Stellar Clusters as the Nurseries of Black Holes

The last decade has seen several quantum leaps in our abilities to study black holes, from the detection of gravitational waves emitted by merging stellar black holes to the images of supermassive black holes taken by the Event Horizon Telescope. As is the case with every major discovery, these breakthroughs raised new questions that we need to answer: How do merging black holes find each other? How do supermassive black holes spring into existence and grow in mass? The answer to these questions may be found in star clusters, dense conglomerations of stars held together by their mutual gravitational forces. We observe star clusters in every major galaxy. In the Milky Way alone, well over a thousand clusters are known, from recently formed open clusters comprised of a few thousand stars to ancient globular clusters composed of several millions of stars. Deciphering the secrets of these star clusters and investigating their link to black holes is the aim of this project.

Black holes form when massive stars exhaust their nuclear fusion "fuel" and collapse under their own gravity. This final episode in the life of a massive star is often accompanied by a bright supernova, visible across intergalactic distances. However, not every massive star will end its life as a black hole. Some are expected to lose most of their mass through powerful winds, whereas others are completely disrupted in the supernova explosions. To complicate matters even further, many massive stars live in binary systems and their fates are altered when they donate mass to their companions. To advance our understanding of the evolution of massive stars, we study them in their natural habitats, namely young star clusters. By measuring their chemical compositions and their velocities using spectroscopy, we determine whether they live in binary systems and whether they have already shed material to their companions. We are also on the hunt for stars that are paired with black holes, as such systems reveal crucial information about the conditions required to form black holes.

Star clusters are not only considered as a primary site for the formation of black holes, but also for their subsequent growth. In these highly dynamic environments, black holes can pair up and merge, resulting in gravitational wave emission and potentially in the growth of black holes that are heavier than even their most massive stellar progenitors. Such "intermediate-mass" black holes are considered as potential seeds from which the supermassive ones observed in the centres of galaxies evolve. This scenario has recently gained in credibility through the discovery that some of the most massive clusters we have found might in fact be the surviving nuclei of galaxies that were accreted by the Milky Way in the past. Indeed, our research has already revealed strong evidence for the presence of a massive black hole in at least one of these clusters. In this project, we are using additional observations to verify this finding and study similar clusters to determine the occurrence rate of intermediate-mass black holes.