A Mid-Thirties Crisis: Dissecting the Properties of Gravitational Wave Sources Near the 35 Solar Mass Peak

TL;DR

This study analyzes the properties of black hole mergers near 35 solar masses to understand their origins, revealing population features that challenge existing formation models and suggesting a need for new explanations.

Contribution

It provides the first detailed population-level analysis of BBH mergers around 35 solar masses, highlighting features incompatible with current formation theories.

Findings

Merger rate peaks around 34 M_sun and declines sharply at 50 M_sun.

Weak preference for equal-mass mergers and positive effective spins.

Current formation models cannot fully explain the observed population features.

Abstract

One striking feature of binary black hole (BBH) mergers observed in the first decade of gravitational-wave astronomy is an excess of events with component masses around $35\,\mathrm{M}_{\odot}$. Multiple formation channels have been proposed to explain this excess. To distinguish among these channels, it is essential to examine their predicted population-level distributions across additional parameters. In this work, we focus on BBH mergers near the $35\,\mathrm{M}_{\odot}$ peak and infer the population distributions of primary mass ($m_1$), mass ratio ($q$), effective spin ($\chi_{\rm eff}$), and redshift ($z$). We observe a gradual increase in the merger rate with $m_1$, rising by a factor of $3$ from $20\,\mathrm{M}_{\odot}$ to a peak around $34\,\mathrm{M}_{\odot}$, followed by a sharp, order-of-magnitude decline by $50\,\mathrm{M}_{\odot}$. This population also shows a weak preference for equal-mass mergers and has a $\chi_{\rm eff}$ distribution skewed toward positive values, with a median of zero excluded at approximately $90\%$ confidence. We find no significant $q-\chi_{\rm eff}$ correlation in the $35\, \mathrm{M}_{\odot}$ peak population, suggesting that lower-mass systems ($m_1<20\,\mathrm{M}_{\odot}$) likely drive the $q-\chi_{\rm eff}$ anti-correlation observed in the full BBH merger catalog. The redshift evolution of the merger rate is consistent with the cosmic star formation rate. We compare our findings with predictions from a wide range of formation channels. We find that common variants of the pair-instability supernova scenario, as well as hierarchical mergers, are incompatible with the observed features of the $35\,\mathrm{M}_{\odot}$ population. Ultimately, none of the formation channels we consider can explain all or even most of the features observed in this population. The ''mid-thirties'' of black hole mergers are in crisis.