Unraveling the Host Galaxies of Gravitational-Wave Sources with a Novel Data-Driven Approach

• Applicant: Dr. Lumen Boco
• Subject Area: Astrophysics and Astronomy
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 555003977

Gravitational waves (GWs) are produced by mergers of compact object binaries, like black holes (BHs). While we have recently started detecting GWs, we are still not able to identify the specific galaxies hosting the merging BHs. Identifying the host galaxy is crucial as it will open the doors to multi-messenger astrophysics, having profound impact on both astrophysics and cosmology. In the realm of astrophysics, the properties of host galaxies can be used to refine our understanding of black hole formation processes. In the realm of cosmology, host galaxy association would allow to determine the redshift of GW events, enabling cosmological studies using standard sirens. This offers an independent way to estimate cosmological parameters, complementing traditional approaches, and addressing tensions and discrepancies of our current cosmological model. The main objective of this research project, named BLACKHOST, is to develop a fully empirical method to statistically associate GW sources with their host galaxies. Currently, this association is made using cosmological simulations, by analyzing simulated galaxies’ properties to identify those most likely to host BH mergers. However, galaxy properties in simulations are inherently model-dependent and rely on a number of assumptions, sub-grid recipes, and other parameters. In contrast, BLACKHOST aims to create a new data-driven, model-independent method to associate host galaxies with GWs. The novel idea is to devise an “effective” approach to galaxy evolution, not attempting to model small-scale processes occurring in galaxies, as done in simulations. Rather, I propose to marginalize over these processes by parametrizing their overall effect on the evolution of galaxies, and to constraint the parameters guided by observational data. The project’s goal is to constrain the galaxies star formation rate, merger rates and quenching timescales using observational relations and statistics, such as the galaxy stellar mass function and the main sequence. This method enables to forecast the stellar mass assembly of galaxies with various masses at different times. By populating these galaxies with stellar binaries and tracking their concurrent evolution, I will assign a probability of potential host galaxies to each observed GW event with different physical parameters, which is the goal of the project. The empirical model developed here offers distinctive advantages: robustness, independence from hypotheses and assumptions, and expedited computational performances. The results will impact many astrophysical research fields, including galaxy evolution, stellar and GW astronomy, and cosmology. The project is timely: its data-driven nature will ensure enough flexibility to readily integrate data from new instruments as Euclid, LSST, and JWST, and to refine its predictions as our observational capabilities improve. This adaptability will ensure this project to remain at the forefront of astrophysical research.

• DFG Programme: WBP Position