Proper Motions in the Center of the Galaxy

Abstract

We report the first measurements of stellar proper motions in the innermost core of the Galaxy. From high resolution near-infrared imaging over the last 4 years we have determined proper motions for 39 stars between 0.03 and 0.3 pc from the compact radio source SgrA*. Proper motion and radial velocity dispersions are in very good agreement indicating that the stellar velocity field on average is close to isotropic. We have detected significant changes in the structure of the innermost complex of stars in the immediate vicinity of SgrA*, suggesting stellar motions of >103 km/s within 0.01 pc of the compact radio source. Taking radial and proper motion data together we find a 2.45(±0.4)×106 M⊙ central dark mass within < 0.015 pc of SgrA*. The density of this central dark mass therefore is in excess of 1012 M⊙pc −3, implying that the central mass concentration is likely a single massive black hole.