Dynamical Formation of Low-Mass Merging Black Hole Binaries like GW151226

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 ($z\lesssim0.2$). 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/Zsun$\gtrsim0.5$. 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/Zsun$\lesssim0.25$. On the other hand, GW150914 is more massive (beyond $1\sigma$) than typical BBHs from even the lowest-metallicity (Z/Zsun=0.005) clusters we consider, but is within $2\sigma$ of the intrinsic mass distribution from our cluster models with Z/Zsun$\lesssim0.05$; of course detection biases also push the observed distributions towards higher masses.