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The LIGO/Virgo collaboration has reported 50 black hole-black hole (BH-BH)mergers and 8 candidates recovered from digging deeper into the detectorsnoise. The majority of these mergers have low effective spins pointing towardlow BH spins and efficient angular momentum transport in massive stars asproposed by several models (e.g., the Tayler-Spruit dynamo). However, out ofthese 58 mergers, 7 are consistent with having high effective spin parameter(chi_eff>0.3). Additionally, 2 events seem to have high effective spins sourcedfrom the spin of the primary (more massive) BH. These particular observationscould be used to discriminate between the isolated binary and dynamicalformation channels. It might seem that high BH spins point to a dynamicalorigin if angular momentum transport in stars is efficient and formslow-spinning BHs. In such a case dynamical formation is required to producesecond and third generations of BH-BH mergers with typically high-spinning BHs.Here we show, however, that isolated binary BH-BH formation also naturallyreproduces such highly spinning BHs. Our models start with efficient angularmomentum transport in massive stars that is needed to reproduce the majority ofBH-BH mergers with low effective spins. Later, some of the binaries are subjectto a tidal spin-up allowing the formation of a moderate fraction (~10%) ofBH-BH mergers with high effective spins (chi_eff>=0.4-0.5). In addition,isolated binary evolution can produce a small fraction of BH-BH mergers withalmost maximally spinning primary BHs. Therefore, the formation scenario ofthese atypical BH-BH mergers remains to be found.

The implications of high BH spins on the origin of BH-BH mergers