The more accurately the metal-dependent star formation rate is modeled, the larger the predicted excess of binary black hole mergers

TL;DR

Accurately modeling the metal-dependent star formation rate significantly influences the predicted binary black hole merger rates, revealing discrepancies with observational data that point to stellar evolution or formation channels as key factors.

Contribution

Developed a data-driven model of metal-dependent star formation rate coupled with binary evolution to assess its impact on BBH merger rate predictions.

Findings

Realistic SFR evolution predicts higher merger rates than observed.

Variations in low-mass galaxy SFRs have limited impact on overall merger rate.

Discrepancies are likely due to stellar evolution models or formation channels.

Abstract

As the number of gravitational-wave detections grows, the merger rate of binary black holes (BBHs) can help us to constrain their formation, the properties of their progenitors, and their birth environment. Here, we aim to address the impact of the metal-dependent star formation rate (SFR) on the BBH merger rate. To this end, we have developed a fully data-driven approach to model the metal-dependent SFR and coupled it to BBH evolution. We have adopted the most up-to-date scaling relations, based on recent observational results, and we have studied how the BBH merger rate density varies over a wide grid of galaxy and binary evolution parameters. Our results show that including a realistic metal-dependent SFR evolution yields a value of the merger rate density which is too high compared to the one inferred from gravitational-wave data. Moreover, variations in the SFR in low-mass galaxies ($M_\ast \lesssim 10^8 \mathrm{M}_{\odot}$) do not contribute more than a factor $\sim 2$ to the overall merger rate density at redshift $z=0$. These results suggest that the discrepancy between the BBH merger rate density inferred from data and theoretical models is not caused by approximations in the treatment of the metal-dependent SFR, but rather stems from stellar evolution models and/or BBH formation channels.