Free Astronomy Magazine November-December 2022

30 NOVEMBER-DECEMBER 2022 ASTRO PUBLISHING by ESO - Bárbara Ferreira U sing the Atacama Large Mil- limeter/submillimeter Array (ALMA), astronomers have spotted signs of a ‘hot spot’ orbiting Sagittarius A*, the black hole at the centre of our galaxy. The finding helps us better understand the enig- matic and dynamic environment of our supermassive black hole. “We think we’re looking at a hot bubble of gas zipping around Sagit- tarius A* on an orbit similar in size to that of the planet Mercury, but making a full loop in just around 70 minutes. This requires a mind blow- ing velocity of about 30% of the speed of light!” says Maciek Wielgus of the Max Planck Institute for Radio Astronomy in Bonn, Germany, who led the study published in Astron- omy & Astrophysics . The observations were made with ALMA in the Chilean Andes — a radio telescope co-owned by the Eu- ropean Southern Observatory (ESO) — during a campaign by the Event Horizon Telescope (EHT) Collabora- tion to image black holes. In April A still image of the supermassive black hole Sagittarius A*, as seen by the Event Horizon Collabo- ration (EHT), with an artist’s illustra- tion indicating where the modelling of the ALMA data predicts the hot spot to be and its orbit around the black hole. [EHT Collaboration, ESO/ M. Kornmesser (Ack.: M. Wielgus)] 2017 the EHT linked together eight existing radio telescopes worldwide, including ALMA, re- sulting in the recently released first ever image of Sagittarius A*. To calibrate the EHT data, Wielgus and his colleagues, who are mem- bers of the EHT Collaboration, used ALMA data recorded simultaneously with the EHT observations of Sagit- tarius A*. To the team’s surprise, there were more clues to the nature of the black hole hidden in the ALMA-only measurements. By chance, some of the observations were done shortly after a burst or flare of X-ray energy was emitted from the centre of our galaxy, which Hot gas bubble swirling around Sagittarius A*