Using numerical models for star clusters spanning a wide range in ages and metallicities ( { Z } ) we study the masses of binary black holes ( BBHs ) produced dynamically and merging in the local universe ( { z } \lesssim 0.2 ) .
After taking into account cosmological constraints on star-formation rate and metallicity evolution , which realistically relate merger delay times obtained from models with merger redshifts , we show here for the first time that while old , metal-poor globular clusters can naturally produce merging BBHs with heavier components , as observed in GW150914 , lower-mass BBHs like GW151226 are easily formed dynamically in younger , higher-metallicity clusters .
More specifically , we show that the mass of GW151226 is well within 1 \sigma of the mass distribution obtained from our models for clusters with { Z } / { { Z } _ { \odot } } \gtrsim 0.5 .
Indeed dynamical formation of a system like GW151226 likely requires a cluster that is younger and has a higher metallicity than typical Galactic globular clusters .
The LVT151012 system , if real , could have been created in any cluster with { Z } / { { Z } _ { \odot } } \lesssim 0.25 .
On the other hand , GW150914 is more massive ( beyond 1 \sigma ) than typical BBHs from even the lowest-metallicity ( { Z } / { { Z } _ { \odot } } = 0.005 ) clusters we consider , but is within 2 \sigma of the intrinsic mass distribution from our cluster models with { Z } / { { Z } _ { \odot } } \lesssim 0.05 ; of course detection biases also push the observed distributions towards higher masses .