Supermassive black holes at the centers of galaxies don’t always merge, new observations show
Almost all massive galaxies harbor a supermassive black hole at their center. When two galaxies merge, their black holes can form a binary pair, meaning they are in a connected orbit. The hypothesis is that these binaries are destined to merge, but this has never been observed. The question of whether such an event is possible has been debated among astronomers for decades. In a recent paper published in the Astrophysical Journal, a team of astronomers presented new data on this question.
The team used data from the Gemini North telescope in Hawaii, one half of the Gemini International Observatory operated by NSF’s NOIRLab and funded by the U.S. National Science Foundation, to analyze a supermassive binary black hole located in the elliptical galaxy B2 0402 is located. +379 . It is the only supermassive black hole binary to be resolved in such detail that both objects can be seen separately, and it holds the record for the closest distance ever directly measured: just 24 light-years. Although this close separation suggests a strong merger, later studies showed that the pair has been stuck at this distance for more than three billion years, leading to the question: What is the reason for this delay?
To better understand the dynamics of this system and its halted merger, the team turned to archival data from the Gemini North Multi-Object Spectrograph (GMOS), which allowed them to determine the speed of stars near black holes. “Thanks to the excellent sensitivity of GMOS, we were able to map the increasing speeds of stars as we approached the center of the galaxy,” explains Roger Romani, professor of physics at Stanford University and co-author of the paper. “This allowed us to determine the total mass of the black holes there.”
The team calculates that the mass of the double star is a whopping 28 billion times that of the Sun, making it the heaviest double star hole ever measured. This measurement not only provides valuable context on the formation of the binary star system and the history of its parent galaxy, but also supports the long-standing theory that the mass of a supermassive binary black hole plays a key role in preventing a possible merger.
“The Gemini International Observatory data archive contains an untapped gold mine of scientific discoveries,” said Martin Still, NSF program director for Gemini International Observatory. “The mass measurements of this extremely supermassive binary black hole are a striking example of the potential impact of new research exploring this rich archive.”
When will they merge?
Understanding how this binary star system formed can help predict if and when it will merge, and a handful of clues suggest that the pair was formed by the merger of multiple galaxies. First, B2 0402+379 is a “fossil cluster,” meaning that it is the result of stars and gas from an entire galaxy cluster merging into a single massive galaxy. Furthermore, the presence of two supermassive black holes, along with their large total mass, suggests that they are the result of the merger of several smaller black holes from several galaxies.
After a galactic merger, supermassive black holes do not collide head-on. Instead, they begin to pass each other as they settle into a confined orbit. With each pass, black holes transfer energy to the stars around them. As they lose energy, the pair is drawn closer together until they are light years apart. At this point, gravitational radiation takes over and they merge. This process has been observed directly in pairs of stellar-mass black holes – the first recorded case occurred in 2015 with the discovery of gravitational waves – but never in a binary star system of the supermassive variety.
Thanks to new insights into the system’s extremely large mass, the team concluded that an exceptionally large number of stars would have been needed to slow the orbit of the binary star system enough for them to be so close to each other. The black holes appear to have expelled almost all of the material from their surroundings, leaving the core of the galaxy starless and gasless. With no material left to slow the pair’s orbit, their final-stage merger has stalled.
“Typically, galaxies with pairs of lighter black holes appear to have enough stars and mass to bring them together quickly,” explains Romani. “Because this pair is so heavy, it took a lot of stars and gas to do the work. But the binary star cleared the central galaxy of said matter, leaving it stagnant and accessible to our research.
It remains to be seen whether the pair will overcome their stalemate and eventually merge within millions of years, or whether they will remain in orbital limbo forever. If they merged, the resulting gravitational waves would be hundreds of millions of times stronger than those produced by stellar-mass black hole mergers. It is possible that the pair could overcome this final distance through another galaxy merger, which would inject additional material into the system or possibly a third black hole to slow the pair’s orbit enough to merge. However, since B2 0402+379 is a fossil cluster, another galactic merger is unlikely.
“We are eager to conduct follow-up studies of the core of B2 0402+379, in which we will analyze the amount of gas present,” says Tirth Surti, a Stanford graduate student and lead author of the paper. “This should give us more information about whether supermassive black holes can merge or remain stranded as binary stars.”
Grades
- Although there is evidence that supermassive black holes are only a few light-years away, none appear to have managed to overcome this final distance. The question of whether such an event is possible is called the final parsec problem and has been a topic of debate among astronomers for decades.
- Previous observations have been made of galaxies containing two supermassive black holes, but in these cases they are thousands of light-years away, too far away to be in a bound orbit like the binary star found in B2 0402+379.
- There are other black hole-fed sources with possibly smaller distances, although these were inferred from indirect observations and are therefore better classified as candidate binaries.
- This theory was first proposed by Begelman and other authors in 1980 and has long been based on decades of observations of the centers of galaxies.
REFERENCE
The central kinematics and mass of the black hole of 4C+37.11