Implication of Spin Constraints by the Event Horizon Telescope on Stellar Orbits in the Galactic Center

Abstract

The center of the Milky Way hosts the closest supermassive black hole, Sgr A*. Decades of near-infrared observations of our Galactic Center have shown the presence of a small population of stars (the so-called S-star cluster) orbiting Sgr A*, which were recently reported to be arranged into two orthogonal disks. In this case, the timescale for the Lense–Thirring precession of S stars should be longer than their age, implying a low spin for Sgr A*. In contrast, the recent results by the Event Horizon Telescope favor a highly spinning Sgr A*, which seems to suggest that the S stars could not be arranged in disks. Alternatively, the spin of Sgr A* must be small, suggesting that the models for its observed image are incomplete.