The origin of merging black holes

LIGO's discovery of gravitational waves from massive merging black hole binaries posed the fundamental question - what is the origin of these black holes? Two models have been proposed: field stellar binaries and capture. In the former, the binary was born as two massive stars. In this case, some level of alignment of the black holes spins with the orbital angular momentum is expected. In the latter, the two black holes evolve individually and were dynamically captured. The black holes' spins and the orbital angular momentum are not correlated and hence they are expected to be isotropically distributed. The effective spin, χ_eff, is probably the best parameter that can distinguish between the models. Recently, independent analysis of the LVC O1-O2 sample revealed, in addition to the original ten identified by LVC, eight new mergers. We present here a concise model for the spin evolution of field binaries and use it to estimate the expected χ_eff distribution. We compare this distribution as well as several isotropic distributions, reflecting capture scenarios, to the observations. While the current data slightly prefers field binaries, isotropic distributions or a combination of both origins are possible. Future detection in O3 and O4 of a few dozens to few hundred mergers will enable us to distinguish with sufficient statistical significance between the different models.