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At four million times heavier than the Sun, the black hole at the centre of the Milky Way might sound impressively large, but it is far from the biggest. The only black hole we have an image of (at time of writing anyway) is the supermassive one at the centre of the M87 galaxy that we saw back in Chapter 10. It’s the galaxy at the very centre of a super-cluster of galaxies that includes the Milky Way; if you could keep zooming out from Earth to see the big picture, then at the centre of everything there would be M87’s supermassive black hole. The age-old phrase ‘all roads lead to Rome’ should really be ‘all roads lead to black holes’.
M87’s black hole is 6.5 billion times more massive than the Sun. It makes the Milky Way’s black hole look like a lightweight. But even that is still not the biggest. The heavyweight crown goes to TON 618, which is 66 billion times the mass of the Sun. It is so big that astronomers had to invent a new word for it – an ultramassive black hole. But as we’ve already heard, black holes aren’t just endless hoovers: they don’t suck. They’re limited to how fast they can grow because of radiation pressure (the Eddington limit).
We know that the majority of black holes don’t accrete at the Eddington limit, at their maximum rate, because of radiation pressure pushing material back out. When we look at the distribution of the growth rate of active supermassive black holes, we find that on average they take on material at about 10 per cent of their maximum possible rate. So can black holes just endlessly grow at that rate with no limit to the mass they can reach? Technically, the theoretical maximum would be a black hole containing all the mass in the entire Universe. That number is a bit difficult to estimate, but it’s somewhere around the 1060 kilogram mark. That’s a 1 with sixty zeros after it; a novemdecillion to use its technical term.
I feel it’s my civic duty to point out here that a black hole with a mass of novemdecillion kilograms is highly unlikely. Space itself is expanding, taking galaxies, and therefore the matter in the Universe, ever further apart. This will reduce the amount of material available for black holes to eventually accrete; once they’ve exhausted the supply their galaxy can give them, then that’s it. It also reduces the likelihood of any galaxy mergers as the Universe ages, and therefore there’ll be fewer supermassive black hole mergers to go with them. A merger can at most double the black hole’s mass, so it’s a very efficient growth process, but the occurrence is getting rarer with every passing day.
The growth of supermassive black holes is very reliant on accretion; the process of taking in more matter through all those collisions between gas particles in the accretion disk, to slowly reduce their energy and bring them closer to the black hole. If you disrupt that process in any way then that’s it for the black hole – it can no longer grow any bigger unless it gets lucky with a merger. So are there any processes that can disrupt this accretion process? And if so, what’s the maximum mass of a black hole then?
The first people to try and put an estimate on this maximum mass were Indian astrophysicist Priya Natarajan (now a professor at Yale University) and Argentinian astrophysicist Ezequiel Treister (now a professor at the University of Chile) in 2008. They argued that a limit to a black hole’s mass naturally occurs because of the co-evolution of supermassive black holes with their galaxies. With continued growth of the black hole comes continued feedback, which eventually blows away the accretion disk around the black hole. They estimated that this would mean that a black hole could only reach up to 10 billion times the mass of the Sun.
But in 2015, British astrophysicist Andrew King entered the chat. King did his PhD at the University of Cambridge during the heyday of black hole research in the 1970s, working with Stephen Hawking. He’s now a professor at the University of Leicester and in 2014 was awarded the coveted Eddington medal from the Royal Astronomical Society for his work on black holes and general relativity. King pointed out a quirk of gravity around a black hole that allowed him to estimate the maximum mass a black hole could grow to via accretion as 50 billion times the mass of the Sun (but that could be pushed to a whopping 270 billion times the mass of the Sun if the black hole was spinning in the same direction as its galaxy).
It’s all to do with the many different ‘spheres’ you can draw around a black hole. We’ve heard about the event horizon already; what we define as the size of the black hole because it’s that point of no return where we no longer receive any light. But there’s a few more distances from the singularity that get thrown about in casual astrophysical conversations. There’s the ergosphere – the region around a black hole you can extract energy from (perhaps obvious to those who speak Greek, ergon means work), for example through a gravitational slingshot like spacecraft use in our own Solar System to steal away a bit of energy from something much more massive than them.
Then there’s the photon sphere – the region around the black hole where gravity is so strong that any photons (particles of light) travelling at the speed of light would have their path curved so much that they’d travel in a perfect circle. Theoretically it would be possible to see the back of your own head at the photon sphere (if you hadn’t been spaghettified111 first). This sphere is just beyond the event horizon, about 1.5 times larger.
But crucial to the process of accretion is the sphere called the Innermost Stable Circular Orbit, or ISCO.112 In Newton’s version of gravity that we all learn at school, all perfectly circular orbits, no matter the distance, are very stable. That means if an object on a circular orbit is perturbed slightly, let’s imagine a rather large asteroid impacts with another asteroid on a perfectly circular orbit, then the orbit can adapt and become slightly elliptical (remember, a circle is just a very special case of an ellipse where the aphelion equals the perihelion). So, that would mean that even if something was somehow orbiting the Sun in a perfect circle just above its surface, and was nudged somehow, it could still adapt its orbit to an elliptical shape to continue orbiting the Sun.
