First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way

We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 × 10$^{6}$ M $_{⊙}$, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 10$^{3}$–10$^{5}$ gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.. Authors: Akiyama, Kazunori [MIT; Natl. Astron. Observ. of Japan; Harvard U. (main); Harvard-Smithsonian Ctr. Astrophys.] (ORCID:0000000294754254); Alberdi, Antxon [Granada U., Theor. Phys. Astrophys.] (ORCID:0000000293711033); Alef, Walter [Bonn, Max Planck Inst., Radioastron.]; Algaba, Juan Carlos [Malaya U.] (ORCID:0000000169931696); Anantua, Richard [Harvard U. (main); Harvard-Smithsonian Ctr. Astrophys.; Texas U., San Antonio] (ORCID:0000000334577660). DOE Contract: AC02-07CH11359. Subjects: 79 ASTRONOMY AND ASTROPHYSICS