The Implications of High Black Hole Spins for the Origin of Binary Black Hole Mergers

The LIGO–Virgo collaboration has reported 50 black hole–black hole (BH–BH) mergers and 8 candidates recovered from digging deeper into the detector noise. The majority of these mergers have low effective spins pointing toward low BH spins and efficient angular momentum (AM) transport in massive stars as proposed by several models (e.g., the Tayler–Spruit dynamo). However, out of these 58 mergers, 7 are consistent with having high effective-spin parameter (χeff > 0.3). Additionally, two events seem to have high effective spins sourced from the spin of the primary (more massive) BH. These particular observations could be used to discriminate between the isolated binary and dynamical formation channels. It might seem that high BH spins point to a dynamical origin if AM in stars is efficient and forms low-spinning BHs. In such a case dynamical formation is required to produce second and third generations of BH–BH mergers with typically high spinning BHs. Here we show, however, that isolated binary BH–BH formation naturally reproduces such highly spinning BHs. Our models start with efficient AM in massive stars that is needed to reproduce the majority of BH–BH mergers with low effective spins. Later, some of the binaries are subject to a tidal spin-up allowing the formation of a moderate fraction (∼10%) of BH–BH mergers with high effective spins (χeff ≳ 0.4–0.5). In addition, isolated binary evolution can produce a small fraction of BH–BH mergers with almost maximally spinning primary BHs. Therefore, the formation scenario of these atypical BH–BH mergers remains to be found.