Hubble Space Telescope/NICMOS Observations of Massive Stellar Clusters near the Galactic Center^*

We report Hubble Space Telescope (HST) Near-Infrared Camera and Multiobject Spectrometer (NICMOS) observations of the Arches and Quintuplet clusters, two extraordinary young clusters near the Galactic center. For the first time, we have identified main-sequence stars in the Galactic center with initial masses well below 10 M_☉. We present the first determination of the initial mass function (IMF) for any population in the Galactic center, finding an IMF slope that is significantly more positive (Γ ≈ -0.65) than the average for young clusters elsewhere in the Galaxy (Γ ≈ -1.4). The apparent turnoffs in the color-magnitude diagrams suggest cluster ages that are consistent with the ages implied by the mixture of spectral types in the clusters; we find τ_age ~ 2 ± 1 Myr for the Arches cluster and τ_age ~ 4 ± 1 Myr for the Quintuplet. We estimate total cluster masses by adding the masses of observed stars down to the 50% completeness limit and then extrapolating down to a lower mass cutoff of 1 M_☉. Using this method, we find ≳10⁴ M_☉ for the total mass of the Arches cluster. Such a determination for the Quintuplet cluster is complicated by the double-valued mass-magnitude relationship for clusters with ages ≳3 Myr. We find a lower limit of 6300 M_☉ for the total cluster mass and suggest a best estimate of twice this value, which accounts for the outlying members of the cluster. Both clusters have masses that place them as the two most massive young clusters in the Galaxy.