Effects of a non‐universal IMF and binary parameter correlations on compact binary mergers

Abstract

Binary population synthesis provides a direct way of studying the effects of different choices of binary evolution models and initial parameter distributions on present‐day compact binary merger populations, which can then be compared to empirical properties such as observed merger rates. Samples of zero‐age main sequence binaries to be evolved by such codes are typically generated from a universal initial mass function (IMF) and simple, uniform, distributions for orbital period P, mass ratio q, and eccentricity e. More recently, however, mounting observational evidence has suggested the non‐universality of the IMF and the existence of correlations between binary parameters. In this study, we implement a metallicity‐ and redshift‐dependent IMF alongside correlated distributions for P, q, and e to generate representative populations of binaries at varying redshifts, which are then evolved with the COMPAS code in order to study the variations in merger rates and overall population properties.

An Overview of Compact Star Populations and Some of Its Open Problems

“…Although it is hard to treat the existing Galactic BH sample as reflective of the real mass distribution with its current size, this also might indicate that high-redshift populations allow for the production of more massive BHs than local populations. If mass-redshift correlations are present in the GW sample, then they might support the aforementioned idea that low-metallicity and high-star formation rate conditions at high redshift lead to the birth of more massive stars, [194] which in turn allow for an increased production of more massive BHs, and consequently of more massive BH mergers, as has been indicated by population synthesis studies that consider environment-dependent initial mass functions and star formation rates ( [193,195], see also Figure 11) and tentatively supported by GW observations [196]. A full consideration will, however, still require accounting for observational biases and for a study of the redshift distribution in GWTC-3 and future catalogs.…”

“…Although a safe conclusion may only be derived after a careful modeling and consideration of observational biases for both samples, this might lend credence to the idea that a Galacticlike population of BHs exists in galaxies at higher redshift (z∼1). BH-BH merger rate (R BBH ) per total binary mass (M tot ) bin in three different ranges of redshift at merger z merger from [193], resulting from binary population synthesis with an IMF that becomes top-heavy at high z.…”

“…Figure11. BH-BH merger rate (R BBH ) per total binary mass (M tot ) bin in three different ranges of redshift at merger z merger from[193], resulting from binary population synthesis with an IMF that becomes top-heavy at high z.…”