Properties of BBH mergers at Z = 10⁻⁴ Z_⊙ in the SMT channel, as probability density functions of the number of BBH mergers per  M_⊙ of star formation. M₁ and M₂ indicate the initial mass at ZAMS of more (top left) and less massive (top right) star, respectively. However, the observed M₁ and M₂ in a BBH merger are defined based on their mass at merger (red line). The mass ratio is defined as q = M₂/M₁, except for the observed q, where q = min(M_BH)/max(M_BH).