ALMA Astrometry of the Nuclear Star Cluster

Atacama Large Millimeter/Submillimeter Array (ALMA) is promising to be a powerful tool for precision astrometry of the area around Sagittarius A* (Sgr A*) because of it’s high angular resolution, high sensitivity, and wide field of view. We have observed t the area around Sgr A* including the Nuclear Star Cluster at 230 GHz with ALMA in October 2017. The angular resolution is ~0.03”.  We determined the positions relative to Sgr A* of ~70 compact objects in the area with the accuracy of ~0.001”. We also analyzed the similar ALMA archival data obtained in June 2019.  We determined the relative positions in these objects and derived the proper motions relative to Sgr A* by comparing these positions.  The positions and proper motions are almost consistent with those by previous infrared observations. The derived proper motions are roughly described with both clockwise and counterclockwise rotations around Sgr A*. The rotation velocities are reproduced by Kepler orbits bounded around Sgr A* without a few exceptions. Moreover, they include co-moving clusters for example IRS13E and IRS13N. Therefore, they demonstrate that ALMA is a powerful tool for precision astrometry of the area.