They are the biggest black holes in the known Universe, billions of times more massive than our Sun, but little is known about how these monsters form and grow so big. New telescopes and techniques are giving us a new way of looking at these giants.
Halfway between the belly of Delphinus the Dolphin and the hind hoof of Pegasus the flying horse, a pristine pinwheel tumbles through space. For billions of years, the flocculent spiral arms of galaxy UCG 11700 have wheeled in peace, undisturbed by the collisions and mergers that have deformed so many other galaxies.
But while a spiral galaxy like UCG 11700 is pleasing to look at, something monstrous lurks in its midst. At the heart of this beautiful cosmic Catherine wheel is one of the most mysterious objects in the Universe – a supermassive black hole.
While standard black holes start at around four times the mass of our Sun, their enormous relatives are millions, and sometimes billions, of times as massive.
Scientists believe almost every large galaxy has a supermassive black hole at its heart. Except nobody knows how they got there.
This is where galaxy UCG 11700 could prove useful.
“The ideal galaxies for my study are the most beautiful, perfect spirals you could possibly think of,” says Becky Smethurst, a junior research fellow at the University of Oxford who studies supermassive black holes. “The prettiest galaxies are the ones that could help us solve the mystery of how these black holes grow.”
Studying something that, by its nature is so dense that even light cannot escape from its centre, makes learning about it difficult. But new techniques that look for the effects supermassive black holes have on the interstellar objects around them, and even at the ripples they create in the fabric of space and time, are providing new clues.
There’s little secret about how conventional – if they can be called that – black holes form and grow. A dying star runs out of fuel, explodes in a supernova, collapses in on itself, and becomes so dense that even light cannot escape its intense gravity. The idea of black holes has been around for a century and is predicted in Albert Einstein’s Theory of General Relativity.
In popular culture, black holes are perfectly dark and endlessly hungry. They barrel through the Universe sucking up everything in their path, growing larger and more voracious as they do. Mystery solved, one might think – supermassive black holes are simply the hungriest and oldest of their kind.
In reality, however, black holes don’t live up to their monstrous reputation. They are surprisingly inefficient at accreting (physicists’ jargon for “sucking up”) surrounding material, even in a dense galactic core. In fact, collapsed stars grow so slowly, they couldn’t possibly become supermassive just by absorbing new material.
“Let’s assume the very first stars formed black holes around 200 million years after the Big Bang,” Smethurst says. “After they’ve collapsed, you’ve then got about thirteen and a half billion years to grow your black hole to billions of times the mass of the Sun. That’s too short a time to get it that big just with accretion.”
Even more mystifying, supermassive black holes already existed when the Universe was still in its relative infancy. Far-distant quasars, some of the brightest objects in the night sky, are actually ancient supermassive black holes that have set the cores of dying galaxies on fire. Some of these giants have been present at least since the Universe was a mere 670 million years old – at a time when some of the oldest known galaxies were forming.
Black holes are the most effective, efficient engines in the Universe.
Marta Volonteri, Researcher, l’Institut d’Astrophysique de Paris
While the heart of a black hole remains unknowable to external observers, supermassive black holes can shine more brightly than an entire galaxy of stars, and can even produce “burps” of ultraviolet radiation as they consume material around them.
Black holes have a spherical boundary known as an “event horizon”. Within this sphere, light, energy, and matter are inescapably trapped. Space and time fold in on themselves, and the physical laws that describe how most of our Universe works break down. But, just outside the event horizon, a spinning black hole can whip nearby material into a spinning, superheated disc. Reaching temperatures higher than 10 million C, the accretion disc in a quasar releases blindingly bright radiation across the electromagnetic spectrum.
“Black holes are the most effective, efficient engines in the Universe,” says Marta Volonteri, a black hole researcher at l’Institut d’Astrophysique de Paris. “They transform mass into energy with up to 40% efficiency. If you think of anything we burn with carbon, or chemical energy, or even what happens in stars, it’s just a small, small fraction of what a black hole produces.”
Supermassive black holes interest scientists for more than just their energy efficiency. Their formation and evolution are clearly connected to the development of galaxies, and to the even larger story of our entire Universe’s history and structure. Solving the mystery of these cosmic giants would represent a significant step in scientists’ ongoing effort to understand why things are the way they are.
