Improving constraints on the extended mass distribution in the Galactic center with stellar orbits

Vol. 692
5. Galactic structure, stellar clusters and populations

Improving constraints on the extended mass distribution in the Galactic center with stellar orbits

The recent “image” of the supermassive black hole of our Galaxy from the Event Horizon Telescope and previous, associated Nobel Prizes have spawned great interest in the very center of our Milky Way. In the present study, the interferometric GRAVITY instrument has been used to precisely monitor the orbits of stars around the central source in the Milky Way, Sgr A*, in order to derive the gravitational potential of its surroundings. The ensuing astrometry and spectroscopy of four stars over the past eight years near their pericenters (on the order of 120 AU and at speeds of tenths of the speed of light) led to a highly significant (10s) detection of their orbital in-plane precession, which provides an impressive doubling of the statistical significance compared to measurements only four years ago. The new data yield further insight into the extended mass distribution near Sgr A*, which, in principle, could consist of various materials such as a stellar cusp made of old stars and stellar remnants, along with a potential dark matter spike. However, such an extended distribution would lead to a retrograde precession, which is at odds with the observations. Accordingly, the derived limits are consistent with a zero mass and leave no room for an extended distribution and, in particular, no dark matter enhancement in excess of 1200 Mo within the orbit of the trademark S2 star. While the Sgr A* hole is black, the future for such dynamical studies is bright: Comparing the current precession rate of 12 arcmin per orbit with S2’s period of 16 years, large-scale precession on degree scales, and thus indisputable evidence of relativistic effects can be witnessed on a human timescale.