HST/NICMOS Observations of Massive Stellar Clusters Near the Galactic Center1

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 which 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 which are consistent with the ages implied by the mixture of spectral types in the clusters; we find τage ∼ 2±1 Myr for the Arches cluster, and τage ∼ 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 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555. University of California, Los Angeles, Division of Astronomy, Department of Physics & Astronomy, Los Angeles, CA, 90095-1562; [email protected], [email protected], [email protected], [email protected] Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Korea Advanced Institute of Science and Technology, Department of Physics, Space Science Laboratory, Daejon, 305-701, Korea JPL 171-113, 4800 Oak Grove Dr., Pasadena, CA 91109; [email protected]