhaps every large galaxy in the observable Universe–including our own barred-spiral Milky Way. When galaxies collide, their supermassive hearts of darkness meet-up as well, and become a single, solitary gravitational beast that weighs-in at the combined masses of the merging black holes that created them. In September 2019, a team of astronomers announced that they have discovered a rare system composed of three galaxies colliding–and taking their resident dark hearts along with them for the ride. Several observatories, including NASA’s Chandra X-ray Observatory, as well as other NASA space telescopes, discovered this exotic system as a result of scientific serendipity.
“We were only looking for pairs of black holes at the time, and yet through our selection technique, we stumbled upon this amazing system. This is the strongest evidence yet found for such a triple system of activity feeding supermassive black holes,” noted Dr. Ryan Pfeifle in a September 26, 2019 Chandra Observatory Press Release. Dr. Pfeifle, who is of George Mason University in Fairfax, Virginia, is the first author of a new paper published in The Astrophysical Journal describing these results.
The unusual system is dubbed SDSS J084905.51+111447 2–SDSS J0849+1114 for short.
Because they lurk in the secretive hearts of perhaps every large galaxy in the observable Universe, supermassive hearts of darkness are common denizens of the Cosmos. Indeed, our own Milky Way Galaxy has its own resident supermassive black hole. The Milky Way’s central gravitational monster is currently dormant, and it is a relative lightweight–at least as far as supermassive black holes go–weighing-in at mere millions (as opposed to billions) of solar masses.
Our Milky Way’s secretive dark heart has been dubbed Sagittarius A* (Sgr A*, for short), and it is on a collision course with another large galaxy. In about 4 billion years or so, our Galaxy is predicted to suffer a titantic collision with the Andromeda galaxy, which is another spiral galaxy of similar size. When this catastrophe occurs, the two supermassive black holes situated in the centers of both galaxies, will also merge. The resulting beast, born from this collision, will weigh-in at the combined mass of the two separate entities that merged to create it.
Our Cosmos is not a peaceful place. Supermassive dark hearts lurk hungrily within their host galaxies, waiting for a feast to come tumbling into their powerful and irresistible gravitational snatching claws. Anything luckless enough to wander too close to one of these frumious bandersnatches of the Universe will become its dinner. Captured victims cannot free themselves from the incredibly powerful gravitational embrace of the predatory black hole. Even light cannot liberate itself if it passes the terrible point of no return called the event horizon.
Doomed stars and clouds of unfortunate gas are some of the tidbits snared by the galactic hearts of darkness. These tragic objects swirl down, down, down as they tumble into the maelstrom of the relentless vortex surrounding the beast–and they will never, never, never return to the outside once they reach the event horizon. As the material travels towards its inevitable end, it creates a ferocious storm composed of glaring matter encircling the black hole–the massive, brilliant accretion disk. In the primordial Universe, these glaring disks dazzled Spacetime in the form of quasars. It is believed that Sgr A* (pronounced saj-a-star) experienced a brilliant quasar stage billions of years ago, when both it, and the Universe itself, were young.
The glaring material that makes up the accretion disk grows ever hotter, as it triggers a raging storm of radiation–especially as it swirls ever closer to the terrible event horizon. The event horizon is located at the innermost portion of the accretion disk.
Although supermassive black holes hide in the hearts of large galaxies, they are not the only members of their bizarre kind. Indeed, black holes come in different sizes. Squeeze enough mass into a small enough space and a black hole will form every time. Smaller black holes of stellar mass are born when an especially massive star runs out of its necessary supply of nuclear-fusing fuel and blasts itself to smithereens in a brilliant core-collapse (Type II) supernova explosion. Intermediate size black holes are also thought to be denizens of the Cosmos and, perhaps, when these middle-weights merge, their supermassive counterparts are born.
In the 18th century, the English scientist John Michell (1724-1793) and the French physicist Pierre-Simon Laplace (1749-1827) proposed that strange beasts like black holes could really lurk in the Universe. Decades later, Albert Einstein, in his Theory of General Relativity (1915), predicted the true existence of such objects. However, at the time, the idea that entities possessing gravitational fields so powerful that nothing could escape from their clutches, seemed so preposterous that Einstein rejected the concept–even though his own calculations suggested otherwise.
