The supermassive black hole at the centre of the Milky Way galaxy is being circled by a bubble of hot gas, according to recent analysis by the Event Horizon Telescope Collaboration. The gas is whipping around the black hole at about 30% the speed of light.
The bubble’s superlative speed means it completes an orbit around Sagittarius A*, a black hole containing the mass of more than 4 million Suns, every 70 minutes. A description of the gas bubble is published in Astronomy & Astrophysics.
“What is really new and interesting is that such flares were so far only clearly present in X-ray and infrared observations of Sagittarius A*,” said Maciek Wielgus, an astrophysicist at the Max Planck Institute for Radio Astronomy, in an ALMA Observatory release. “Here we see for the first time a very strong indication that orbiting hot spots are also present in radio observations.”
The bubble was discovered in data taken by the massive ALMA radio telescope in 2017. The ALMA observations came shortly after NASA’s Chandra Space Telescope detected an X-ray flare from the centre of the Milky Way.
Such X-ray flares are associated with hot spots in the galactic core — areas where the superheated gas circling the galaxy’s centre is particularly superheated. The timing was serendipitous, but showed how radio telescopes like ALMA can detect the same behaviour as X-ray observatories.
Since then, the EHT Collaboration released the first-ever image of a black hole (in 2019) and, this year, the first image of our own black hole. ALMA, high in the Chilean desert, was involved in imaging both black holes.
Supermassive black holes have such intense gravitational pulls that not even photons of light can escape them. That’s why images of black holes show bright orange haloes — superheated gas and dust around the holes — with black splotches at the centre. Supermassive black holes have long been thought to reside at the centres of galaxies; the images taken by the EHT are the best evidence yet that the theory is correct.
The research team believes the gas bubble circling Sagittarius A* is a product of how the superheated gas around the black hole interacts with the hole’s magnetic field. The observations also “give us a clue about the geometry of the process,” according to Monika Mościbrodzka, an astrophysicist at Radboud University in the Netherlands, in the same release.
The bubble wasn’t directly observed; rather, the researchers saw signs of it in how the brightness and angle of light from Sagittarius A* changed during the bubble’s orbit.
The team hopes that the EHT will be able to directly observe the gas, potentially even closer to the black hole and its extreme physics. Whether we’ll ever glean information from beyond the event horizon is tough to say, but for now we can settle for closer.
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