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

Abstract

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 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 ∼ >10 4 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 -2estimate of twice this value which accounts for the outlying members of the cluster. Both clusters have masses which place them as the two most massive clusters in the Galaxy.

High spatial resolution mid-infrared spectroscopy of NGC 5253: The stellar content of the embedded super-star cluster

“…Selman et al 1999). Towards the Arches cluster, Figer et al (1999a) find that there more than 10 stars with masses larger than 120 M . If true, the case of a low upper mass cutoff in NGC 5253 C2 would greatly differ with the high values found in these massive clusters.…”

“…There are only minor indications of clusters with top-heavy IMF's. For instance, Figer et al (1999a) find an index α ∼ 1.6 for the massive Arches and Quintuplet clusters situated near the Galactic Centre. There also indications of a top-heavy IMF in the massive M 82-F cluster located in the starburst galaxy M 82.…”

Simulated X-Ray Images and Spectra of the Arches Cluster

“…Ozernoy, Genzel, & Usov (1997) did analytic modeling, while Cantó, Raga, & Rodríguez (2000) did both analytic and numerical studies for the Arches cluster. The Arches cluster (G0.121ϩ0.017; Nagata et al 1995;Cotera et al 1996;Serabyn, Shupe, & Figer 1998;Figer et al 1999) is a remarkable object, with about 100 massive stars within a ∼0.2-0.3 pc radius. Cantó et al (2000) carried out a prediction of the X-ray flux from their models, suggesting that this emission could be detected observationally.…”

Studying the Nearby Universe with Chandra

“…A considerable fraction of the emission may come directly from individual massive stars. The total luminosity of the cluster (L bol ∼ 4×10 41 ergs s −1 ; Figer et al 1999) and the "canonical" relation between X-ray and total luminosity of L x /L bol ∼ 10 −7 (Chlebowski 1989) predicts an integrated X-ray luminosity L x ∼ 4 × 10 34 ergs s −1 , or about 10% of the observed X-ray luminosity of the Arches cluster. This fraction could be higher if the relation under-estimates the X-ray luminosity of stars with high metal abundances (∼ 3× solar) and in a dense stellar environment (e.g., Chlebowski 1989).…”

“…Located at only 12 ′ away from Sgr A * , the cluster contains more than 100 O-type and Wolf-Rayet stars within a region of ∼ < 1 pc across (Figer et al 1999;Fig. 2).…”