In Einstein’s general relativity, though, that’s not the case. As you get closer to an object, and in particular a very compact object like a black hole, there is a point where if you nudge something on a circular orbit, it can’t correct and it ends up spiralling inwards to the black hole. This is the ISCO, and it sits at three times larger than the event horizon (although if the black hole is spinning that can shrink slightly). Anything that has mass (i.e. not photons of light) cannot form a stable orbit around a black hole any closer in than the ISCO. Usually this marks the rough edge of the accretion disk around the black hole. Just like with the event horizon, the ISCO is related to how massive the black hole is. As the black hole grows in mass, the ISCO gets pushed further out.
There’s one more circle around a black hole to mention: the self-gravitational radius. Now, this also depends on the object that’s creeping too close, along with the mass of the black hole, but essentially it marks the point at which the pull of gravity holding the object together (self-gravity) is stronger than the pull from the black hole. This is a really crucial point because it explains why we even have galaxies of stars surrounding supermassive black holes in the first place; beyond this radius gas in a galaxy is attracted to itself, more than to the supermassive black hole in the centre, and so the gas can get denser before collapsing in on itself to form stars. If this wasn’t the case then we wouldn’t be here at all, our atoms would all just be part of one giant accretion disk around the supermassive black hole of the Milky Way.
What Andrew King pointed out in 2015 was that, as supermassive black holes grow ever bigger (via accretion and co-evolution with their galaxies), the ISCO gets pushed beyond the self-gravitational radius. What that means is that any gas particles in the accretion disk, no matter how many collisions they have, will never lose enough energy to reduce their orbit enough that they will reach the ISCO and spiral inwards to grow the mass of the black hole. Instead, the pull of gravity from all the other particles in the accretion disk will always be stronger than the pull of gravity from the black hole.
In fact, at this point, an accretion disk isn’t even going to form. Instead, if you have an influx of gas, its self-gravity will hold it together and it will loop around the black hole relatively unscathed; akin to the G2 gas cloud’s trajectory around the Milky Way’s black hole. Unless material is on a direct trajectory with the black hole at the bullseye (which is rare given how big space is and how relatively small black holes are, even the ultramassive ones) it won’t become part of the black hole. This lack of accretion disk means that we also won’t be able to spot an ultramassive black hole, as there’ll be no luminous matter around it lighting up like a Christmas tree.
This is what makes TON 618 so interesting; with an ultramassive black hole of 66 billion times the mass of the Sun, it lies above King’s estimate of the maximum limit for a non-spinning black hole (which was 50 billion times the mass of the Sun). Most black holes are spinning (angular momentum, you just can’t shake it), so that’s not unsurprising, but it does mean it could be nearing its maximum mass.
TON 618’s peculiarity was noted well before it was recognised for what it was. It was spotted on photographic plates taken in 1957 at the Tonantzintla Observatory in Mexico by Mexican astronomers Braulio Iriarte and Enrique Chavira, who noted that it looked violet in colour. It was finally identified as a quasar in 1970 by a group of Italian astronomers conducting a radio survey of the sky in Bologna. By 1976, French astronomer Marie-Helene Ulrich had managed to use the McDonald Observatory in Texas to calculate its distance (the light left it 10.8 billion years ago) and work out it was one of the most luminous quasars ever known (the more luminous the quasar, i.e. the accretion disk, the more massive the black hole).
It’s by using the measured speed of the gas in the accretion disk that the estimate of TON 618’s mass is derived: 66 billion times the mass of the Sun. I know I keep repeating that number but it really is huge. It’s more than the total mass of stars in the entire Milky Way (estimated at 64 billion times the mass of the Sun). Its event horizon is 1,300 times larger than the Earth–Sun distance (forty times the distance of Neptune from the Sun). It’s a behemoth: massive enough to spark fear into the hearts of us puny humans, and yet unless you launched yourself out of a canon, Zazel-style,113 directly into TON 618, there’s absolutely nothing to fear from it. Almost as if the Universe finally put a stopper in the sink plug hole.
It’s fascinating to consider the implications of this maximum mass a black hole can grow to via accretion, and that TON 618 has even come near to it. It means we could be approaching the epoch of the Universe, where black holes reach their limit. As black holes reach this limit and quit growing, or glowing, quasars across the Universe will begin to wink out. If this had happened just a few million years earlier, we as humans may never have even known supermassive black holes existed. It could even be the case that there are some black holes that have reached ultramassive status, but we don’t know they’re there. Without some sort of light from the accretion disk, we cannot hope to measure the mass of the black holes in the centres of distant galaxies. Perhaps ultramassive black holes are already hiding among us.
I am both equally amazed and at the same time slightly disappointed that we are right now living through the epoch of the Universe, where some black holes might never grow any bigger. It’s as if these big, scary, mysterious, infuriatingly interesting black holes are past their heyday, over the hill, senescent. I don’t know whether to laugh or to cry at the thought. And yet they might just have the last laugh.