The Low Effective Spin of Binary Black Holes and Implications for Individual Gravitational-wave Events

While the Advanced LIGO and Virgo gravitational-wave (GW) experiments now regularly observe binary black hole (BBH) mergers, the evolutionary origin of these events remains a mystery. Analysis of the BBH spin distribution may shed light on this mystery, offering a means of discriminating between different binary formation channels. Using the data from Advanced LIGO and Virgo's first and second observing runs, here we seek to carefully characterize the distribution of effective spin chi(eff) among BBHs, hierarchically measuring the distribution's mean mu and variance sigma(2) while accounting for selection effects and degeneracies between spin and other black hole parameters. We demonstrate that the known population of BBHs have spins that are both small, with mu approximate to 0, and very narrowly distributed, with sigma(2) <= 0.07 at 95% credibility. We then explore what these ensemble properties imply about the spins of individual BBH mergers, reanalyzing existing GW events with a population-informed prior on their effective spin. Under this analysis, the BBH GW170729, which previously excluded chi(eff) = 0, is now consistent with zero effective spin at similar to 10% credibility. More broadly, we find that uninformative spin priors generally yield overestimates for the effective spin magnitudes of compact binary mergers.