A mid-thirties crisis: dissecting the properties of gravitational wave sources near the 35 solar mass peak

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 M ⊙ . 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 M ⊙ peak and infer the population distributions of primary mass ( m 1 ), mass ratio ( q ), effective spin ( χ eff ), and redshift ( z ). We observe a gradual increase in the merger rate with m 1 , rising by a factor of 3 from 20 M ⊙ to a peak around 34 M ⊙ , followed by a sharp, order-of-magnitude decline by 50 M ⊙ . This population also shows a weak preference for equal-mass mergers and has a χ eff distribution skewed toward positive values, with a median of zero excluded at approximately 90% confidence. We find no significant q − χ eff correlation in the 35 M ⊙ peak population, suggesting that lower-mass systems ( m 1 < 20 M ⊙