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arXiv 1406.2849astro-ph.GA
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Milky Way核星团的大尺度运动学和动力学模拟

Large scale kinematics and dynamical modelling of the Milky Way nuclear star cluster

A. Feldmeier, N. Neumayer, A. Seth, R. Schödel, N. Lützgendorf, P. T. de Zeeuw, M. Kissler-Patig, S. Nishiyama, C. J. Walcher

摘要:

在我们银河系的中央10 pc内有一个致密的核星团(NSC),在大多数附近的星系中都发现了类似的NSC。对NSCs结构和运动学的研究揭示了星系核的质量吸积历史。由于银河系(MW)NSC的距离只有8 kpc,我们可以在亚pc尺度上空间分辨MWNSC。这使得MWNSC成为了解所有NSC形成的参考对象。我们已经使用近红外长缝光谱仪ISAAC(VLT)在漂移扫描,构建一个积分场光谱图的中央9.5 × 8 pc的银河系。我们使用这个数据集提取恒星运动学的个别恒星和未解决的综合光谱。我们提出了一个速度和色散图,从集成的光和模型,这些运动学使用kinemetry和轴对称牛仔裤模型。我们还测量了来自单个恒星的1,375个光谱的CO带头强度。我们发现运动学的复杂性在NSC的径向速度地图,包括9度逆时针相对于银河系平面的运动学位置角的错位,和旋转的子结构垂直于银河系平面的半径为20”或0.8pc的迹象。我们确定了NSC的质量,其范围在r = 4.2pc到1.4 × 10^7 Msun之间。我们还表明,我们的运动学数据的结果在一个显着低估的超大质量黑洞(SMBH)的质量。运动的子结构和位置角偏差可能暗示在不同的吸积事件。这表明MWNSC的成长至少有一部分是由大质量星团的合并造成的。与其他神经干细胞相比,MWNSC在r_eff-M_NSC关系的紧凑侧。SMBH质量的低估可能是由运动失调和恒星星族梯度引起的。但也有可能在积分光获得的SMBH质量测量中存在偏差。

英文摘要:

Within the central 10pc of our Galaxy lies a dense nuclear star cluster (NSC), and similar NSCs are found in most nearby galaxies. Studying the structure and kinematics of NSCs reveals the history of mass accretion of galaxy nuclei. Because the Milky Way (MW) NSC is at a distance of only 8kpc, we can spatially resolve the MWNSC on sub-pc scales. This makes the MWNSC a reference object for understanding the formation of all NSCs. We have used the NIR long-slit spectrograph ISAAC (VLT) in a drift-scan to construct an integral-field spectroscopic map of the central 9.5 x 8pc of our Galaxy. We use this data set to extract stellar kinematics both of individual stars and from the unresolved integrated light spectrum. We present a velocity and dispersion map from the integrated light and model these kinematics using kinemetry and axisymmetric Jeans models. We also measure CO bandhead strengths of 1,375 spectra from individual stars. We find kinematic complexity in the NSCs radial velocity map including a misalignment of the kinematic position angle by 9 degree counterclockwise relative to the Galactic plane, and indications for a rotating substructure perpendicular to the Galactic plane at a radius of 20" or 0.8pc. We determine the mass of the NSC within r = 4.2pc to 1.4 x 10^7 Msun. We also show that our kinematic data results in a significant underestimation of the supermassive black hole (SMBH) mass. The kinematic substructure and position angle misalignment may hint at distinct accretion events. This indicates that the MWNSC grew at least partly by the mergers of massive star clusters. Compared to other NSCs, the MWNSC is on the compact side of the r_eff - M_NSC relation. The underestimation of the SMBH mass might be caused by the kinematic misalignment and a stellar population gradient. But it is also possible that there is a bias in SMBH mass measurements obtained with integrated light.

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