Focus on First Sgr A* Results from the Event Horizon Telescope

EHT image of Sgr A* (top; Paper I). Ring-like images dominate the wide range of images obtained across multiple methods, however, variability and sparse visibility domain coverage make selection of a single image impossible (Paper III). The inset images represent different imaging solutions and their associated frequency (histograms).

We present the first image of the Galactic Center black hole, Sagittarius A*. Identified nearly 50 years ago as the nearest supermassive black hole candidate and among the most studied astrophysical objects, Sgr A* is the ultimate laboratory for black hole astrophysics. In six papers, the Event Horizon Telescope Collaboration presents observations, images, and analysis that spark new insights into accretion, outflow, and gravitational physics on scales not accessible through any other observation. These results are the culmination of a multi-year effort by the EHTC and a decades-long journey by the astronomy community to approach the event horizon through high-resolution imaging.

The Sgr A* image (Figure 1) reveals the same ring-like structure and shadow seen in the M87* black hole. The observed ring is the result of lensed emission with a diameter precisely predicted by general relativity using only the mass and distance of the black hole. By a cosmic coincidence, the observed Sgr A* angular diameter differs only slightly from that of M87*, which is 1500 times more massive and 2000 times more distant. Together, the Sgr A* and M87* results establish that lensed rings are universal features of black holes and that general relativity can consistently predict this across three orders of magnitude in black hole mass.

The lower mass and, hence, shorter dynamical scale of Sgr A* introduced significant complexity to the imaging and analysis of these EHT data (Papers II, III, and IV). The orbital period at the innermost stable circular orbital is 30 minutes or less, significantly shorter than the 12-hour observation we would ideally use to obtain an Earth-rotation synthesis image. Additionally, the image of Sgr A* is partially blurred by turbulent plasma along the line of sight to the Galactic Center, an effect which prevents resolution of the ring at longer wavelengths. The collaboration employed a number of traditional and novel techniques to assess and correct for these effects. Across all techniques, we demonstrate very strong evidence for a ring with a diameter of 52 microarcseconds.

Joining the EHT results with extensive multi-wavelength constraints provides a powerful probe of accretion and outflow physics (Paper V). In spite of the extraordinary depth and range of the theoretical simulations produced in our analyses, none of the models meet all of the observational constraints. Variability stands out as providing particularly challenging criteria to meet in future modeling efforts. Magnetically arrested disks at low inclination angle are among the best performing models.

Infrared astrometry of stellar orbits constrains the mass, distance, and, therefore, ring diameter of Sgr A* to approximately 1% accuracy, enabling precision explorations of gravitational physics (Paper VI). We find remarkably good consistency with a black hole described by the Kerr metric and including a genuine event horizon.

These observations were obtained by a global array of millimeter wavelength telescopes in April 2017 and analyzed by an international research team that now numbers over 300 people. Since those observations, the EHT has continued to observe and to grow in its capabilities through the addition of new stations, the widening of bandwidth, and the introduction of a higher frequency capability. Existing and new observations with the EHT of Sgr A* and M87* coupled with innovations in analysis and theoretical modeling will drive discovery in these unique laboratories for black hole physics.