A new study, published in the Astrophysical Journal, has revealed that scientists have discovered a galaxy hosting the heftiest pair of supermassive black holes we've ever seen. Yes, you read that right!
According to the research, the two supermassive black holes lurking in a galaxy called B2 0402+379, which is about 750 million light-years away, have a combined mass of a whopping 28 billion times the mass of the Sun. Though there are several individual black holes that exceed the similar amount of mass, this pair represent the chonkiest black hole binary that have been found.
The scientists note that the two supermassive black holes exhibit several peculiar properties that are helping the astronomers to figure out what exactly happens to this gargantua when they come together.
The black holes' growth to supermassive sizes is a mysterious process which is largely unknown. The study revealed that small black holes form from the collapsed cores of massive stars that have burned through their atomic fuel and can no longer shine. These stellar mass black holes can grow by colliding with each other to produce objects too massive to form via the core collapse pathway.
Yes, there must be a method by which black holes can grow to supermassive proportions – millions to billions of times the mass of the Sun. While it seems reasonable to assume that, if small black holes can collide and merge, big ones should be able to as well, a series of hierarchical mergers that eventually produce the giant black holes that sit at the heart of every galaxy. But according to theory, there's a potential problem.
As per research, black holes in binaries grow closer together by shedding their orbital momentum, transferring it onto nearby gas stars that shoot off to parts unknown, and losing it in the form of gravitational waves. As the orbital distance reduces, so does the region of space to which they can shed their energy. At a distance of around a parsec, or 3.2 light-years, there is no longer enough room to shed further momentum, so the orbital decay stalls and stabilizes. This is known as the final parsec problem.
According to a team led by astrophysicist Tirth Surti of Stanford University, B2 0402+379 might just be an excellent example of the final parsec problem in action.
The researchers carefully studied the archival data, collected by the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope, and made a new analysis that calculates the properties and behavior of the two black holes embedded in the center of B2 0402+379.
The results showed the mass of the binary – 28 billion solar masses – and find that the galaxy itself is the 'fossil' of a cluster of galaxies. B2 0402+379 was once a group of galaxies hanging out; eventually they smooshed together and became B2 0402+379.
The binary supermassive black hole is what is left of the cluster of black holes that fell into the galactic center and remained there. Those two black holes are separated by a distance of 7.3 parsecs, or 24 light-years. That's not quite the final parsec, nor is it the tightest supermassive black hole binary we've seen.
However, the scientists analysed the orbital decay has stalled. Those black holes have been separated by that distance, in a stable orbit, for some 3 million years. This indicates that the high mass might have a role to play in the final parsec problem.
According to the researchers, the previous orbital decay of the binary ejected so many stars from their vicinity that there are simply now none left onto which they can transfer their orbital momentum. They are pretty well stuck, for now.
Astrophysicist Roger Romani of Stanford University said, "Normally it seems that galaxies with lighter black hole pairs have enough stars and mass to drive the two together quickly. Since this pair is so heavy it required lots of stars and gas to get the job done. But the binary has scoured the central galaxy of such matter, leaving it stalled and accessible for our study."
But what about the future? The researchers said that it could gain a kick in the right direction from the injection of material that would follow another galactic merger, sending a third supermassive black hole to the party; but all the galaxies that made up the initial cluster have already merged to form B2 0402+379, so that doesn't seem to be on the cards. However, there might be some material within the galaxy that could give the stalled union a helping hand.
Astrophysicist Tirth Surti of Stanford University said, "We're looking forward to follow-up investigations of B2 0402+379's core where we'll look at how much gas is present. This should give us more insight into whether the supermassive black holes can eventually merge or if they will stay stranded as a binary."
