Milky Way black hole revealed at last
The first picture of the glowing surroundings around Sagittarius A* – the black hole at the centre of the Milky Way – could offer clues to the inner workings of supermassive black holes. Will Gater explains
Astronomers working on the Event Horizon Telescope (EHT) have captured the first-ever image of the “shadow” – and glowing surroundings – of the supermassive black hole at the centre of the Milky Way. The achievement is another huge success for the EHT project, which in 2019 released a similar image of the black hole in the core of the galaxy Messier-87.
The hefty black hole within the heart of our galaxy – known as Sagittarius A* or Sgr A* – lies some 27,000 light-years away from Earth and had been scrutinized, albeit indirectly, by astronomers for decades. Based on the motions of stars zipping around a seemingly empty spot at the Milky Way’s centre, researchers had calculated that a body roughly four million times the mass of our Sun must sit there.
Other studies, including recent observations from the Hubble Space Telescope, had also detected features in the centre of our galaxy and beyond that were assumed to be the hallmarks of an immense, hidden black hole. But to take the image of Sgr A*, astronomers have had to combine the capabilities of several radio telescopes – including those belonging to the European Southern Observatory in Chile, as well as observatories in Europe, the US and the South Pole.
They did this using “very long baseline interferometry”, or VLBI, which effectively creates a single, vast array. It has allowed astronomers to obtain an image resolution of several tens of micro-arcseconds – equivalent to spotting an orange on the surface of the Moon. This resolution means that researchers can image and pick out the features of a supermassive black hole such as Sgr A*.
“This endeavour has required an unprecedented level of international collaboration and coordination,” says Ziri Younsi, an astrophysicist at University College London, who is one of more than 350 people in the EHT collaboration. “It has been exciting and on occasion nerve-wracking, but I’m delighted that we’ve reached this milestone and finally obtained an image of our own supermassive black hole.”
The resulting first EHT image of Sgr A* confirms that this gravitational leviathan, inferred for so long by scientists, is indeed the object predicted by astrophysical theories. The picture shows a glowing ring around a dark, inner area, which is the shadow of the black hole. “[It] denotes the boundary where light can no longer orbit the black hole multiple times without being eventually captured”, says Younsi.
Inside the shadow, albeit invisible in the new image, is the location of the event horizon – the mathematically defined “edge” of the black hole. In the case of Sgr A*, the event horizon is expected to measure somewhere between 12 to 24 million kilometres across, depending on how the black hole is spinning.
The doughnut-shaped swathe of light around the shadow, meanwhile, is thought to arise from a mix of two sources whose appearance has been smeared by gravitational lensing. The first is a maelstrom of photons whirling around close to Sgr A* – a phenomenon astrophysicists call the “photon ring” – while the second is a superheated disc of glowing material that likely encircles the black hole. EHT astronomers found that the measured size of the photon ring is consistent with the predictions of Einstein’s general theory of relativity.
Acquiring this first image of Sgr A* required more than just gathering and analysing the petabytes of data created by the EHT: researchers also had to contend with the dust and gas suspended throughout the Milky Way. “The interstellar medium in our galaxy is a major confounding factor in reconstructing images of Sgr A*,” says Younsi, who adds that it acts like a screen that scatters radiation emanating from close to the black hole. “Given the unknown structure and distribution of this screen, mitigating for its effects has proven to be challenging.”
Sgr A* itself was a tricky target to observe – weighing much less than the 6.5 billion solar-mass black hole that the EHT imaged in the centre of the galaxy Messier 87. As Younsi explains, the mass of the black hole “sets a characteristic timescale over which material around the black hole evolves, as well as a timescale over which light and information take to propagate”. In other words, features around Sgr A* change in just a matter of minutes, in contrast to the days- and weeks-long variations of the M87 black hole. “The source structure and light produced from it is rapidly changing and it is not easy to obtain a clear image,” Younsi adds.
The EHT picture of Sgr A* is based on observations that were made in April 2017, but astronomers hope that even more detailed views could be forthcoming. “We’ve recorded data a few times since then, including this year,” says Younsi. “These are with additional telescope sites so they promise to yield better images.” Such improvements could enable EHT astronomers to get a measure of the spin and mass of Sgr A*.
- The results are published in a series of 10 papers in Astrophysical Journal Letters.