Structure and Mass of a Young Globular Cluster in NGC 6946
Structure and Mass of a Young Globular Cluster in NGC 6946
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NGC 6946 中年轻球状星团的结构和质量
DOI：
10.1086/321620
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发表时间：
2001
期刊：
影响因子：
--
通讯作者：
T. Richtler
中科院分区：
文献类型：
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作者：
S. Larsen;J. Brodie;B. Elmegreen;Y. Efremov;P. Hodge;T. Richtler
利用哈勃太空望远镜上的宽视场行星照相机2，我们拍摄了附近螺旋星系NGC 6946中一个明亮的年轻星星星团。在65 pc的半径范围内，这个星团的绝对视星等为MV =-13.2，可以与银河并合星系中最明亮的年轻“超级星星星团”相媲美。UBV颜色表示年龄约为1500万年。该星团有一个致密的核心（半径约1.3 pc），被一个具有幂律光度分布的扩展包络所包围。星系团轮廓的外部逐渐与一般场合并，使得难以测量精确的半光半径Re，但我们估计Re约为13 pc。结合星族合成模型，星系团的光度和年龄意味着，当Salpeter初始质量函数（IMF）向下延伸到0.1 M时，星系团的质量为8.2 × 105 M。如果IMF在0.4 M以下为对数正态分布，则质量减少到5.5 × 105 M。根据模型的假设，星团的中心密度在5.3 × 103到1.7 × 104 M <$pc-3之间，与其他高密度恒星形成区相当。我们还估计了动态质量的集群使用高色散光谱HIRES摄谱仪上的凯克我望远镜。HIRES数据表明，速度色散为10.0 ± 2.7 km s-1，表明星团总质量在65 pc × 106 M的范围内。将动力学质量与基于测光和布居合成模型的质量估计值进行比较，我们发现质光比至少与Salpeter IMF延伸到0.1 M的量一样高，尽管IMF在0.4 M的量仍然有可能在~1 σ的误差范围内。这个星团大概会保持束缚，演变成一个球状星团状的物体。
Using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have imaged a luminous young star cluster in the nearby spiral galaxy NGC 6946. Within a radius of 65 pc, the cluster has an absolute visual magnitude, MV = -13.2, comparable to the most luminous young "super star clusters" in the Antennae merger galaxy. UBV colors indicate an age of about 15 Myr. The cluster has a compact core (radius ~1.3 pc) surrounded by an extended envelope with a power-law luminosity profile. The outer parts of the cluster profile gradually merge with the general field, making it difficult to measure a precise half-light radius Re, but we estimate Re ~ 13 pc. Combined with population synthesis models, the luminosity and age of the cluster imply a mass of 8.2 × 105 M☉ for a Salpeter initial mass function (IMF) extending down to 0.1 M☉. If the IMF is lognormal below 0.4 M☉, then the mass decreases to 5.5 × 105 M☉. Depending on model assumptions, the central density of the cluster is between 5.3 × 103 and 1.7 × 104 M☉ pc-3, comparable to other high-density star-forming regions. We also estimate a dynamical mass for the cluster using high-dispersion spectra from the HIRES spectrograph on the Keck I telescope. The HIRES data indicate a velocity dispersion of 10.0 ± 2.7 km s-1 and imply a total cluster mass within 65 pc of × 106 M☉. Comparing the dynamical mass with the mass estimates based on the photometry and population synthesis models, we find that the mass-to-light ratio is at least as high as for a Salpeter IMF extending down to 0.1 M☉, although a turnover in the IMF at 0.4 M☉ is still possible within the ~1 σ errors. The cluster will presumably remain bound, evolving into a globular cluster-like object.