Explaining LIGO's observations via isolated binary evolution with natal kicks

TL;DR

This paper compares binary evolution models with different black-hole natal kick assumptions to LIGO's gravitational-wave data, estimating kick velocities and spin distributions that best match observations.

Contribution

It introduces a method to estimate black-hole natal kicks and spins by comparing models with LIGO data, exploring a range of spin and kick assumptions.

Findings

Black holes likely receive natal kicks around 200 km/s with spin realignment.

Modest kicks are necessary to produce observed spin-orbit misalignments.

Observations favor low black-hole natal spins across models.

Abstract

We compare binary evolution models with different assumptions about black-hole natal kicks to the first gravitational-wave observations performed by the LIGO detectors. Our comparisons attempt to reconcile merger rate, masses, spins, and spin-orbit misalignments of all current observations with state-of-the-art formation scenarios of binary black holes formed in isolation. We estimate that black holes (BHs) should receive natal kicks at birth of the order of $\sigma\simeq 200$ (50) km/s if tidal processes do (not) realign stellar spins. Our estimate is driven by two simple factors. The natal kick dispersion $\sigma$ is bounded from above because large kicks disrupt too many binaries (reducing the merger rate below the observed value). Conversely, the natal kick distribution is bounded from below because modest kicks are needed to produce a range of spin-orbit misalignments. A distribution of misalignments increases our models' compatibility with LIGO's observations, if all BHs are likely to have natal spins. Unlike related work which adopts a concrete BH natal spin prescription, we explore a range of possible BH natal spin distributions. Within the context of our models, for all of the choices of $\sigma$ used here and within the context of one simple fiducial parameterized spin distribution, observations favor low BH natal spin.