SG A* feeds on hot material pushed off of massive stars at the galactic center. That gas, drawn toward SG A* by its gravitational pull, flows into a surrounding disk of glowing material, called an accretion disk. The disk, the stars and an outer bubble of X-ray light “are like an ecosystem,” says astrophysicist Daryl Haggard of McGill University in Montreal and a member of the ELT collaboration. “They’re completely tied together.” That accretion disk is where the action is — as the gas moves within immensely strong magnetic fields — so astronomers want to know more about how the disk works. Like the majority of supermassive black holes, SG A* is quiet and faint (SN: 6/5/19). The black hole eats only a few morsels fed to it by its accretion disk. Still, “it’s always been a bit of a puzzle why it’s so, so faint,” says astrophysicist Meg Urey of Yale University, who is not part of the ELT collaboration. M87’s black hole, in comparison, is a monster gorging on nearby material and shooting out enormous, powerful jets (SN: 11/10/21). But that doesn’t mean SG A* isn’t producing light. Astrophysicists have seen its region feebly glowing in radio waves, jittering in infrared and burping in X-rays. In fact, the accretion disk around SG A* seems to constantly flicker and simmer. This variability, the constant flickering, is like a froth on top of ocean waves, Mark off says. “And so we’re seeing this froth that is coming up from all this activity, and we’re trying to understand the waves underneath the froth.” The big question, she adds, has been if astronomers would be able to see something changing in those waves with ELT. In the new work, they’ve seen hints of those changes below the froth, but the full analysis is still ongoing. By combining about 3.5 petabytes of data, or the equivalent of about 100 million TikTok videos, captured in April 2017, researchers could begin to piece together the picture. To tease out an image from the initial massive jumble of data, the ELT team needed years of work, complicated computer simulations and observations of various types of light from other telescopes. Those “multiwavelength” data from the other telescopes were crucial to assembling the image. “By looking at these things simultaneously and all together, we’re able to come up with a complete picture,” says theorist Gibwa Muscle of the University of Amsterdam. SG A*’s variability, the constant simmering, complicated the analysis because the black hole changes on timescales of just a few minutes, changing as the researchers were imaging it. “It was like trying to take a clear picture of a running child at night,” astronomer José L. Gómez of Instituto de Astrofísica de Andalucía in Granada, Spain, said at a news conference announcing the result. M87 was easier to analyze because it changed over the course of weeks. Ultimately, a better understanding of what is happening in the disk so close to SG A* could help scientists learn how many other similar supermassive black holes work. The new ELT observations also confirm the mass of SG A* at 4 million times that of the sun. If the black hole replaced our sun, the shadow ELT imaged would sit within Mercury’s orbit. The researchers also used the image of SG A* to put general relativity to the test (SN: 2/3/21). Einstein’s steadfast theory of gravity passed: The size of the shadow matched the predictions of general relativity. By testing the theory in extreme conditions — like those around black holes — scientists hope to pinpoint any hidden weaknesses.
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