Astronomers have spotted three supermassive black holes (SMBHs) at the center of three colliding galaxies a billion light years away from Earth. That alone is unusual, but the three black holes are also glowing in x-ray emissions.
This is evidence that all three are also active galactic nuclei (AGN,) gobbling up material and flaring brightly.
This discovery may shed some light on the "final parsec problem," a long-standing issue in astrophysics and black hole mergers.
Astronomers found the three SMBHs in data from multiple telescopes, including the Sloan Digital Sky Survey (SDSS,) the Chandra X-ray Observatory, and the Wide-field Infrared Survey Explorer (WISE.)
The three black holes are wrapped up in an almost unimaginably epic event; a merger of three galaxies. These triplet mergers may play a critical role in how the most massive black holes grow over time.
The astronomers who found it were not expecting to find three black holes in the center of a triple-galaxy merger.
"We were only looking for pairs of black holes at the time, and yet, through our selection technique, we stumbled upon this amazing system," said Ryan Pfeifle of George Mason University in Fairfax, Virginia, the first author of a new paper in The Astrophysical Journal describing these results.
"This is the strongest evidence yet found for such a triple system of actively feeding supermassive black holes."
Triple black hole systems are difficult to spot because there's so much going on in their neighbourhood. They're shrouded in gas and dust that makes it challenging to see into. In this study, it took several telescopes operating in different parts of the electromagnetic spectrum to uncover the three holes. It also took the work of some citizen scientists.
They're not only difficult to spot, but rare.
"Dual and triple black holes are exceedingly rare," said co-author Shobita Satyapal, also of George Mason, "but such systems are actually a natural consequence of galaxy mergers, which we think is how galaxies grow and evolve."
The SDSS was the first to spot this triple-merger in visible light, but it was only through Galaxy Zoo, a citizen science project, that it was identified as a system of colliding galaxies.
Then WISE saw that the system was glowing in the infrared, indicating that it was in a phase of galaxy merger when more than one of the black holes was expected to be feeding.
The Sloan and WISE data were just tantalizing clues though, and astronomers turned to the Chandra Observatory and the Large Binocular Telescope (LBT) for more confirmation. Chandra observations showed that there were bright x-ray sources in the center of each galaxy. That's exactly where scientists expect to find SMBHs.
More evidence showing that SMBHs were there arrived from Chandra and NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) satellite. They found evidence of large amounts of gas and dust near one of the black holes.
That's expected when black holes are merging. Other optical light data from the SDSS and the LBT provided spectral evidence that's characteristic of the three SMBHs feeding.
"Optical spectra contain a wealth of information about a galaxy," said co-author Christina Manzano-King of University of California, Riverside. "They are commonly used to identify actively accreting supermassive black holes and can reflect the impact they have on the galaxies they inhabit."
With this work, the team of astronomers have developed a way to find more of these triple black hole systems.
"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," said Pfeifle. "We hope to extend our work to find more triples using the same technique."
They may have also shed some light on the final parsec problem.
The Final Parsec Problem
The final parsec problem is central to our understanding of binary black hole mergers. It's a theoretical problem that says when two black holes approach each other, their excessive orbital energy stops them from merging. They can get to within a few light years, then the merging process stalls.
When two black holes initially approach each other, their hyperbolic trajectories carry them right past each other. Over time, as the two holes interact with stars in their vicinity, they slingshot the stars gravitationally, transferring some of their orbital energy to a star each time they do it. The emission of gravitational waves also decreases the black holes' energy.
Eventually the two black holes shed enough orbital energy to slow down and approach each other more closely, and come to within just a few parsecs of each other.
The problem is, as they close the distance, more and more matter is ejected from their vicinity via sling-shotting. That means there's no more matter for the black holes to interact with and shed more orbital energy. At that point, the merging process stalls. Or it should.
Yet astrophysicists know that black holes merge because they've witnessed the powerful gravitational waves. In fact, LIGO (Laser Interferometry Gravitational-Wave Observatory) is discovering a black hole merger about once a week. How they merge with each other at the end is called the final parsec problem.
The team behind this study thinks that they might have an answer. They think that a third black hole, like they've observed in this system, could provide the boost needed to get two holes to merge.
As a pair of black holes in a trinary system approach each other, the third hole could influence them to close the final parsec and merge.
According to computer simulations, about 16% of pairs of supermassive black holes in colliding galaxies will have interacted with a third supermassive black hole before they merge.
Those mergers would produce gravitational waves, but the problem is that those waves would be too low-frequency for LIGO or the VIRGO observatory to detect.
To detect those, scientists may have to rely on future observatories like LISA, ESA/NASA's Laser Interferometer Space Antenna. LISA will observe lower frequency gravitational waves than LIGO or VIRGO and is better-equipped to find super-massive black holes merging.
The paper presenting these results is titled "A Triple AGN in a Mid-Infrared Selected Late Stage Galaxy Merger."