Black hole mergers: connecting stellar physics and global star formation – COSMERGE

The detection of gravitational waves (GWs) in 2015 has been labeled as one of the “major scientific discoveries of the 21st century” by most science magazines. GWs have opened a new window on our universe, at the intersection between fundamental physics and astrophysics. The first detection, the merger of two 30 Solar mass black holes, came as an immense surprise to the astrophysical world as all black holes we knew off had masses around 10-15 Solar mass. This discovery immediately raised the questions: “where do merging black holes come from?” Black hole mergers are thought to come from the evolution of isolated massive binary stars and from N-body interactions in dense star clusters but the relative contribution of both formation channels is still unclear. This is because our knowledge of the evolution of massive stars, and their binary interactions is still very partial and the study of clusters is limited by uncertain initial conditions. Because of their different initial conditions and different evolutionary pathways, the binary and cluster merger channels have never been compared in a consistent, systematic fashion.

In 2022, the next Virgo/LIGO observations (O4) should yield several hundreds of binary black hole mergers. With this statistical sample, we expect to finally be able to answer to the question of the origins of black hole mergers. Provided accurate models and appropriate statistics are used, the interpretation of GW events has the potential to provide information on massive stars which is unavailable with current electromagnetic observations. The first goal of this ANR project COSMERGE is to determine the importance of each formation channel and the main parameters ruling massive binary evolution. The second goal of COSMERGE is to determine the typical host galaxies and formation times of black hole merger progenitors. Based on our findings about how, when and where GW progenitors form, our final goal will be to make discriminating predictions for detections with the sensitivities of future instruments.

The key to the astrophysical interpretation of GW events is the multidisciplinary combination of a robust statistical analysis with state-of-the-art astrophysical models. With COSMERGE, we will infer the main astrophysical parameters leading to black hole mergers using hierarchical Bayesian analysis of the O4 data, based on our newly developed astrophysical prior models. We will use a large scale high resolution cosmological simulation to model star formation both in isolated binaries and massive star clusters, with a novel technique we have been developing over the past years. In combination with various binary population synthesis models and simulations for cluster evolution, we will provide the first self-consistent end-to-end comparison of large scale models of black hole mergers in both clusters and binaries. These will be used as priors for a hierarchical Bayesian analysis of the O4 detections, and lead to a confident determination of the main channels leading to black hole mergers.

The COSMERGE project will be lead by Astrid Lamberts, who will hire a postdoctoral researcher and PhD student at the Laboratoire Lagrange in Nice. Astrid Lamberts is one of the only GW astrophysicists in France. With her record for interdisciplinary research projects, the COSMERGE project will build on local expertise and develop GW astrophysics in Nice. This project will strongly improve our understanding of the origin of binary black hole mergers, which is one of the core science objectives of the Virgo and LIGO projects. Its results will be relevant to different astrophysical communities such as massive stellar evolution and global star formation and will also be disseminated within a public audience.