The supermassive black hole at the center of the Milky Way plays host to a massive, young cluster that may have formed in one of the most inhospitable environments in the Galaxy. We present new measurements of the global properties of this cluster, i
ncluding the initial mass function (IMF), age, and cluster mass. These results are based on Keck laser-guide-star adaptive optics observations used to identify the young stars and measure their Kp-band luminosity function as presented in Do et al. 2013. A Bayesian inference methodology is developed to simultaneously fit the global properties of the cluster utilizing the observations and extensive simulations of synthetic star clusters. We find that the slope of the mass function for this cluster is alpha = 1.7 +/- 0.2, which is steeper than previously reported, but still flatter than the traditional Salpeter slope of 2.35. The age of the cluster is between 2.5-5.8 Myr with 95% confidence, which is a younger age than typically adopted but consistent within the uncertainties of past measurements. The exact age of the cluster is difficult to determine since our results show two distinct age solutions (3.9 Myr and 2.8 Myr) due to model degeneracies in the relative number of Wolf-Rayet and OB stars. The total cluster mass is between 14,000 - 37,000 msun above 1 msun and it is necessary to include multiple star systems in order to fit the observed luminosity function and the number of observed Wolf-Rayet stars. The new IMF slope measurement is now consistent with X-ray observations indicating a factor of 10 fewer X-ray emitting pre-main-sequence stars than expected when compared with a Salpeter IMF. The young cluster at the Galactic center is one of the few definitive examples of an IMF that deviates significantly from the near-universal IMFs found in the solar neighborhood.
We present new high angular resolution near-infrared spectroscopic observations of the nuclear star cluster surrounding the Milky Ways central supermassive black hole. Using the integral-field spectrograph OSIRIS on Keck II behind the laser-guide-sta
r adaptive optics system, this spectroscopic survey enables us to separate early-type (young, 4-6 Myr) and late-type (old, >1 Gyr) stars with a completeness of 50% down to K = 15.5 mag, which corresponds to ~10 msun for the early-type stars. This work increases the radial extent of reported OSIRIS/Keck measurements by more than a factor of 3 from 4 to 14 (0.16 pc to 0.56 pc), along the projected disk of young stars. For our analysis, we implement a new method of completeness correction using a combination of star-planting simulations and Bayesian inference. We assign probabilities for the spectral type of every source detected in deep imaging down to K = 15.5 mag using information from spectra, simulations, number counts, and the distribution of stars. The inferred radial surface-density profiles, $Sigma(R) propto R^{-Gamma}$, for the young stars and late-type giants are consistent with earlier results ($Gamma_{early} = 0.93 pm 0.09$, $Gamma_{late} = 0.16 pm 0.07$). The late-type surface-density profile is approximately flat out to the edge of the survey. While the late-type stellar luminosity function is consistent with the Galactic bulge, the completeness-corrected luminosity function of the early-type stars has significantly more young stars at faint magnitudes compared to previous surveys with similar depth. This luminosity function indicates that the corresponding mass function of the young stars is likely less top-heavy than that inferred from previous surveys.
We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within 3.5 (0.14 pc) of the supermassive black hole at the Galactic Center. Our proper motion measurements have uncertainties of onl
y 0.07 mas/yr (3 km/s), which is ~7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas/yr^2 (7 km/s/yr). Using these measurements, line-of-sight velocities from the literature, and 3D velocities for additional young stars in the central parsec, we constrain the true orbit of each individual star and directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one of the previously proposed disks of young stars using a method that is capable of detecting disks containing at least 7 stars. The detected disk contains 50% of the young stars, is inclined by ~115 deg from the plane of the sky, and is oriented at a position angle of ~100 deg East of North. Additionally, the on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger projected radii as propto r^-2. The disk has an out-of-the-disk velocity dispersion of 28 +/- 6 km/s, which corresponds to a half-opening angle of 7 +/- 2 deg, and several candidate disk members have eccentricities greater than 0.2. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple, thin circular disk.
