THE FIRST LASER GUIDE STAR ADAPTIVE OPTICS OBSERVATIONS OF T HE GALACTIC CENTER: SGR A*’S INFRARED COLOR AND THE EXTENDED RED EMISSION IN ITS VICINITY

We present the first Laser Guide Star Adaptive Optics (LGS-AO ) observations of the Galactic center. LGSAO has dramatically improved the quality and robustness wit h hich high angular resolution infrared images of the Galactic center can be obtained with the W. M. Keck II 10 -meter telescope. Specifically, Strehl ratios of 0.7 and 0.3 at L’[3.8μm] and K’[2.1μm], respectively, are achieved in these LGS-AO images; thes e are at least a factor of two higher and a factor of four to five more stable ag ainst atmospheric fluctuations than the Strehl ratios delivered thus far with the Keck Natural Guide Star AO system on the Galactic center. Furthermore, these observations are the first that cover a large area (76 ×76) surrounding the central black hole at diffractionlimited resolution for an 8-10 meter class telescope. Durin g our observations, the infrared counterpart to the central supermassive black hole, Sgr A*-IR, showed signific ant infrared intensity variations, with observed L’ magnitudes ranging from 12.6 to 14.5 mag and a decrease in fl ux density of a factor of two over an 8 minute interval. The faintest end of our L’ detections, 1.3 m Jy (dereddened), is the lowest level of emission yet observed for this source by a factor of 3. No significant varia tion in the location of SgrA*-IR is detected as a function of either wavelength or intensity. Previous claim s of such positional variations are easily attributable to a nearby (0.09 or 720 AU, projected), extended, very red source, which we suggest arises from a locally heated dust feature. Near a peak in its intensity, we obtaine d the first measurement of SgrA*-IR’s K’-L’ color; its K’-L’ of 3.0 ± 0.2 mag (observed) or 1.4 ± 0.2 (dereddened) corresponds to an intrinsic spectral inde x of α -0.5± 0.3 for Fν ∼ ν. This is significantly bluer than other recent infrared meas urements from the literature, which suggest α = -4± 1. Because our measurement was taken at a time when Sgr A* was ∼6 times brighter in the infrared than the other measurements, we posit that th e spectral index of the emission arising from the vicinity of our Galaxy’s central black hole may depend on the str ngth of the flare, with stronger flares giving rise to a higher fraction of high energy electrons in the emit ting region. Subject headings: black hole physics – Galaxy:center — infrared:stars – techn iques:high angular resolution