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

点击翻译按钮获取中文摘要

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.