Testing Black-Hole Formation with LIGO: Evidence for a Bimodal Black-Hole Mass Distribution

Abstract: Dramatic progress in recent years in our understanding of core-collapse supernovae has, for the first time, allowed us to connect supernova progenitors with their explosions and compact remnants. Among other insights, this framework yields unique predictions for the black-hole mass spectrum, which can be directly tested through LIGO’s gravitational-wave detections of binary black-hole mergers.

Specifically, theory predicts a peak in the black-hole mass distribution around 9 solar masses (M☉), driven by late nuclear burning phases—specifically carbon burning and the onset of neon burning—and showing only a weak dependence on metallicity. This leads to a predicted peak in the LIGO chirp-mass distribution around 8 M☉, followed by a distinct gap or deficiency of systems just beyond.

Using the binary population synthesis code COMPAS, we generated detailed predictions for observational rates. The recent release of results from the LIGO/Virgo/KAGRA O4a run brings the total number of detected black-hole mergers to 158. The observed chirp-mass distribution displays a peak-gap structure in striking agreement with our predictions, effectively ruling out several commonly used black-hole formation prescriptions.

This match confirms our foundational understanding of late stellar evolution and key aspects of the core-collapse mechanism. Furthermore, it demonstrates that forming black holes in binaries is significantly more difficult than often assumed in the literature, naturally explaining observational puzzles such as the lack of Be X-ray binaries containing black holes. This lecture will present these findings alongside broader implications for modeling black-hole binary systems.

Speaker: Prof. Philipp Podsiadlowski, research of London Centre for Stellar Astrophysics & University of Oxford