Chemical Evolution of the Universe and its Consequences for Gravitational‐Wave Astrophysics

Abstract

We are now routinely detecting gravitational waves (GW) emitted by merging black holes and neutron stars. Those are the afterlives of massive stars that formed all across the Universe -at different cosmic times and with different metallicities (abundances of elements heavier than helium). Birth metallicity plays an important role in the evolution of massive stars. Consequently, the population properties of mergers are sensitive to the metallicity dependent cosmic star formation history (fSFR(Z,z)). In particular, within the isolated formation scenarios (the focus of this paper), a strong low metallicity preference of the formation of mergers involving black holes was found. The origin of this dependence and its consequences are discussed. Most importantly, uncertainty in the fSFR(Z,z) (substantial even at low redshifts, especially at low metallicity) cannot be ignored in the models. This poses a significant challenge for the interpretation of the observed GW source population properties. Possible paths for improvements and the role of future GW detectors are considered. Recent efforts to determine fSFR(Z,z) and the factors that dominate its uncertainty are summarized. Many of those factors are related to the properties of galaxies that are faint and distant and therefore difficult to observe. The fact that they leave imprint on the properties of mergers as a function of cosmic time makes future GW observations a promising (and complementary to electromagnetic observations) tool to study chemical evolution of galaxies.

The Mass Density of Merging Binary Black Holes over Cosmic Time

“…The BBHs that the LVK observes are stellar remnants; consequently, the rate and masses of these BBH mergers depend on the SFR (Chruślińska 2024). In this work, we use The mass in mergers where the primary component is <40M e is indistinguishable from the total mass density on this plot, and the mergers where m 1 > 60M e is multiple orders of magnitude smaller than the presented mass density.…”

“…The efficiency ò is not constant with redshift because it likely depends on metallicity (Chruślińska 2024). Here, we assume that the efficiency as a function of metallicity takes the form of a step function with a constant low-metallicity efficiency, <Z t  , and a constant high-metallicity efficiency, >Z t  separated by a metallicity threshold Z t .…”

“…Hence, we expect the progenitor formation rate to trace the cosmic SFR (see, e.g., Madau & Dickinson 2014, for a review). Only a small fraction of all stars will become black holes (BHs) that merge within a Hubble time, and this fraction depends on metallicity (Kudritzki & Puls 2000;Belczynski et al 2010;Brott et al 2011;Fryer et al 2012;Chruślińska 2024). High metallicities contribute to increased stellarmass loss via line-driven winds, increased radial expansion, and higher natal kicks, which reduce the BBH merger rate and the mass of merging BBHs (see, e.g., Chruślińska 2024, for a review).…”

The Metallicity Dependence and Evolutionary Times of Merging Binary Black Holes: Combined Constraints from Individual Gravitational-wave Detections and the Stochastic Background

“…Compact binaries are expected to preferentially arise from massive stars in low-metallicity environments (Fryer et al 2012;Belczynski et al 2016;Chruslinska et al 2018;Mapelli et al 2019;Santoliquido et al 2020;Liotine et al 2023). Both the underlying metallicity-specific star formation rate and the metallicity dependence of compact object formation, however, remain poorly understood (Chruslinska & Nelemans 2019;Broekgaarden et al 2021Broekgaarden et al , 2022Chruślińska et al 2023;van Son et al 2023;Chruślińska 2024). Therefore, gravitational-wave observations might provide one of the best observational routes to constraining or measuring the metallicity-specific star formation rate and the formation efficiency of compact objects.…”