The sky at night can be deceptive. At a glance, it appears serene, but in those vast expanses of space, swirling vortices of gas particles are producing high-energy radiation, so energetic that we can measure it from Earth. The enormous volumes of energy, produced within the cores of some galaxies, is in the form of radio waves, similar to those used for broadcasting purposes. These core regions give out unimaginably large amounts of energy every second. If we could learn their secrets, our load-shedding and fossil-fuel woes could end.
For decades, the cores of galaxies have been a source of bemusement and curiosity for those who study them. We measure the intensity of their radio emission and image their extensive structures using radio maps but we can’t yet fully explain how they convert gas into energy. Black holes in these regions may contain secrets that could inspire a new way of generating energy on Earth.
Black holes are compact celestial objects which exert a gravitational pull so strong, that not even light can escape them, hence their name. It also means that we can’t “see” them and can only detect their presence by examining how objects such as stars orbit around them.
Radio waves measured from the centre of our home galaxy, the Milky Way, and the motion of stars in this region led to the discovery of a black hole millions of times the mass of our sun. We call these colossal entities supermassive black holes.
A black hole forms when a star collapses gravitationally under its own weight: the nuclear reactions that once powered the star and put outward pressure on the star to counteract gravity, cease to burn. Within galactic cores, supermassive black holes are the engines that produce the tremendous power that we can even measure from Earth, by way of an “accretion disc”. Accretion discs form when gas particles come together in a rotating, disc-shaped cloud around the black hole.
Supermassive black holes are the central engines of active galaxies that generate strong radio and X-ray waves of light. All active galaxies contain these compact cores. The term “active galactic nuclei” (AGN) stands for the compact central regions which produce all of this energy. Although a supermassive black hole is a pre-requisite for a galaxy to host AGN, not all galaxies are active – the Milky Way, for example, has a super-massive black hole at its centre, but is not active. The main distinguishing factor is that AGN emit energy in frequencies spanning from the radio to X-ray emissions.
The powering mechanisms behind AGN are a highly contentious and complex field because we’re just not sure how to accurately measure properties across space. Although there is some universal agreement on their general structure (an active core is surrounded by a disc of rotating gas that fuels it), we don’t really understand how their components are formed and evolve.
The best explanation, so far, is that gravity funnels gas into orbit on the black hole’s surface, so that it spins around the black hole like a frisbee with a hole in the middle.
Friction between individual gas particles heats up the gas and separates hydrogen atoms into their basic components, electrons and protons. The result is the formation of a plasma, which is a superheated gas that contains these charged particles.
When plasma particles and magnetic fields interact, streaming jets form, like arms jutting out of the disk. These jets produce the strong radio waves we measure and ascribe to the AGN. But this is only our best guess. We accept it because there has yet to be a decent alternative, but to find more meaningful clues, we make further observations.
My search for answers began last year under the guidance of two experienced astrophysicists, Dr Kim McAlpine and Professor Matt Jarvis from the University of Western Cape. We compiled a dataset containing a group of almost 20 000 galaxies identified by a collection of radio telescopes in Socorro, New Mexico in the United States. Together, the telescopes are called the Very Large Array (VLA) and are similar to our own MeerKAT project.
We’re trying to find out what influences the power of AGN, so we are investigating the environment around the nucleus, which supplies fuel to the black hole in the form of gas. The distance of the AGN host galaxy from its neighbours may very well affect the amount of energy it generates.
Perhaps the gravity galaxies exert on each other when they pass by or collide with one another affects the energy output? Maybe how hot or cold the gas is in the galaxy’s immediate environment determines its energy generation? We do not know.
To study this, we use data from Wide-Field Infrared Survey Explorer (WISE), NASA’s orbiting satellite that measures infrared emission from the sky. We also use a ground-based telescope at the Apache Point Observatory, New Mexico. Called the Sloane Digital Sky Survey, it scans the sky for sources that emit visible light waves. This information allows us to determine the various properties of the gas surrounding an AGN and find out whether there is a link between an AGN’s power and whether it resides in a more populated or remote region in space.
Despite the fact that astrophysicists have produced hundreds of journal publications on AGN feeding and powering, the process still remains largely opaque. The quest to continue studying these mysterious objects is motivated, in part, by human curiosity and also perhaps the idea that if we could mimic processes in nature that generate energy, we would not have to rely so heavily on unsustainable methods like fossil fuels.
Perhaps the “holy grail” to solve our energy problems lies in one of the universe’s great mysteries: how active galactic nuclei are powered.
Sthabile Kolwa attends the University of Western Cape.