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Gravitational-wave detectors are starting to reveal the redshift evolution of the binary black hole (BBH) merger rate,R_BBH(z). We make predictions forR_BBH(z) as a function of black hole mass for systems originating from isolated binaries. To this end, we investigate correlations between the delay time and black hole mass by means of the suite of binary population synthesis simulations,COMPAS. We distinguish two channels: the common envelope (CE), and the stable Roche-lobe overflow (RLOF) channel, characterized by whether the system has experienced a common envelope or not. We find that the CE channel preferentially produces BHs with masses below about 30M_⊙and short delay times (t_delay≲ 1 Gyr), while the stable RLOF channel primarily forms systems with BH masses above 30M_⊙and long delay times (t_delay≳ 1 Gyr). We provide a new fit for the metallicity-dependent specific star formation rate density based on the Illustris TNG simulations, and use this to convert the delay time distributions into a prediction ofR_BBH(z). This leads to a distinct redshift evolution ofR_BBH(z) for high and low primary BH masses. We furthermore find that, at high redshift,R_BBH(z) is dominated by the CE channel, while at low redshift, it contains a large contribution (∼40%) from the stable RLOF channel.more »Our results predict that, for increasing redshifts, BBHs with component masses above 30M_⊙will become increasingly scarce relative to less massive BBH systems. Evidence of this distinct evolution ofR_BBH(z) for different BH masses can be tested with future detectors.

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Abstract The recent detection of GW190521 stimulated ideas on how to populate the predicted black hole (BH) pair-instability (PI) mass gap. One proposal is the dynamical merger of two stars below the PI regime forming a star with a small core and an oversized envelope. We outline the main challenges this scenario faces to form one BH in the gap. In particular, the core needs to avoid growing during the merger, and the merger product needs to retain enough mass, including in the subsequent evolution, and at core collapse (CC). We explore this scenario with detailed stellar evolution calculations, starting with ad hoc initial conditions enforcing no core growth during the merger. We find that these massive merger products are likely to be helium-rich and spend most of their remaining lifetime within regions of instabilities in the Herzsprung–Russell diagram, such as luminous blue variable eruptions. An energetic estimate of the amount of mass loss neglecting the back reaction of the star suggests that the total amount of mass that can be removed at low metallicity is ≲1 M ⊙ . This is small enough that at CC our models are retaining sufficient mass to form BHs in the PI gapmore »similar to the recent ones detected by LIGO/Virgo. However, mass loss at the time of merger, the resulting core structure, and the mass loss at CC still need to be quantified for these models to confirm the viability of this scenario.« less