Nearly every massive galaxy hosts a supermassive black hole at its center. When two galaxies merge, their black holes can form a binary pair, meaning they’re in a sure orbit with each other. It’s hypothesized that these binaries are fated to eventually merge, but this has never been observed [1]. The query of whether such an event is feasible has been a subject of debate amongst astronomers for many years. In a recently published paper in The Astrophysical Journal, a team of astronomers have presented recent insight into this query.
The team used data from the Gemini North telescope in Hawai’i, one half of the International Gemini Observatory operated by NSF’s NOIRLab, which is funded by the U.S. National Science Foundation, to investigate a supermassive black hole binary positioned inside the elliptical galaxy B2 0402+379. That is the one supermassive black hole binary ever resolved in enough detail to see each objects individually [2], and it holds the record for having the smallest separation ever directly measured — a mere 24 light-years [3]. While this close separation foretells a strong merger, further study revealed that the pair has been stalled at this distance for over three billion years, begging the query; what is the hold-up?
To higher understand the dynamics of this technique and its halted merger the team looked to archival data from Gemini North’s Gemini Multi-Object Spectrograph (GMOS), which allowed them to find out the speed of the celebs inside the vicinity of the black holes. “The wonderful sensitivity of GMOS allowed us to map the celebs’ increasing velocities as one looks closer to the galaxy’s center,” said Roger Romani, Stanford University physics professor and co-author of the paper. “With that, we were in a position to infer the full mass of the black holes residing there.”
The team estimates the binary’s mass to be a whopping 28 billion times that of the Sun, qualifying the pair because the heaviest binary black hole ever measured. Not only does this measurement give useful context to the formation of the binary system and the history of its host galaxy, nevertheless it supports the long-standing theory that the mass of a supermassive binary black hole plays a key role in stalling a possible merger [4].
“The info archive serving the International Gemini Observatory holds a gold mine of untapped scientific discovery,” says Martin Still, NSF program director for the International Gemini Observatory. “Mass measurements for this extreme supermassive binary black hole are an awe-inspiring example of the potential impact from recent research that explores that wealthy archive.”
Understanding how this binary formed might help predict if and when it’ll merge — and a handful of clues point to the pair forming via multiple galaxy mergers. The primary is that B2 0402+379 is a ‘fossil cluster,’ meaning it’s the results of a complete galaxy cluster’s price of stars and gas merging into one single massive galaxy. Moreover, the presence of two supermassive black holes, coupled with their large combined mass, suggests they resulted from the amalgamation of multiple smaller black holes from multiple galaxies.
Following a galactic merger, supermassive black holes don’t collide head-on. As an alternative they start slingshotting past one another as they settle right into a sure orbit. With each pass they make, energy is transferred from the black holes to the encircling stars. As they lose energy, the pair is dragged down closer and closer until they are only light-years apart, where gravitational radiation takes over they usually merge. This process has been directly observed in pairs of stellar-mass black holes — the primary ever recorded instance being in 2015 via the detection of gravitational waves — but never in a binary of the supermassive variety.
With recent knowledge of the system’s extremely large mass, the team concluded that an exceptionally large variety of stars would have been needed to slow the binary’s orbit enough to bring them this close. In the method, the black holes appear to have flung out nearly all of the matter of their vicinity, leaving the core of the galaxy starved of stars and gas. With no more material available to further slow the pair’s orbit, their merger has stalled in its final stages.
“Normally evidently galaxies with lighter black hole pairs have enough stars and mass to drive the 2 together quickly,” said Romani. “Since this pair is so heavy it required a number of stars and gas to get the job done. However the binary has scoured the central galaxy of such matter, leaving it stalled and accessible for our study.”
Whether the pair will overcome their stagnation and eventually merge on timescales of tens of millions of years, or proceed in orbital limbo ceaselessly, is yet to be determined. In the event that they do merge, the resulting gravitational waves can be 100 million times more powerful than those produced by stellar-mass black hole mergers. It’s possible the pair could conquer that final distance via one other galaxy merger, which might inject the system with additional material, or potentially a 3rd black hole, to slow the pair’s orbit enough to merge. Nevertheless, given B2 0402+379’s status as a fossil cluster, one other galactic merger is unlikely.
“We’re looking forward to follow-up investigations of B2 0402+379’s core where we’ll take a look at how much gas is present,” says Tirth Surti, Stanford undergraduate and the lead creator on the paper. “This could give us more insight into whether the supermassive black holes can eventually merge or if they are going to stay stranded as a binary.”
Notes
[1] While there’s evidence of supermassive black holes coming inside a number of light-years of one another, it seems none have been in a position to overcome that final distance. The query of whether such an event is feasible is generally known as the final-parsec problem and has been a subject of debate amongst astronomers for many years.
[2] Previous observations have been made from galaxies containing two supermassive black holes, but in these cases they’re 1000’s of light-years apart — too far to be in a sure orbit with each other just like the binary present in B2 0402+379.
[3] Other black hole-powered sources exist with possible smaller separations, though these have been inferred using indirect observations and due to this fact can best be classified as candidate binaries.
[4] This theory was first put forth in 1980 by Begelman et al. and has long been argued to occur based on many years of observations of the centers of galaxies.
