Event Horizon Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way With the Inner Stellar Disk

Abstract

Observations of the black hole in the center of the Milky Way with the Event Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is smaller than the size of the black-hole shadow. This can be reconciled with the spectral properties of the source, if the accretion flow is seen at a relatively high inclination (50◦–60◦). Such an inclination makes the angular momentum of the flow, and perhaps of the black hole, nearly aligned with the angular momenta of the orbits of stars that lie within 3 from the black hole. We discuss the implications of such an alignment for the properties of the black hole and of its accretion flow. We argue that future Event Horizon Telescope observations will not only refine the inclination of Sgr A∗ but also measure precisely its orientation on the plane of the sky.