Understanding how fast black holes spin by analyzing data from the second gravitational-wave catalogue

The Advanced LIGO and Virgo detectors have now observed approximately 50 black hole binary mergers, from which we can begin to infer how rapidly astrophysical black holes spin. The LIGO-Virgo Collaboration (LVC) analysis of detections up to the end of the first half of the third observing run (O3a) appeared to uncover a distribution of spin magnitudes that peaks at ∼0.2. This is surprising: is there a black hole formation mechanism that prefers a particular, nonzero spin magnitude, or could this be the cumulative effect of multiple formation processes? We perform an independent analysis of the most recent gravitational-wave (GW) catalog, and find that (a) the support for the LVC spin magnitude is tenuous; in particular, adding or removing just one signal from the catalog can remove the statistical preference for this distribution; and (b) we find potential evidence for two spin subpopulations in the observed black holes: one with extremely low spins and one with larger spin magnitudes. We make the connection that these spin subpopulations could be correlated with the mass of the binary, with more massive binaries preferring larger spin magnitudes, and argue that this may provide evidence for hierarchical mergers in the second GW catalog