Bayesian analysis of the spin distribution of LIGO/Virgo black holes

Gravitational wave detection from binary black hole (BBH) inspirals has become routine thanks to the LIGO/Virgo interferometers. The nature of these black holes remains uncertain. We study here the spin distributions of LIGO/Virgo black holes from the first catalog GWTC-1 and the first four published BBH events from run O3. We compute the Bayes evidence for several independent priors: flat, isotropic, spin-aligned and anti-aligned. We find strong evidence for low spins in all of the cases, and significant evidence for small isotropic spins versus any other distribution. When considered as a homogeneous population of black holes, these results give support to the idea that LIGO/Virgo black holes are primordial.