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>High-Dispersion Spectroscopy of a Luminous, Young Star Cluster in NGC 1705: Further Evidence for Present-Day Formation of Globular Clusters
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High-Dispersion Spectroscopy of a Luminous, Young Star Cluster in NGC 1705: Further Evidence for Present-Day Formation of Globular Clusters
We present evidence that some of the compact, luminous, young star clusters discovered recently through images taken with the Hubble Space Telescope (HST) have masses comparable to those of old Galactic globular clusters. The "super star cluster" in the center of the nearby amorphous galaxy NGC 1705 has been observed with high dispersion at optical wavelengths using the HIRES echelle spectro-graph on the Keck 10 m telescope. Numerous weak metal lines arising from the atmospheres of cool supergiants have been detected in the integrated spectrum, permitting a direct measurement of the line-of-sight stellar velocity dispersion through cross-correlation with a template star; the result is σ* 11.4 ± 1.5 km s–1. Assuming that the system is gravitationally bound and using a cluster size measured from HST images, we apply the virial theorem to obtain the dynamical mass. Its derived mass [(8.2 ± 2.1) × 104 M☉], mass density (2.7 ± 104 pc–3), and predicted mass-to-light ratio after aging by 10-15 Gyr [0.6-1.6(M/LV)☉] closely resemble those of the majority of evolved Galactic globular clusters. The central cluster in NGC 1705 appears to be very similar in nature to one of the bright clusters in NGC 1569, which was discussed earlier this year by Ho & Filippenko. We also observed the brightest cluster in the Magellanic irregular galaxy NGC 4214, but no lines suitable for measuring its velocity dispersion were detected, most likely because of the very young age of the cluster. Although these observations need to be extended to a much larger sample of objects before generalizations can be made concerning the nature of similar clusters observed in other galaxies, our preliminary results are tantalizing and strongly suggest that, at least in two cases, we are witnessing globular clusters in their extreme youth.
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机译：我们提供的证据表明，最近通过哈勃太空望远镜（HST）拍摄的图像发现的一些紧凑，发光的年轻恒星团的质量与老银河系球状星团的质量相当。使用Keck 10 m望远镜上的HIRES echelle光谱仪，已观察到在附近的非晶星系NGC 1705中心的“超级星团”在光波长处具有高色散。在综合光谱中已检测到由超巨星大气产生的许多弱金属线，从而可以通过与模板星的互相关来直接测量视线恒星速度色散;结果为σ* 11.4±1.5 km s-1。假设系统受重力约束并且使用从HST图像测量的簇大小，我们应用病毒定理来获得动力学质量。其派生质量[（8.2±2.1）×104M☉]，质量密度（2.7±104 pc–3），以及老化10-15 Gyr [0.6-1.6（M / LV）之后的预测的质光比☉]与大多数演化的银河系球状星团非常相似。 NGC 1705中的中央星团在性质上似乎与NGC 1569中的亮星团之一非常相似，今年早些时候，Ho＆Filippenko对其进行了讨论。我们还观察到麦哲伦不规则星系NGC 4214中最亮的星团，但未检测到适合测量其速度色散的线，这很可能是因为该星团年龄很小。尽管在对涉及其他星系的相似星团的性质进行归纳之前，需要将这些观测值扩展到更大的物体样本，但我们的初步结果令人着迷，并强烈表明，至少在两种情况下，我们正在目睹球状聚集在他们的极端青年时期。
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