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

Black holes are among the boldest and most profound predictions of Einstein’s theory of General Relativity [28, GR;]. Originally studied as a mathematical consequence of GR rather than as physically relevant objects, they are now believed to be generic and sometimes inevitable outcomes of gravitational collapse. In GR, the spacetime around astrophysical black holes is predicted to be uniquely described by the Kerr metric, which is entirely specified by the black hole’s mass and angular momentum or “spin”.

The first empirical evidence for their existence was through stellar-mass black holes, beginning with observations of X-ray binary orbits and culminating in the detection of gravitational waves from merging stellar-mass black holes. In parallel, the discovery that quasars are not stellar in nature but are rather extremely luminous, compact objects located in the centers of distant galaxies led to an intensive effort to identify and measure the supermassive black holes (SMBHs) energetically favored to power them. Observations now suggest that SMBHs not only lie at the center of nearly every galaxy but also may play a role in their evolution [79, 43, 70, see, e.g.,], though how exactly the ebbs and flows of black hole activity and growth proceed is a major outstanding question in the field.

With the advent of the Event Horizon Telescope (EHT), SMBHs can now be studied with direct imaging [30, 31, 32, 33, 34, 35, 36, 37, hereafter M87∗ Paper I-VIII]. The combination of an event horizon and strong lensing near black holes is predicted to produce distinctive gravitational signatures in their images [61, 6, 77, 64, 46, e.g.,]. In particular, simulated images of black holes typically have a central brightness depression encircled by a bright emission ring. The ring usually lies near the gravitationally lensed photon orbits that define the boundary of what we hereafter refer to as the black hole “shadow.” The shadow has an angular diameter dsh≈10GM/(c²D)≡10θg, where G is the gravitational constant, c is the speed of light, M is the black hole mass, and D is the black hole distance.

From the first realization that SMBHs could power bright radio cores in many galactic nuclei [78, and references therein], the search has been on to identify them. Within our own galaxy, the compact source Sgr A∗ has been intensely studied as a candidate SMBH since its discovery as a bright source of radio emission located near the Galactic Center. Decades of monitoring its proper motion, as well as of motions of individual stars in orbit around it, have revealed Sgr A∗ to be an extremely dense concentration of mass (M≈4×10⁶M⊙) that is located at and nearly motionless with respect to the dynamical center of the Galaxy (D≈8kpc), providing strong evidence that it is the nuclear SMBH in our Galaxy [59, 23, 104, e.g.,]. As the nearest supermassive black hole, Sgr A∗ provides a unique opportunity to directly image such an object, together with its accretion system, in the most common, quiescent state of SMBHs across the Universe. It also provides the chance to elucidate some of the drivers of observed cycles in accretion power and jet launching, via comparison with the more “active” galactic nucleus M87∗.

In this paper, we present the first EHT observations of Sgr A∗ and put them into context with our previous results on M87∗. In section II, we describe what was previously known about the physical properties of Sgr A∗ and compare them to M87∗. We then summarize our Sgr A∗ observations with the EHT and other observatories in section III and discuss its variability in section IV. In section V, we present the first EHT images of Sgr A∗ and analyze its event-horizon-scale structure. In section VI, we discuss the astrophysical interpretation of these results using an extensive suite of general relativistic magnetohydrodynamic (GRMHD) simulations, and in section VII we present the constraints that these results give for GR and black hole alternatives. We provide the overall conclusions and outlook in section VIII. The five companion papers of this series provide a more comprehensive discussion of all these topics [38, 39, 40, 41, 42, hereafter Papers II-VI].