Kavli Affiliate: George Efstathiou
| First 5 Authors: Boyuan Liu, Tilman Hartwig, Nina S. Sartorio, Irina Dvorkin, Guglielmo Costa
| Summary:
The gravitational wave (GW) signal from binary black hole (BBH) mergers is a
promising probe of Population III (Pop III) stars. To fully unleash the power
of the GW probe, one important step is to understand the relative importance
and features of different BBH evolution channels. We model two channels,
isolated binary stellar evolution (IBSE) and nuclear star cluster-dynamical
hardening (NSC-DH), in one theoretical framework based on the semi-analytical
code A-SLOTH, under various assumptions on Pop III initial mass function (IMF),
initial binary statistics and high-$z$ nuclear star clusters (NSCs). The NSC-DH
channel contributes $sim 8-95%$ of Pop III BBH mergers across cosmic history,
with higher contributions achieved by initially wider binary stars, more
top-heavy IMFs, and more abundant high-$z$ NSCs. The dimensionless stochastic
GW background (SGWB) produced by Pop III BBH mergers has peak values
$Omega^{rm peak}_{rm GW}sim 10^{-11}-8times 10^{-11}$ around
observer-frame frequencies $nusim 10-100 rm Hz$. The Pop III contribution
can be a non-negligible ($sim 2-32%$) component in the total SGWB at
$nulesssim 10 rm Hz$. The estimated detection rates of Pop III BBH mergers
by the Einstein Telescope are $sim 6-230 rm yr^{-1}$ and $sim 30-1230 rm
yr^{-1}$ for the NSC-DH and IBSE channels, respectively. Pop III BBH mergers in
NSCs are more massive than those from IBSE, so they dominate the Pop III SGWB
below $20$ Hz in most cases. Besides, the detection rate of Pop III BBH mergers
involving at least one intermediate-mass BH above $100 rm M_odot$ by the
Einstein Telescope is $sim 0.5-200 rm yr^{-1}$ in NSCs but remains below
$0.1 rm yr^{-1}$ for IBSE.
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