Gravitational Waves from the Cosmic Dawn: Tracing Cosmic Black Hole Binaries with ET, LGWA and LISA

TL;DR

This paper predicts gravitational wave signals from early black hole mergers at high redshift using semi-analytic models, exploring how different accretion scenarios influence detectability by future observatories.

Contribution

It introduces a comprehensive analysis of high-redshift black hole mergers considering various seed and accretion models, predicting detection rates for upcoming GW detectors.

Findings

SE accretion leads to higher LISA detection rates (~64 yr^-1) compared to EL (~32 yr^-1).

EL models produce mostly equal-mass binaries, while SE models yield lower mass ratios.

Detection rates vary across detectors, with LGWA showing similar rates in both scenarios.

Abstract

Next generation detectors, such as LISA, LGWA, and ET will, for the first time, probe the high redshift Universe, offering unique insight into the birth, growth, and dynamics of the first black holes (BHs) during their earliest stages formation. We aim to predict merger rates and gravitational wave (GW) signatures of "cosmic" binary BHs, forming as a result of galaxy mergers, at z>=4. We investigate how BH seeding, accretion physics and dynamical delays affect their properties and detectability across cosmic epochs. We use the semi-analytic model Cosmic Archaeology Tool (CAT) to trace the evolution and delayed-mergers, driven by dynamical friction, of BH binaries formed from light, medium-weight and heavy seeds, under Eddington-limited (EL) and super-Eddington (SE) accretion prescriptions. We employ the GWFish package to evaluate their GW signals and detectability by LISA, LGWA and ET. Our results show the impact of BH accretion and seeding prescriptions on the properties and distribution of detectable sources. In the EL model, the detected populations are dominated by nearly equal-mass binaries. In contrast, SE growth leads to lower mass ratios for LISA detections and medium ratios for ET and LGWA. We present the total detection rates predicted under the two accretion scenarios. The SE model allows BHs to grow faster, transferring a significant fraction of detectable systems from the ET band to the LISA band, compared to the EL model. As a result, the predicted LISA detection rate increases from ~32 yr^-1 in the EL case to ~64 yr^-1 in the SE scenario, and the ET detection rate reduces from ~64 yr^-1 in the EL model to only ~4 yr^-1 in the SE scenario. LGWA yields comparable detection rates in both scenarios (~21 yr^-1 in EL and ~12 yr^-1 in SE). The combined information encoded in mass ratios, redshift evolution and merger rates emerge as a promising diagnostic of early BH growth.