Energy release is one of many ways black holes divulge their secrets. When black holes merge or collide with slightly less dense objects like neutron stars, the events create ripples in spacetime called gravitational waves. These waves move across the cosmos at the speed of light and were first detected on Earth in 2015. Since then huge observatories like the Laser Interferometer Gravitational-wave Observatories (Ligo) in the United States, and the Virgo facility near Pisa, Italy have been picking up the ripples created by these collisions.
But while these observatories use instruments several kilometres in size, they can only detect waves from relatively modest-sized black holes.
“Ligo has detected mergers up to only about 150 solar masses,” says Nadine Neumayer, who leads the Galactic Nuclei research group at the Max Planck Institute for Astronomy. “There is a gap in data about what people call ‘intermediate-mass black holes’ of about 10,000 solar masses or so. And those actually could be the seeds for supermassive black holes.”
Intermediate-mass black holes, she says, could have formed in the very early Universe from collapsing giant gas clouds or runaway collisions of stars. In the cramped environment of the young Universe, successive collisions between these medium-sized black holes, combined with a rapid accretion of the surrounding material, could have accelerated their growth to supermassive scales.
Still, the intermediate-mass black-hole seed theory has problems. The early, small Universe was also very hot. Gas clouds would have been bathed in radiation, possibly giving them too much energy to collapse in on themselves. And even in a dense cosmos, the laws of physics still limit the maximum rate at which black holes can absorb matter.
Volonteri says that every current explanation for supermassive black holes has “bottlenecks and drawbacks” that prevent scientists from converging on a definitive answer.
“The theories involving what we call ‘dynamical processes’, meaning you form a black hole from many, many stars rather than just one are possible, but these processes need to happen in very specific conditions,” she says. “There are also theories about ‘primordial black holes’, which could have come into existence and begun growing before there were stars. But this is completely unknown territory. We don’t have any observational proof to test this principle.”
She says she loves the physics of dynamical processes, but acknowledges that it is very difficult for the theory to credibly predict anything growing larger than about 1,000 solar masses.
“When you consider quasars that already had a billion solar masses when the Universe was a billion years old, it’s very hard to get to those numbers,” she says. She believes the true story of how supermassive black holes came to be is yet to be told. “The more we dig, the more we find there are issues with what we thought we had understood. We’re missing something fundamental.”
The current generation of observational instruments have started to fill in the gaps. Virgo, Ligo and similar observatories are providing increasingly in-depth “demographic information” about the size, age and locations of the Universe’s black hole population.
But to fill in this kind of data on supermassive black holes, researchers are going to need even bigger detectors.
In the 2030s, Nasa and the European Space Agency (Esa) will launch the ambitious Laser Interferometer Space Antenna (Lisa), which comprises three satellites flying in a triangle with sides 2.5 million kilometres long. This array will work on similar principles to Ligo and Virgo, but its massive scale will allow it to detect gravitational waves from very large black holes beyond the reach of existing technology.
There are already hints that gravitational waves created by supermassive black holes are washing over us. At the start of 2021, astronomers announced they had detected small discrepancies in the pulses of radiation coming from 45 pulsars – compact stars that release beams of light at regular intervals. Although the results have yet to be confirmed, the researchers suggest this could be due to a “gravitational wave background” that is likely created by supermassive black hole mergers.
But there are other more direct ways of seeing black holes. The Event Horizons Telescope recently captured the first photographic images of black holes, coaxing these mysterious objects out of the shadows and revealing more about their nature and their effects of their gravity and magnetism on the galaxies they inhabit. Astrophysicists can also track the movement of stars in close orbits around black holes in the galactic core, extrapolating information about the massive objects at the centre.
Most observations of this type rely on ground-based telescopes that use a technology called “adaptive optics”. Observers analyse a bright star (or human-generated laser beam) to measure atmospheric distortions that would otherwise reduce image quality. Computer controlled signals correct for these distortions through tiny adjustments to the physical shape of the telescope’s mirror. The results are precise observations of the hearts of galaxies billions of light years away, and a wealth of data on their supermassive black holes.
Neumayer was one of the first scientists to use adaptive optics to study galactic cores.
