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For the first time, astronomers have detected evidence for black hole-scale magnetic fields near the black hole at the centre of the Milky Way galaxy. Their finding helps validate the theory of how black holes grow.
Were these magnetic fields not there, “a lot theoretical astrophysics would have to go back to the drawing board,” says Avery Broderick (left), an Assistant Professor in the Department of Physics and Astronomy at the University of Waterloo and jointly appointed at the Perimeter Institute.
The discovery, published in the journal Science, moves the understanding of how black holes grow from the realm of theoretical speculation to the territory of empirical fact.
Broderick was part of a collaboration that discovered high levels of polarization in the radio emission from Sagittarius A*, the bright radio source believed to be the astronomical manifestation of the 4.5-million-solar-mass black hole at the centre of the Milky Way.
The discovery was made using the Event Horizon Telescope, a linked array of millimeter-wavelength telescopes that spans the globe and is set to take the highest-resolution images in the history of astronomy. When trained on the black hole at the centre of our galaxy, Sagittarius A*, it can see the structural details in the accretion flow that surrounds the black hole horizon. Broderick’s work involves analyzing and interpreting the data that comes from the array.
The current observations are from only three of the sites in the global EHT array, comparable to having just a handful of pixels of the larger picture that will eventually be produced. Nevertheless, these few pixels are already writing the preface to the coming revolution in our understanding of black holes. Researchers are able to begin the process of putting our best current ideas of what is happening near the black hole to the test.
Black holes are regions of spacetime where gravity is so strong that “what goes into them does not come out,” Broderick says.
As the name implies, black holes are intrinsically dark, with no light or matter able to escape once they have passed the threshold of no return known as the event horizon. But as the black holes feast on the surrounding gas and stars, their accretion disks can shine and produce extraordinary energy. They can even outshine their host galaxies in some instances.
Compared to some black holes, Sagittarius A* is a much more anemic and fails to outshine a single bright star despite its comparatively enormous mass. But the data from the Event Horizon Telescope opens a window on the inner workings of how material spirals towards black holes, finally disappearing across their event horizons, and growing into what Broderick calls “monsters lurking in the night.”
It will also shed light on the reverse process, whereby some black holes are capable of launching outflows of energy and material at nearly the speed of light, extending the black hole’s impact to intergalactic scales.
Decades of theoretical work, including enormous computer simulations, painted a picture of how strong magnetic fields near the black hole horizon contribute to the processes that enable a black hole to grow. But now, with the data from the Event Horizon Telescope, scientists can begin to see how these processes work in practice.
To explain how a black hole to grows, the theoretical models had to deal with angular momentum. The Earth doesn’t fall into the sun because of the conservation of angular momentum. So for a black hole to grow, some physical process must remove angular momentum from the gas that is falling in, presumably transferring it outward to material that ultimately escapes gravity’s tug.
That’s where magnetic fields come in. The dancing magnetic fields can loop and twist and pull the material that is falling in and couple it material that is flowing away from the black hole. This tugging and pulling generates the viscosity that is needed to mediate the flow of angular momentum and permit black holes to grow. Without these magnetic fields, the models predict, accretion would halt and jets would falter.
Although this was described in the models, what was needed was observational evidence. The large polarization measurements provide that proof.
The radio emission in Sagittarius A* is generated by high-energy electrons zipping around magnetic field lines. This produces highly polarized emission on microscopic scales, tied to the local orientation of the magnetic field, so the polarization traces the structure of the magnetic fields. Detecting high polarization on the size of the black hole horizon at Sagittarius A* does two things: It verifies that these magnetic fields are there and they must be ordered. Second, it provides a measurement of the typical size of these magnetic structures.
There is much more to come.
Taking images of the accretion disk around the 4.5-million-solar-mass black hole at the centre of our own galaxy – something that is extremely compact and with an event horizon that is smaller than the orbit of Mercury – is a feat akin to trying to image a grapefruit on the moon. But the Event Horizon Telescope array will be able to accomplish that.
There are now enough telescopes in the array, in principle, to make images in the next couple of years,” Broderick adds.
Those images will enable astrophysicists to transform our understanding of how black holes grow, how they interact with their surroundings, and even the nature of gravity.
By studying the details of the cosmic “traffic jam” caused by gas as it rushes headlong towards the black hole, researchers will be able to check if Albert Einstein’s theory of general relativity, one of the pillars of modern physics, holds up in the extreme gravity conditions around black holes.
It will help to validate a lot of the ideas about black holes.
All of a sudden the stories that we tell our students are grounded in a reality,” says Broderick.