A Mid-Thirties Crisis: Dissecting the Properties of Gravitational Wave Sources Near the 35 Solar Mass Peak
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 ($χ_{\rm eff}$), 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 $χ_{\rm eff}$ distribution skewed toward positive values, with a median of zero excluded at approximately $90\%$ confidence. We find no significant $q-χ_{\rm eff}$ correlation in the $35\, \mathrm{M}_{\odot}$ peak population, suggesting that lower-mass systems ($m_1<20\,\mathrm{M}_{\odot}$) likely drive the $q-χ_{\rm eff}$ 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.