State of the field: Binary black hole natal kicks and prospects for isolated field formation after GWTC-2

TL;DR

This paper investigates whether the observed spin misalignments in binary black hole mergers can be explained by isolated field formation with natal kicks, or if alternative formation channels are necessary.

Contribution

It demonstrates that explaining the observed spin misalignments solely through isolated binary evolution requires extreme natal kicks or non-zero black hole spins at birth, suggesting multiple formation channels.

Findings

Extreme natal kicks are needed if black holes are born with small spins.

Non-zero natal spins or alternative formation channels can better explain observations.

High velocity dispersions are required for CE formation to match the data.

Abstract

Advanced LIGO and Advanced Virgo's newly-released GWTC-2 catalog of gravitational-wave detections offers unprecedented information about the spin magnitudes and orientations of merging binary black holes (BBHs). Notably, analysis of the BBH population suggests the presence of binaries whose component spins are significantly misaligned with respect to their orbital angular momenta. Significantly misaligned spins are typically predicted to be at odds with isolated field formation via standard common envelope (CE) evolution, and hence a "smoking gun" signature of dynamical binary formation inside dense stellar clusters. Here, we explore whether the LIGO/Virgo observation of spin-orbit misalignment indeed rules out the possibility that BBHs are formed entirely in the field via standard CE evolution. In particular, we seek to understand whether, by varying the natal kicks black holes receive upon formation, we can invoke the CE scenario to self-consistently explain both the observed spin distribution and merger rate of BBHs. We find that, if isolated black holes are born with small natal spins, then BBHs formed through CE require extreme natal kicks to match the observed BBH population, with a velocity dispersion $\sigma = 9.7^{+26.7}_{-5.9}\times10^2\,\mathrm{km}\,\mathrm{s}^{-1}$ and $\sigma>260\,\mathrm{km}\,\mathrm{s}^{-1}$ at 99% credibility. To avoid this condition, we argue that it is necessary to assume that isolated black holes are born with non-vanishing natal spins, that one or more alternative channels contribute to the observed BBH population, and/or that other unforeseen mechanisms serve to yield large spin-orbit misalignment in the field.