A binary black hole merger rate comparison within the same metallicity - star formation rate framework

Abstract

Recent studies have suggested that binary population synthesis models, when coupled with observationally based, metallicity-dependent star formation rate density, overpredict the observed local binary black hole (BBH) merger rate density. The significance of this tension might vary depending on the specific code and parameters adopted. Thus, a more extensive exploration of the parameter space is required. In this work, we perform such an extended analysis by considering BBH merger efficiencies coming from multiple population synthesis codes across a wide range of physical assumptions and parameter's choices. We adopt an observationally motivated metallicity distribution, exploring several variations to encompass observational uncertainties. We find that the tension persists: in almost all our metallicity variations, 14 out of 18, none of the models considered predicts a local BBH merger rate within or below the observed $90\%$ credible interval. Even in the four most favorable metallicity variations, only $\lesssim 10\%$ of the models are consistent with the observational constraints. We show that such a discrepancy originates from the low-metallicity tail contributed by low-mass galaxies and starbursts, as well as from the use of iron abundance rather than oxygen abundance in deriving the metallicity distribution. Even literature models that predict moderate BBH merger rates shift toward higher merger rates when combined with observationally motivated metallicity distributions. Although not comprehensive of all the literature models, our analysis suggests that models featuring stronger natal kicks and/or non-standard treatments of mass-transfer and common-envelope physics provide the most promising avenue for alleviating the tension with the observed local BBH merger rate.