For 90 years, astronomers have known there is something powerful lurking in the crowded heart of our galaxy, the Milky Way. Now, at last, they have seen it.
On Thursday, an international team of researchers reported they had succeeded in piercing the dusty veil that conceals the galactic centre from view and producing the first direct image of the giant black hole that resides there.
Their achievement – a scientific tour de force involving co-ordinated observations made at multiple locations followed by years of data processing and analysis – lays bare the most intriguing and exotic object in humanity’s corner of the universe. And it sets the stage for years of measurements to reveal the details of a black hole that is almost four million times more massive than our sun and the complex ways in which it interacts with and shapes its immediate environment.
“It’s the supermassive black hole next door,” said Avery Broderick, a researcher at the University of Waterloo and the Perimeter Institute for Theoretical Physics.
Dr. Broderick is a long-time member of the Event Horizon Telescope collaboration, a project that harnesses radio observatories around the globe for the sole purpose of revealing what a black hole really looks like.
The goal may seem like a contradiction in terms. A black hole, by definition, is an object so dense that not even light can escape its powerful gravitational pull. But black holes also draw in hot, ionized gas torn from nearby stars or clouds of interstellar material. The glowing gas creates a backdrop against which the dark boundary of the black hole – its event horizon – can be seen in silhouette.
It is this kind of image that Dr. Broderick and his colleagues have now created: a glowing, gaseous doughnut with a dark void at the centre.
The shape of the image “is the telltale sign of the black hole,” said Feryal Ozel, a project member and astronomer at the University of Arizona, during a news conference in Washington, D.C.
The images marks the second time the project has made astronomical history.
In 2019, the collaboration garnered worldwide attention when it produced the first direct image of a black hole. On that occasion the target was a far more distant object – a behemoth with 6.5 billion times the sun’s mass located in the galaxy M87. That image proved possible simply because that black hole is so large – close to the theoretical limit for how big one can get. A beam of light takes an entire week to circle its impressive girth.
In contrast, the black hole at the centre of the Milky Way is 1,500 times smaller but, by cosmic coincidence, about 2,000 times closer. That means it looks roughly the same size from Earth’s point of view, which makes it equally accessible to the Event Horizon Telescope. But while the black hole in M87 can be observed from across 55 million light-years of empty intergalactic space, the Milky Way’s black hole is shrouded by layers of dust in the densest part of our galaxy, about 26,000 light-years away. The contrast is similar to peering at a large building across an open lake and seeing it more easily than spotting a birdhouse hidden in the woods nearby.
Also, because the Milky Way’s black hole is smaller, it changes on time scales that make observing it a challenge. The electrified gas swirling around it can flare up suddenly as magnetic field lines rupture and reconnect, releasing vast amounts of energy and causing minute-to-minute variations that confound the imaging process.
The black hole “burbled and gurgled as we looked at it,” Dr. Ozel said.
Only after surmounting the twin Everests of obscuration and variability was the team prepared to release its image of the black hole, five years after the observations were first made, Dr. Broderick said. The results were simultaneously published in a series of six scientific papers in the Astrophysical Journal Letters.
At first blush, the image of the M87 black hole and that of the Milky Way look remarkably similar in a side-by-side comparison. Team members said this offers confidence that what they are seeing is physical reality and not an artifact of the imaging process. The shapes and dimensions of each black hole are products of the same physical rules, spelled out in the mathematics of Einstein’s general theory of relativity.
“We know now that what we see in both cases is the heart of the black hole – the point of no return,” Dr. Ozel said.
In each instance, the resulting image is not an optical photograph but a map of the radio energy emitted by ionized gas around the black holes. The images were produced by combining observations from multiple radio telescopes located around the world, including at the South Pole. Through a process known as interferometry, astronomers combined data from each of the observatories to build a picture as sharp as though it were produced by a single radio dish almost as large as Earth.
The Milky Way photo marks a high point for the Event Horizon Telescope as well as a far longer odyssey by astronomers to understand the energy source at our galaxy’s centre.
That odyssey began in the early 1930s when Karl Jansky, a U.S. physicist working for Bell Telephone Laboratories, was given the task of ferreting out natural sources of radio interference. He discovered that one of those sources tracked with the sky. After ruling out the sun, he concluded the source was astronomical, located in the constellation Sagittarius, which coincides with the centre of the Milky Way.
Radio waves can penetrate the dust clouds that block the galaxy’s centre from view. As technology matured, radio astronomers became aware that there were multiple sources of energy at the galactic core, including one particularly strong and compact source dubbed Sagittarius A* (pronounced “Sagittarius A-star”).
Starting in the 1990s, astronomers including Andrea Ghez at UCLA were able to use infrared observations to track the motions of large, luminous stars in the vicinity Sagittarius A*. The observations allowed Dr. Ghez and her colleagues to confirm that the object is massive enough to be a black hole – a result for which she was named co-winner of the Nobel prize in physics in 2020.
Now the Event Horizon Telescope has added a new chapter to the story, providing a direct view of Sagittarius A* that can be connected to other observations and used to test theories of gravity and other astrophysical processes.
“It’s really exciting,” said Daryl Haggard, an astronomer at McGill University and part of a team that conducted parallel X-ray satellite observations of the black hole while the Event Horizon Telescope was gathering its data for the image.
“For many of us, Sagittarius A* is the target that got us into this work,” she added. “To have an exciting result to share about our own galaxy’s black hole is sort of a holy grail. It’s a culminating moment.”
The Globe and Mail, May 12, 2022