Event Horizon Telescope Illuminates Black Hole Growth

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The international EHT research team, including Perimeter’s Avery Broderick, has measured for the first time the magnetic fields that contribute to black hole growth.

Finally, the stories we tell about black holes are being validated.

For the first time, astronomers have detected evidence of black-hole-scale magnetic fields near the black hole at the centre of the Milky Way galaxy.

Were these magnetic fields not there, “a lot of theoretical astrophysics would have to go back to the drawing board,” says Avery Broderick, an Associate Faculty member at Perimeter Institute jointly appointed at the University of Waterloo.

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 says.

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 millimetre-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 come 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 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 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 EHT, scientists can begin to see how these processes work in practice.

To explain how a black hole grows, the theoretical models had to deal with angular momentum. The Earth doesn’t fall into the sun, thanks to 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 with 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. First, it verifies that magnetic fields are there and that 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 Sagittarius A*, which has 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 EHT array should 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,” Broderick says.

− Rose Simone


black holes at lowest order are exactly


what you think stuff goes in and it


doesn't come out and that's as good a


definition of a black hole as any and


many of the questions about how exactly


they grow how do they accrete stuff how


does that stuff shine all critically


depend upon uncertain physics near the


horizon and right now we just


extrapolate and for the first time we're


gonna be able to check the first time


we're gonna be able to validate or even


more exciting maybe invalidate some of


these ideas about how black holes


operate in practice and that's gonna


happen because we have this brand-new


resource the event horizon telescope so


broad international collaboration to


build a global array of telescopes that


has the capability to the for the first


time in the history of astronomy


resolved a horizon of a black hole make


a picture in which the horizon shows up


as a shadow on all the surrounding


luminous material and to do this this


necessarily has to be the highest


resolution instrument in the history of



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Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.