"Birth kick" estimates of second-generation black holes observed by LIGO-Virgo: implications for formation channels and host environments

According to stellar formation and evolution models, black holes in the mass range of about 60-130 solar masses cannot form from stellar collapse. However, gravitational-wave detections, such as GW190521, display component black holes within this instability gap, challenging traditional models of stellar-origin black hole formation. Plausible scenarios, like hierarchical mergers in dense environments such as stellar clusters and active galactic nuclei, emerge to explain the presence of these high-mass black holes. Such scenarios, however, require the remnant black hole (later observed by our detectors) to inherit a gravitational recoil smaller than the escape velocity of the host environment.

 

Given the masses and spins of “parent” merging black holes, General Relativity allows us to predict the mass, spin and gravitational recoil of the remnant “child”. In this talk, I will wind back the clock. I will present estimates for the “parental” masses and spins of the component black holes of GW190521 and similar events and, crucially, for the corresponding “child” gravitational recoils. Then, I will show that the recoil estimate places strong constraints on the hierarchical-formation origin of the studied black holes and the corresponding environments.