The Alma radio telescope finally made it possible to visualize the cold part of the accretion disk surrounding the central black hole of 4 million solar masses of the Milky Way, Sagittarius A *.
The Event Horizon Telescope collaboration (EHT) brilliantly imaged M87 * despite a bad surprise in the case of Sgr A *. But this is only a postponement, and we should soon have images of the supermassive black hole of the Milky Way. Even the quality and resolution of images already taken with M87 * should quickly improve. Meanwhile, one of the radio telescopes that has contributed to the success of the EHT, the Atacama Large Millimeter / submillimeter Array (Alma), already helps us to clarify our knowledge of the environment around Sgr A *.
As an article published in the Nature journal explains, a team of American astrophysicists managed to use Alma and its gaze in the field of millimeter radio waves to finally reveal a relatively cold disc around a black hole of the galaxy, to probe regions which are only one hundredth of a light-year (that is to say about 1,000 astronomical units ) of its horizon of events. The temperature is relatively cold compared to that of about 10 million Kelvin, which is that of plasma the size of a few light-years encompassing Sgr A *. This disk, which is also largely a plasma, is still about 10,000 Kelvin.
Sgr A *, a laboratory for the accretion of black holes
We already had observations in the field of X-rays radiated by such hot plasmas occupying a few light-years at most around Sgr A *, but never again an image clearly showing the presence of the disc today revealed by Alma. The instrument also makes it possible to show and measure movements associated with this disc and to evaluate their density and mass. We thus find that it contains the equivalent of about a tenth of the mass of Jupiter, which makes 100,000 to 1,000,000 atoms of hydrogen and helium per cubic centimeter.
“We were the first to be imaged that elusive disc and study its rotation,” said astrophysicist Murchikova Elena, a member of the Institute for Advanced Study in Princeton, New Jersey, and lead author of the article. The researcher adds, “We are also probing the accretion process in a supermassive black hole. It’s important because it’s closest to us. Despite this, we still do not understand how this accretion works. We hope these new observations with Alma will help us reveal some of the secrets of the black hole.”