LIGO-Virgo-KAGRA's Oldest Black Holes: Probing star formation at cosmic noon with GWTC-3

TL;DR

This paper uses gravitational wave data from LIGO-Virgo-KAGRA to infer the formation history of binary black holes and their relation to cosmic star formation and metallicity evolution at high redshifts.

Contribution

It introduces a method to estimate black hole progenitor formation redshifts from GW data, linking GW observations to cosmic star formation and metallicity history.

Findings

At least one BBH formed before redshift 4.4.

The BBH formation rate at z=4 is approximately 6.4e-6 per solar mass.

The average metallicity at z=4 was about one-half solar metallicity.

Abstract

In their third observing run, the LIGO-Virgo-KAGRA gravitational-wave (GW) observatory was sensitive to binary black hole (BBH) mergers out to redshifts $z_\mathrm{merge}\approx1$. Because GWs are inefficient at shrinking the binary orbit, some of these BBH systems likely experienced long delay times $\tau$ between the formation of their progenitor stars at $z_\mathrm{form}$ and their GW merger at $z_\mathrm{merge}$. In fact, the distribution of delay times predicted by isolated binary evolution resembles a power law $p(\tau)\propto\tau^{\alpha_\tau}$ with slope $-1\lesssim\alpha_\tau\lesssim-0.35$ and a minimum delay time of $\tau_\mathrm{min}=10$ Myr. We use these predicted delay time distributions to infer the formation redshifts of the $\sim70$ BBH events reported in the third GW transient catalog GWTC-3 and the formation rate of BBH progenitors. For our default $\alpha_\tau=-1$ delay time distribution, we find that GWTC-3 contains at least one system (with 90\% credibility) that formed earlier than $z_\mathrm{form}>4.4$. Comparing our inferred BBH progenitor formation rate to the star formation rate (SFR), we find that at $z_\mathrm{form}=4$, the number of BBH progenitor systems formed per stellar mass was $6.4^{+9.4}_{-5.5}\times10^{-6}\,M_\odot^{-1}$ and this yield dropped to $0.134^{+1.6}_{-0.127}\times10^{-6}\,M_\odot^{-1}$ by $z_\mathrm{form}=0$. We discuss implications of this measurement for the cosmic metallicity evolution, finding that for typical assumptions about the metallicity-dependence of the BBH yield, the average metallicity at $z_\mathrm{form}=4$ was $\langle\log_{10}(Z/Z_\odot)\rangle=-0.3^{+0.3}_{-0.4}$, although the inferred metallicity can vary by a factor of $\approx3$ for different assumptions about the BBH yield. Our results highlight the promise of current GW observatories to probe high-redshift star formation.