The cosmic merger rate density of compact objects: impact of star formation, metallicity, initial mass function and binary evolution

TL;DR

This paper assesses how various astrophysical uncertainties affect the predicted merger rates of compact binary systems across cosmic time, highlighting the dominant factors and constraints from gravitational-wave observations.

Contribution

It systematically quantifies the impact of star formation, metallicity, and binary evolution uncertainties on merger rate predictions, providing constraints consistent with GW observations.

Findings

Common envelope ejection uncertainty greatly affects BNS merger rates.

Metallicity evolution impacts BBH merger rate by an order of magnitude.

BNS merger rates are insensitive to metallicity, making them ideal for constraining binary evolution.

Abstract

We evaluate the redshift distribution of binary black hole (BBH), black hole - neutron star binary (BHNS) and binary neutron star (BNS) mergers, exploring the main sources of uncertainty: star formation rate (SFR) density, metallicity evolution, common envelope, mass transfer via Roche lobe overflow, natal kicks, core-collapse supernova model and initial mass function. Among binary evolution processes, uncertainties on common envelope ejection have a major impact: the local merger rate density of BNSs varies from $\sim{}10^3$ to $\sim{}20$ Gpc$^{-3}$ yr$^{-1}$ if we change the common envelope efficiency parameter from $\alpha_{\rm CE}=7$ to 0.5, while the local merger rates of BBHs and BHNSs vary by a factor of $\sim{}2-3$. The BBH merger rate changes by one order of magnitude, when $1 \sigma$ uncertainties on metallicity evolution are taken into account. In contrast, the BNS merger rate is almost insensitive to metallicity. Hence, BNSs are the ideal test bed to put constraints on uncertain binary evolution processes, such as common envelope and natal kicks. Only models assuming values of $\alpha_{\rm CE}\gtrsim{}2$ and moderately low natal kicks (depending on the ejected mass and the SN mechanism), result in a local BNS merger rate density within the 90% credible interval inferred from the second gravitational-wave transient catalogue.