Who Ordered That? Unequal-mass Binary Black Hole Mergers Have Larger Effective Spins

Hierarchical analysis of binary black hole (BBH) detections by the Advanced LIGO and Virgo detectors has offered an increasingly clear picture of their mass, spin, and redshift distributions. Fully understanding the formation and evolution of BBH mergers will require not just the characterization of these marginal distributions, but the discovery of any correlations that exist between the properties of BBHs. Here, we hierarchically analyze the ensemble of BBHs discovered by LIGO and Virgo with a model that allows for intrinsic correlations between their mass ratios q and effective inspiral spins chi (eff). At 98.7% credibility, we find that the mean of the chi (eff) distribution varies as a function of q, such that more unequa-mass BBHs exhibit systematically larger chi (eff). We find a Bayesian odds ratio of 10.5 in favor of a model that allows for such a correlation over one that does not. Finally, we use simulated signals to verify that our results are robust against degeneracies in the measurements of q and chi (eff) for individual events. While many proposed astrophysical formation channels predict some degree correlation between spins and mass ratio, these predicted correlations typically act in an opposite sense to the trend we observationally identify in the data.