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 (2006-2012) 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 Brγ 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 Brγ 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, 1600 R_s), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L' astrometry. It is shown that G2 has no K' counterpart down to K' ~ 20 mag. G2's L' continuum and the Brγ line emission appears unresolved in almost all epochs, which implies that the bulk of the emission resides in a compact region. The observations altogether suggest that while G2 has a gaseous component that 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.