“It was just mind-blowing that you could have better resolution from the Earth than from the Hubble Space Telescope,” she says. “I worked on measuring specific black hole masses. There is a tight correlation: the more mass a galaxy has, the more massive its central supermassive black hole is. Somehow these objects grow in step.”
Despite this correlation, there’s no clear evidence that massive galaxies create massive black holes, or vice versa. They are connected, but the nature of that connection remains a mystery.
One piece of the explanation might involve collisions between galaxies. Most of the observable Universe’s two trillion galaxies are accelerating away from one another, but many collisions occur, creating opportunities for two very large central black holes to merge into something even bigger. Some scientists believe this could be how the truly monstrous supermassive black holes are formed.
When comparatively tiny stellar black holes collide, they release huge amounts of energy for a fraction of a second, producing a flash so bright that it briefly outshines everything else in the sky. If we were to see a similar event involving supermassive black holes, it would be one of the most cataclysmic events to be detected in the night’s sky.
But, while scientists suspect mergers between supermassive black holes do occur, they may be made less common due to another problematic aspect of black hole dynamics.
Black holes on a collision course spin around one another with increasing speed as they draw closer. But very large black holes reach a point at about one parsec (3.26 light years) apart where their orbital velocity starts to balance out gravitational attraction. The decay of their orbits would happen so slowly that the actual merger could not happen within the current age of the Universe.
Nevertheless, physicists do believe such mergers happen, which requires new theories about how to overcome the so-called “final parsec problem“. Some additional force or energy is required to bring the orbiting black holes together.
The Universe is littered with galaxies that are thought to have formed through mergers, including our own Milky Way, suggesting they do occur. When galaxies do collide their original spiral structure gets destroyed as their stars, gas clouds, dark matter, and black holes interact. Even glancing blows between galaxies can mess with their structures, which makes them easy to spot.
But it means supermassive black holes at the centre of perfect pinwheel galaxies like UCG 11700 can’t be explained by collisions. Their structures suggest they have never brushed up against another galaxy.
“I pick out very rare galaxies that have been left alone their entire lives, that have been very, very isolated in the Universe,” Becky Smethurst says. “In those we can be sure that the black hole in the middle has never grown by merging with something else.”
It means they must have formed in another way.
Smethurst works backwards to determine how big these black holes would have had to have been at the beginning in order to grow to their current size. Her best models suggest a black hole that formed early in the Universe with 1,000 to 10,0000 solar masses could possibly do the trick — numbers that jibe with Neumayer’s theories about intermediate-sized “seed” black holes. But black holes of that size wouldn’t likely come from collapsing stars.
Astrophysicists are also exploring the possibility that supermassive black holes form directly from dark matter, the mysterious material that holds galaxies together. But dark matter, which is a theoretical type of particle that interacts with gravity, but is invisible to light and electromagnetism, is itself poorly understood. Combining the mysteries of black holes and dark matter only makes the physics more challenging.
“There’s still a lot we don’t know,” says Smethurst. “I think it would be arrogant of us to conclude that the only way to form a black hole is by supernova, because we don’t know that for sure. Maybe the explanation is something else entirely we just haven’t thought of yet. I am looking forward to the day the Universe surprises us with the answer. I think it will be a good day for science.”
More advanced observational instruments are on the way. This autumn, Nasa plans to launch the James Webb Space Telescope (though there is currently a push to rename the instrument due to the homophobic policies enforced by its eponymous Nasa director), whose unprecedented size and sensory capabilities will make it a valuable tool in the study of supermassive black holes. The Lisa mission, when it is launched, will also give scientists new ways of looking at supermassive black holes through their gravitational waves.
Other scientists are creating increasingly detailed maps of the locations, movements, shapes, and sizes of millions of galaxies, which feeds into the research of both observers and theoreticians.
“The pace of work is just phenomenal,” says Smethurst. “We have a hundred years’ worth of research on black holes. But compared to the 14 billion years that the Universe has been around, that’s not enough to crack all the mysteries. I set out to answer one question and end up with five more. And that’s okay with me.”
Neumayer agrees with Smethurst that the most exciting discoveries about black holes will likely relate to questions that nobody yet has asked.
“It’s been an amazing century of technical developments that make these discoveries possible,” she says. “We have a lot of known problems we want to solve. But we will also see new things we cannot even imagine. And I think that’s amazing.”