Nuclear Young Star Cluster in our Galactic Center
The formation of the young stars near the Supermassive Black Hole in our galactic center has been a mystery for a long time. In my thesis, I will present how we use the improved astrometric measurements, combined with well-measured radial velocities to to study their dynamical structures and constrain their formation scenario.
First, we present an improved relative astrometry for stars within the central half parsec of our Galactic Center based on data obtained with the 10 m W. M. Keck Observatory from 1995 to 2017. The new methods used to improve the astrometric precision and accuracy include correcting for local astrometric distortions, applying a magnitude dependent additive error, and more carefully removing instances of stellar confusion. Additionally, we adopt jackknife methods to calculate velocity and acceleration uncertainties. The resulting median proper motion uncertainty is 0.05 mas/yr for our complete sample of 1184 stars in the central 1000(0.4 pc). We have detected 24 accelerating sources, 2.6 times more than the number of previously published accelerating sources, which extend out to 400(0.16 pc) from the black hole. Based on S0-2’s orbit, our new astrometric analysis has reduced the systematic error of the supermassive black hole (SMBH) by a factor of 2. The linear drift in our astrometric reference frame is also reduced in the North-South direction by a factor of 4. We also find the first potential astrometric binary candidate S0-27 in the Galactic center. These astrometric improvements provide a foundation for future studies of the origin and dynamics of the young stars around the SMBH, the structure and dynamics of the old nuclear star cluster, the SMBH’s properties derived from orbits, and tests of General Relativity (GR) in a strong gravitational field.
Second, we measure the 3D kinematic structures of the young stars within the central 0.5 parsec of our Galactic center using the 10m Keck telescope with a time baseline from 1995 to 2015. Using these high-precision astrometric measurements of positions and proper motions, plus radial velocities from literature, we are able to constrain the orbital parameters for each young star. Our results clearly show the significant clockwise stellar disk as has been proposed before but with a better disk membership measurement. Besides the previously identified clockwise stellar disk, we have found another almost edge-on stellar disk, but we cannot rule out the possibility that it is a stream associated with the IRS 13 group. Based on their disk membership, the young stars are divided into different dynamical subgroups, and the disk properties, e.g. eccentricity and semi-major axis, are compared between these three dynamical subgroups. In order to understand the asymmetric 2D density profile, we simulated a single-age, single-metallicity star cluster with observational disk properties to reproduce the spatial distribution for each dynamical subgroup, and then applied a differential extinction map. It is the first time that stellar populations are simulated and compared to observational density profile, which provides us with a deeper understanding of their dynamical structures and helps us distinguish different star formation scenarios around the supermassive black hole at our Galactic center.