Binary black holes ( BBHs ) detected by gravitational wave ( GW ) observations could be broadly divided into two formation channels : those formed through field binary evolution and those assembled dynamically in dense stellar systems .
Each of these formation channels , and their sub-channels , populate a distinct region in the effective spin-mass ( \chi _ { eff } - M ) plane .
Depending on the branching ratio of different channels , an ensemble of BBHs could show a trend in this plane .
Here we fit a mass-dependent distribution for \chi _ { eff } to the GWTC-1 BBHs from the first and second observing runs of Advanced LIGO and Advanced Virgo .
We find a negative correlation between mass and the mean effective spin ( \bar { \chi } _ { \mathrm { eff } } ) , and positive correlation with its dispersion ( \sigma _ { \chi _ { \mathrm { eff } } } ) at 75 % and 80 % confidence .
This trend is robust against the choice of mass variable , but most pronounced when the mass variable is taken to be the chirp mass of the binary .
The result is consistent with significant contributions from both dynamically assembled and field binaries in the GWTC-1 catalog .
The upcoming LIGO O3a data release will critically test this interpretation .