Probing cosmic chemical enrichment with next-generation gravitational-wave observatories

By observing binary black hole (BBH) mergers out to the edge of the Universe, next-generation (XG) ground-based gravitational-wave detectors like Cosmic Explorer and Einstein Telescope will map the BBH merger rate across all of cosmic history. This merger rate traces the formation rate of their progenitor stars convolved with a delay time distribution. Given theoretically-motivated priors on the delay time distribution, we show how XG observations can measure the BBH progenitor formation rate, probing the star formation rate (SFR) up to z > 15. However, the progenitor formation rate does not directly give a measurement of the SFR, but rather a combination of the SFR and its metallicity distribution as a function of redshift. Fortunately, the metallicity-dependence of BBH formation likely varies as a function of BBH mass and/or formation channel. We find that if different BBH subpopulations with distinct metallicity biases can be identified, comparing their rates as a function of redshift yields a simultaneous measurement of the SFR and its metallicity distribution. Given optimistic theoretical priors and one year of observation, this may provide a measurement of the SFR at its peak and a 0.2 dex (0.7 dex) measurement of the median metallicity out to z = 10 (z = 15) at 90% credibility, although the uncertainties scale with theoretical uncertainties on BBH delay times and formation efficiencies.