Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?

We present new observations and analysis of G2 – the intriguing red emission-line object which is quickly approaching the Galaxy’s central black hole. The observations were obtained with the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes and include spectroscopy (R ∼ 3600) centered on the Hydrogen Brγ line as well as K ′ (2.1 μm) and L (3.8 μm) imaging. Analysis of these observations shows the Br-γ line emission has a positional offset from the L continuum. This offset is likely due to background source confusion at L. We therefore present the first orbital solution derived from Br-γ line astrometry, which when coupled with radial velocity measurements, results in a later time of closest approach (2014.21 ± 0.14), closer periastron (130 AU, 1900Rs), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L astrometry. The new orbit casts doubt on previous associations of G2 and a low surface brightness “tail.” It is shown that G2 has no K ′ counterpart down to K ∼20 mag. G2’s L continuum and the Br-γ line-emission is unresolved in almost all epochs; however it is marginally extended in our highest quality Br-γ data set from 2006 and exhibits a clear velocity gradient at that time. While the observations altogether suggest that G2 has a gaseous component which is tidally interacting with the central black hole, there is likely a central star providing the self-gravity necessary to sustain the compact nature of this object. Department of Physics and Astronomy, University of California, Los Angeles Dunlap Institute for Astronomy and Astrophysics, University of Toronto Institute for Astronomy, University of Hawaii NASA-Ames Research Center Division of Physics, Mathematics, and Astronomy, California Institute of Technology NSF Graduate Student Fellow, Grant DGE-0707424 Dunlap Fellow Leichtman-Levine Chair in Astrophysics Preston Fellow NSF Astronomy and Astrophysics Postdoctoral Fellow, Grant AST-1102791