In 1916, the German physicist and astronomer Karl Schwarzschild (1873-1916) developed the first modern solution to General Relativity that describes a black hole. However, its characterization as a region of space from which absolutely nothing could ever escape, was not fully understood for another half-century. Up until then, black holes were interpreted to be mere mathematical oddities. Indeed, it was not until the 1960s that theoretical work showed that black holes are a generic prediction of General Relativity.
When Three’s A Crowd
In order to reveal the rare system composed of triplet black holes, scientists needed to combine information derived from telescopes–both ground-based and space-borne. First, the Sloan Digital Sky Survey (SDSS) telescope, which searches large regions of the sky in optical light from New Mexico, imaged the unusual SDSS JO849+11114. Citizen scientists then contributed to the hunt by participating in a project called Galaxy Zoo. The end result tagged the oddity as a system of three colliding galaxies.
Next, data collected from NASA’s Wide-field Infrared Sky Survey Explorer (WISE) mission showed that the system was intensely glowing in infrared light, during a phase in the galaxy merger when more than one of the dark hearts was predicted to be on a feeding frenzy. In order to perform a follow-up to these enticing clues, astronomers then used the Chandra X-ray Observatory and the Large Binocular Telescope (LBT) in Arizona.
The Chandra data uncovered X-ray sources situated within the bright centers of each of the three galaxies involved in the massive merger. This was a tattle-tale clue that material was being devoured by a black hole, since the sources originated exactly where astronomers expected them to reside. Both Chandra and NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) satellite also detected evidence for large quantities of gas and dust surrounding one of the black holes, typical for this type of a merging system.
At the same time, optical light data derived from SDSS and LBT revealed characteristic spectral signatures of material being devoured by the trio of supermassive black holes.
“Optical spectra contain a wealth of information about a galaxy. They are commonly used to identify actively accreting supermassive black holes and can reflect the impact they have on the galaxies they inhabit,” commented study co-author Dr. Christina Manzano-King in the September 26, 2019 Chandra Observatory Press Release. Dr. Manzano-King is of the University of California, Riverside.
It is difficult for astronomers to detect a trio of supermassive black holes because they are likely to be heavily blanketed by obscuring gas and dust, that blocks out much of their light. The infrared images obtained from WISE, the infrared spectra from LBT and the X-ray images from Chandra were able to avoid the problem. This is because both X-ray and infrared light slice through clouds of gas much more easily than optical light.
“Through the use of these major observatories, we have identified a new way of identifying triple supermassive black holes. Each telescope gives us a different clue about what’s going on in these systems. We hope to extend our work to find more triples using the same techniques,” Dr. Pfeifle explained in the Chandra Press Release.
“Dual and triple black holes are exceedingly rare, but such systems are actually a natural consequence of galaxy mergers, which we think is how galaxies grow and evolve,” added study co-author Dr. Shobita Satyapal, who is also of George Mason University.
A trio of supermassive black holes merging do not behave the same way as a colliding duo of their kind. When there are three black holes influencing one another, a pair should merge into a larger black hole considerably faster than if there were only two actors in the drama. This may provide a solution to a theoretical conundrum referred to as the “final parsec problem”–in which a duo of supermassive black holes can approach to within a few light-years of one another, but would require some extra pull inwards in order to merge. This is because of the excess energy they carry in their orbits. The influence of a third black hole, as in SDSS J0849+1114, could be the entity that finally brings the duo together.
Computer simulations have demonstrated that 16% of supermassive duos in colliding galaxies will have interacted with a third member of their kind before they merge. Such mergers will trigger ripples through Spacetime termed gravitational waves. These waves will possess lower frequencies than the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European Virgo gravitational-wave detector can detect. However, they may be detectable with radio observations of pulsars, which are rapidly and regularly spinning newborn neutron stars. The gravitational waves may also be detectable by future space observatories, such as the European Space Agency’s Laser Interferometer Space Antenna (LISA), which will be able to detect black holes that weigh-in up to one million times solar-mass.
The paper describing these results has been published in The Astrophysical Journal.