Speaker
Description
Since 2016, the GRAVITY interferometer at ESO’s Very Large Telescope has provided astrometric data with unprecedented accuracy for the S-stars orbiting Sagittarius A*, providing a powerful means to probe the gravitational potential surrounding the supermassive black hole at the center of our Galaxy. Notably, we have detected the in-plane, prograde Schwarzschild precession of the orbit of the star S2 and the gravitational redshift of its spectral lines, as predicted by General Relativity.
In this presentation, I will discuss the implications of an extended mass distribution around Sagittarius A, primarily consisting of a dynamically relaxed cusp of old stars and stellar remnants, along with a potential dark matter spike. By analyzing S-stars data, we establish stringent upper limits on the enclosed mass within S2’s orbit—approximately 1200 solar masses within the central 10 milliparsecs of our Galaxy—assuming a smooth, spherically symmetric mass distribution. Our observational constraint aligns closely with theoretical predictions for a dynamically relaxed stellar cusp, leaving little room for a significant enhancement of dark matter density near Sagittarius A.
I will then discuss the impact of granularity in the mass distribution on the orbit of the star S2, assuming it consists of a cluster of equal-mass objects surrounding Sagittarius A*. We find that this granularity can induce significant deviations from the orbit in case of a smooth potential, leading to precession of the orbital plane and variations in the in-plane precession. Specifically, I will show that if a cluster of stellar-mass black holes resides within S2’s orbit with a total mass consistent with our derived upper limit, the astrometric residuals during S2’s next apocenter passage in 2026 may exceed the accuracy threshold of GRAVITY. This presents a unique opportunity to detect scattering effects on S2’s orbit caused by stellar-mass black holes, leveraging the exceptional precision of GRAVITY and its future upgrade, GRAVITY+.