标题： 在球状星团NGC 6441中寻找中等质量黑洞：天体测量学研究 显示英文标题

标题： Hunting for intermediate-mass black holes in globular clusters: an astrometric study of NGC 6441

摘要： 我们对球状星团NGC 6441核心区域的自行（PMs）进行了天体测量研究。该星团的核心区域密度很高，使用现有仪器进行观测非常具有挑战性。我们将地面观测的高角分辨率NACO@VLT图像与哈勃空间望远镜ACS/HRC数据相结合，并在15年的时间基线上测量了星团中心最内15角秒区域内约1400颗恒星的自行。我们对于明亮且测量良好的恒星的自行精度达到$\sim$30$\mu$每年$^{-1}$。我们的速度弥散结果与其他研究结果一致，并将已有的关于NGC 6441恒星运动学的分析扩展到了此前从未探测过的最核心区域。在星团最内侧的角秒范围内，我们测得演化恒星的速度弥散为（19.1$\pm$2.0）公里每秒$^{-1}$。由于其高质量，NGC 6441是一个可能包含中等质量黑洞（IMBH）的有希望的候选者。我们将测量结果与文献中的其他数据相结合，并计算了星团的动力学模型。我们得到了一个上限为$M_{\rm IMBH} < 1.32 \times 10^4\,\textrm{M}_\odot$，但无法确认或排除其存在。我们还精确确定了星团的动力学距离为$12.74^{+0.16}_{-0.15}$千秒差距。尽管在NGC 6441中寻找IMBH的工作尚未结束，但我们的结果表明，未来使用极大望远镜的观测将受益于现有高角分辨率数据提供的长时间基线。 显示英文摘要

摘要： We present an astrometric study of the proper motions (PMs) in the core of the globular cluster NGC 6441. The core of this cluster has a high density and observations with current instrumentation are very challenging. We combine ground-based, high-angular-resolution NACO@VLT images with Hubble Space Telescope ACS/HRC data and measure PMs with a temporal baseline of 15 yr for about 1400 stars in the centermost 15 arcseconds of the cluster. We reach a PM precision of $\sim$30 $\mu$as yr$^{-1}$ for bright, well-measured stars. Our results for the velocity dispersion are in good agreement with other studies and extend already-existing analyses of the stellar kinematics of NGC 6441 to its centermost region never probed before. In the innermost arcsecond of the cluster, we measure a velocity dispersion of (19.1 $\pm$ 2.0) km s$^{-1}$ for evolved stars. Because of its high mass, NGC 6441 is a promising candidate for harbouring an intermediate-mass black hole (IMBH). We combine our measurements with additional data from the literature and compute dynamical models of the cluster. We find an upper limit of $M_{\rm IMBH} < 1.32 \times 10^4\,\textrm{M}_\odot$ but we can neither confirm nor rule out its presence. We also refine the dynamical distance of the cluster to $12.74^{+0.16}_{-0.15}$ kpc. Although the hunt for an IMBH in NGC 6441 is not yet concluded, our results show how future observations with extremely-large telescopes will benefit from the long temporal baseline offered by existing high-angular-resolution data.