Are LIGO-Virgo-KAGRA's black holes made from smaller black holes? Limits from binary black hole effective spin observations

It has been proposed that some black holes in binary black hole systems are born from “hierarchical mergers;" i.e. earlier mergers of smaller black holes. These hierarchical merger products have spin magnitudes χ~0.7, and, if they are dynamically assembled into binary systems, their spin orientations will be sometimes anti-aligned with the binary orbital angular momentum. In fact, ~16% of binary black holes systems that include hierarchical merger products will have an effective inspiral spin parameter, χ_eff < -0.3. Nevertheless, the LIGO-Virgo-KAGRA gravitational-wave detectors have yet to observe a binary black hole system with χ_eff < -0.2. The absence of observed binary black holes with large, misaligned spins automatically limits how many black holes are made from smaller black holes. I will discuss implications for the formation of the most massive black holes.