Galactic Nuclei as Nurseries for Gravitational Wave Sources

We are in a similar position as Galileo when he first peered through a telescope: mankind has developed a new way to study the Universe, we can now see gravitational waves! Advance LIGO has provided unique access to the properties of space-time in the strong field of gravity together with a direct confirmation of the existence of black holes and in particular of binary black holes that merge within the age of the Universe. The Advanced LIGO detections have not only confirmed the existence of gravitational waves but have also ushered astrophysics into a new era of observing cosmic events that were previously invisible. This new set of eyes on the Universe is expected to provide important clues to the properties of black holes and to the environments in which they form. But a proper theoretical understanding of the astrophysical processes leading to black hole coalescence is required to make sense of the large amount (hundreds to thousands) of new detections expected in the next few years from Advanced LIGO and other interferometers on the ground.

In this project I will study the formation and dynamical evolution of black hole binaries that are likely to merge and become gravitational waves sources. It has long been recognised that dense star clusters are a preferred site for the formation of compact objects binaries. Of particular relevance among star clusters are the so called nuclear clusters, massive stellar systems that are observed at the centre of most nearby galaxies, including our own. Nuclear clusters have similar sizes to galactic globular clusters but are much more massive, and as such represent the densest systems observed in the local Universe - they contain between 1 to 100 million stars within the same distance that separates our Sun from its closest star, alpha Centauri. Due to these extreme densities, close dynamical encounters involving black holes, stars, and binaries are common and lead to the formation of black hole binaries. It is likely that many, or perhaps even most, of the gravitational waves sources detectable by Advanced LIGO are binary black holes produced dynamically in nuclear clusters. Therefore, nuclear star clusters are ideal nurseries for binaries of compact objects and high frequency gravitational waves sources.

The main objective of this proposal is to make detailed predictions for the properties (e.g. mass, eccentricity, spin distributions) of black hole binaries in nuclear star clusters and of their merger rate. I will characterize how such properties can be related to the global properties of the host cluster, opening the door to more quantitative comparisons between theoretical models and observations. The method will include a combination of analytical calculations and state-of-the-art numerical simulations, including high accuracy simulations of nuclear star clusters with realistic stellar and binary populations.

My goals go beyond a characterization of the gravitational wave sources. I will build hundreds to thousands of cluster models that I will make publicly available to the community. These will represent the most detailed theoretical framework for the interpretation of a variety of interesting additional phenomena, including stellar collisions and blue stragglers, radio pulsars, planetary systems in star clusters, and the dynamical formation of stellar triples.