Dynamically probing the dark side of globular clusters: from central black holes to the Milky Way halo

球状星团是稠密而古老的（约120亿年），由多达100万颗恒星通过引力聚集在一起，通常在星系的外部发现。这些特性的结合使它们成为研究天体物理学中一些非常及时的问题的理想实验室。这项研究计划旨在利用银河系中的球状星团来更好地了解黑洞的起源和演化，并绘制银河系外围神秘暗物质的地图。这将通过研究星团内恒星的运动和位置来完成，这些运动和位置受到不可见质量（黑洞和暗物质）通过引力的影响。最近，来自合并的恒星质量黑洞（质量高达几十个太阳质量）的引力波（“时空涟漪”）的历史性发现，引发了许多关于这些黑洞（大质量恒星生命的最终产物）如何能在如此近的地方找到自己的问题。其中一个主要的假设是，紧密的双黑洞系统是通过球状星团密集核心的相互作用形成的。虽然这些密集的相互作用随着时间的推移可以用强大的计算机模拟，但许多进入计算的假设是不确定的。这项研究计划的第一个目标是，通过比较恒星运动和位置的观测结果，与更简单（但计算速度更快）的“静态”模型（捕捉恒星残骸和正常恒星之间的相互作用），推断出目前星团中黑洞和其他恒星残骸的内容。这将为更密集的计算模拟提供重要的边界条件，并使对球状星团中黑洞合并的产生进行更可靠的估计成为可能。鉴于球状星团的高中心密度，它们也可能形成并容纳所谓的“中等质量黑洞”（几百到数万个太阳质量），这可能是在所有大质量星系中心发现的超大质量黑洞成长过程中缺失的一环。该项目的第二个组成部分将测试并应用对中心中等质量黑洞敏感的新的快速“静态”模型。这些模型将面对在假定的中等质量黑洞附近的恒星运动的观测，以限制这些黑洞的存在。这些努力也将为下一代仪器铺平道路，这些仪器将更灵敏地探测中等质量黑洞附近恒星的运动，如果它们存在的话。该计划的第三个组成部分将集中在球状星团外缘恒星的运动上，这些恒星对银河系中暗物质的分布很敏感（银河系本身就掌握着暗物质性质的线索）。一种推测银河系暗物质分布的有希望的方法将被开发出来，并通过对星团外部恒星的新观测进行测试。

Globular clusters are dense and old (~12 billion years) collections of up to a million stars held together by gravity and typically found in the outer parts of galaxies. This combination of properties makes them ideal laboratories to study some very timely questions in astrophysics. This research program aims to use globular clusters in the Milky Way to better understand the origin and evolution of black holes, and map the mysterious dark matter in the outskirts of our galaxy. This will be done by studying the motions and positions of stars within clusters, which are affected by the invisible mass (black holes and dark matter) through gravity. The recent historic discovery of gravitational waves ("ripples in space-time") from merging stellar-mass black holes (with masses up to a few tens of solar masses) has led to many questions about how these black holes (the end products of a massive star's life) can find themselves in such close vicinity. One of the leading hypotheses is that close binary black hole systems form via interactions in the dense core of globular clusters. While these interactions in dense clusters over time can be modelled with powerful computers, many of the assumptions entering the calculations are uncertain. The first goal of this research program is to infer the content of black holes and other stellar remnants in clusters at the present day by comparing observations of stellar motions and positions with simpler (but faster to compute) "static" models that capture the interplay between stellar remnants and normal stars. This will provide important boundary conditions for more computationally intensive simulations and enable more reliable estimates of the production of black hole mergers in globular clusters. Given their high central densities, globular clusters may also form and harbour so-called "intermediate-mass black holes" (several hundred to tens of thousands of solar masses), a possible missing link in the growth of the supermassive black holes found at the centre of all massive galaxies. The second component of this program will test and apply new fast "static" models sensitive to a central intermediate-mass black hole. The models will be confronted to observations of stellar motions in the vicinity of a putative intermediate-mass black holes to constrain the existence of these black holes. These efforts will also pave the way for the future generation of instruments that will probe more sensitively the motion of stars in the vicinity of intermediate-mass black holes, should they exist. The third component of the program will focus on the motions of stars at outer edges of globular clusters, which are sensitive to the distribution of dark matter in the Milky Way (itself holding clues about the nature of dark matter). A promising method for inferring the dark matter distribution in the Milky Way will be developed and tested with new observations of stars in the outer parts of clusters.