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.
