Evidence for the evolution of black hole mass function with redshift

We investigate the joint primary mass, mass ratio, and redshift observed distribution of astrophysical black holes using the gravitational wave events detected by the LIGO-Virgo-KAGRA collaboration and included in the third gravitational wave transient catalogue. We reconstruct this distribution using Bayesian non-parametric methods, which are data-driven models able to infer arbitrary probability densities under minimal mathematical assumptions. We find evidence for the evolution with redshift of both the primary mass and mass ratio distribution: our analysis shows the presence of two distinct sub-populations in the primary mass - redshift plane, with the lighter population, $\lesssim$ 20 $M_\odot$, disappearing at higher redshifts, $z > 0.4$. The mass ratio distribution shows no support for symmetric binaries. The observed population of coalescing binary black holes evolves with look-back time, suggesting a trend in metallicity with redshift and/or the presence of multiple, redshift-dependent formation channels.