“The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time,” said astrophysicist Subrahmanyan Chandrasekhar. Chandrasekhar first demonstrated that at the end of a star’s life, if its remaining mass is greater than 1.4 times our sun’s mass, then its ultimate fate will be rather strange, collapsing under its own gravity to reach enormous density as a neutron star or black hole.
Today, we know that in the center of most galaxies lies a supermassive black hole millions or even billions times more massive than our Sun. Infalling gas and dust onto supermassive black holes heat up to extreme temperatures in an accretion disk, expelling excess energy as powerful jets and outflows, seen as quasars across the entire observable Universe. A new study led by astronomers at the Cosmic Dawn Center reviewed this process using new techniques—and the results may change how we think about the diets of these cosmic behemoths whose extreme gravitational field engulfs vast amounts of gas, dust, and doomed stars that wander into their grasp.
Mystery of Radiation 30 Times More Efficient than Nuclear Fusion
Physics tells us that this material tends to form a disk as it is drawn towards the black hole in a phenomenon called “accretion.” Now these accretion disks are some of the most uninviting, violent places in the known Universe, with velocities approaching the speed of light, and temperatures far in excess of the surface of our Sun. This heat produces radiation which we see as light, but the conversion of heat to light is so efficient—about 30 times more efficient than nuclear fusion—that physicists don’t quite understand how.
Varied Diets and Missing Accretion Disks
The dietary patterns of black holes have a wide range. Some, like the one in our own Galaxy, aren’t very hungry and don’t seem to have accretion disks. But we see other galaxies with ravenous hunger whose supermassive black holes have grown extremely hot accretion disks so bright that they outshine all of the stars in their galaxy.
EHT’s First Image of an Accretion Disk
Only recently have we obtained our first picture of an accretion disk from the Event Horizon Telescope, a worldwide network of radio telescopes. However, this accretion disk belongs to a very nearby galaxy. We cannot repeat this experiment with more distant galaxies as the disks are simply too small and so are unresolved, even by the largest telescopes.
The supermassive black hole in the center of the galaxy M87. The streaks show the polarized light from the electric field of the gas plummeting into the black hole. Credit: EHT Collab. et al. 2021 The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. Event Horizon Telescope Collaboration.
New Picture from Variations in the Disks’ Light
Fortunately another method of probing the size and structure of distant accretion disks seems promising. Although we cannot resolve the disks’ various components, we can study how its intensity varies in time. By studying the variations in the disks’ light we can piece together a picture of the accretion disks of even the most distant galaxies.
This is what DAWN Ph.D. Fellow John Weaver has done, looking into past observations of more than 9,000 galaxies with bright accretion disks—the so-called quasars—from the observational program “Sloan Digital Sky Survey.”
When the source is not resolved, the observed light from the accretion disk will be “contaminated” by light from the galaxy hosting the black hole. This unwanted light from the host galaxies has largely been ignored by previous studies. However, by using a new model for the variations in the quasar light, Weaver and his collaborator Keith Horne, professor of astronomy at the University of St Andrews, were able to separate the light of the accretion disk from that of the host galaxy.
In other words, the model allowed them to more directly see the light from the accretion disk around supermassive black holes, even in galaxies billions of lightyears away.
Cosmic Dust was Blocking the View
What Weaver and Horne found was that cosmic dust near the accretion disk was likely blocking their view. Using several different models of cosmic dust to account for, and remove, its obscuring effects, they were able to determine how hot the accretion disk is, both near the black hole and far from it at the edges of the disk.
Temperature Even Hotter than Predicted
This difference in temperature between the hot inner disk and the cold outer disk has been theoretically predicted. However, what Weaver and Horne found observationally was a very different picture of the temperature of the disk: the disks turned out to be even hotter near the black hole than predicted. These unexpected findings were published today in the Monthly Notices of The Royal Astronomical Society and suggest that our assumptions and theoretical models need to be revised—with consequences for our understanding of supermassive black holes altogether.
“The quasar brightness variations are bluer than expected, implying higher temperatures in the inner disk region. Horne wrote in an email to The Daily Galaxy. “This might mean that the quasar brightness variations are caused by changes in magnetic links connecting the disk and the black hole,” He explained. “The black hole then spins up and down as the magnetic links move energy and angular momentum back and forth between the disk and the hole.”
The Last Word –”Accretion Disk is Stealing Energy from the Black Hole”
“Accretion is more efficient than nuclear fusion in converting rest mass to energy. Fusing 4 hydrogen nuclei to make helium, as in the core of the Sun, converts less than 1% of the rest mass into energy (because the neutron’s rest mass is slightly less than the proton’s),” Horne explained in his email. “In an accretion disk feeding gas into a black hole, friction heats the gas enough to radiate away up to 42% of the rest mass, depending on the spin of the black hole, before the gas plunges into the hole.“
“Just to be super clear,” John Weaver wrote to The Daily Galaxy, “the idea that accretion disks are ‘hotter’ may be true, but this is largely an over-simplification. The findings show that the temperature structure of the discs may be steeper. In other words, the inner regions of the disc may be bluer (indeed hotter) than the outer regions (which may be redder — i.e. colder). But we are not finding that they are hotter in an absolute sense, but rather hotter in their inner regions relative to their outer regions (so that the temperature drops off more quickly).
“In that sense,” Weaver continues in his email, “we’re not claiming that the discs are necessarily more efficient energy producers than previously thought, so that hasn’t changed (from my thinking, at least!). The extreme efficiency of accretion processes (theoretically) is a well known result (see these slides) and so the physics are well understood, with the question being put: are real accretion discs actually behaving according to these (fairly simple) physics? It’s probably more complicated than that in the real world, but they are definitely efficient beasts!”
“What I can say is this,” Weaver concludes in his email: “The environment and physics of the accretion discs around supermassive black holes are not well understood. Long standing theoretical models are powerful, but perhaps missing all of the ingredients that nature provides. As such, these models make several assumptions about how the disc behaves. With this new method we are able to gather a more precise picture about accretion discs, which may indicate that this theoretical picture needs some additional ingredients.
“It seems,” Weaver concludes, “based on recent theoretical advancements, that our observations support the idea that the disc is stealing energy (angular momentum) from the black hole — increasing its temperature in the innermost region. If confirmed, this picture has significant implications for how we estimate the mass of the black hole, for example. Future facilities such as the Rubin Observatory will enable further investigations in this direction.`
Image at top of the page: artist’s impression of the quasar ULAS J1120+0641. Credit: ESO/M. Kornmesser