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

There has been dramatic progress in recent years in our understanding of core-collapse supernovae. This, for the first time, has allowed us to connect supernova progenitors with their supernova explosions (if successful) and their remnants. Among other things, this makes unique predictions about the black-hole mass spectrum which can be tested through detections of gravitational waves by LIGO caused by the merger of two black holes. Specifically, it predicts a peak in the mass distribution around 9 solar masses. I will explain the origin of this peak (it is related earlier nuclear burning phases, specifically carbon burning and the onset of neon burning) and show that it is only weakly dependent on metallicity. This leads to a peak in the predicted LIGO chirp-mass distribution around 8 solar masses and a gap/deficiency of systems just beyond. Using the binary population synthesis code COMPAS, we have made predictions for the rates LIGO should observe. The LIGO/Virgo/KAGRA just a few weeks ago released the results from the O4a LIGO run, bringing the number of detected black-hole mergers to 158. The observed chirp-mass distribution now shows a peak-gap structure that is in striking agreement with our predictions and rules out other commonly used black-hole formation prescriptions. This confirms our basic understanding of late stellar evolution and key aspects of the core-collapse mechanism. It also shows that, as predicted, black holes are much harder to form in binaries than has often been assumed in the literature. This explains, e.g., the lack of Be X-ray binaries containing black holes. I will discuss this and other implications for modeling black-hole binary systems.