SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month^*

We report on 75 mas resolution, near-IR imaging spectroscopy within the central 30 lt-days of the Galactic center, taken with the new adaptive optics-assisted integral-field spectrometer SINFONI on the ESO VLT. To a limiting magnitude of K ~ 16, 9 of 10 stars in the central 04, and 13 of 17 stars out to 07 from the central black hole have spectral properties of B0-B9 main-sequence stars. Based on the 2.1127 μm He I line width, all brighter early-type stars have normal rotation velocities, similar to solar neighborhood stars. We combine the new radial velocities with SHARP/NACO astrometry to derive improved three-dimensional stellar orbits for six of these "S stars" in the central 05. Their orientations in space appear random. Their orbital planes are not co-aligned with those of the two disks of massive young stars 1''-10'' from Sgr A*. We can thus exclude the hypothesis that the S stars as a group inhabit the inner regions of these disks. They also cannot have been located/formed in these disks and then migrated inward within their planes. From the combination of their normal rotation and random orbital orientations, we conclude that the S stars were most likely brought into the central light-month by strong individual scattering events. The updated estimate of distance to the Galactic center from the S2 orbit fit is R₀ = 7.62 ± 0.32 kpc, resulting in a central mass value of (3.61 ± 0.32) × 10⁶ M_☉. We happened to catch two smaller flaring events from Sgr A* during our spectral observations. The 1.7-2.45 μm spectral energy distributions of these flares are fit by a featureless, "red" power law of spectral index α' = -4 ± 1 (S_ν ~ ν). The observed spectral slope is in good agreement with synchrotron models in which the infrared emission comes from accelerated, nonthermal, high-energy electrons in a radiatively inefficient accretion flow in the central R ~ 10R_S region.