Supermassive black hole at the center of the Milky Way: From the first infrared observations to the Nobel Prize in Physics

The 2020 Nobel Prize in Physicswas awarded jointly to Roger Penrose (½) for the theoreticalinvestigation of the formation of black holes and Reinhard Genzel (½)and Andrea Ghez (¼) for the experimental confirmation of thesupermassive compact objectin the center of the Milky Way. In my talk, I will focus on thecontribution of Genzel &  Ghez and their associated collaboratorsin terms of discoveringhigh-velocity stars around the compact radio source Sgr A*. Themonitoring of these stars in the near-infrared domain during the last30 years, including measuring their proper motions and radialvelocities, has brought a firm evidence of the presence of compactmass of 4 million Solar masses. In combination with variability ofSgr A* in the near-infrared and X-ray domain, and the recentdetection of orbiting components on microarcsecond scales from SgrA*, the compact mass is consistent with being a supermassive blackhole beyond any reasonabledoubt. I will also point outsome future directions of the investigation of Sgr A*. While the massof Sgr A* is well constrained, its spin and potentially also chargeare uncertain andwill potentiallybe determinedwith future observations of fainter stars and plasma components onthe scale of 10 to 100 gravitational radii.