BPASS predictions for Binary Black-Hole Mergers
  Using the Binary Population and Spectral Synthesis code BPASS, we have
calculated the rates, timescales and mass distributions for binary black hole
mergers as a function of metallicity. We consider these in the context of the
recently reported 1st LIGO event detection. We find that the event has a very
low probability of arising from a stellar population with initial metallicity
mass fraction above Z=0.010 (Z>0.5Zsun). Binary black hole merger events with
the reported masses are most likely in populations below 0.008 (Z<0.4Zsun).
Events of this kind can occur at all stellar population ages from ~3 Myr up to
the age of the universe, but constitute only 0.1 to 0.4 per cent of binary BH
mergers between metallicities of Z=0.001 to 0.008. However at metallicity
Z=0.0001, 26 per cent of binary BH mergers would be expected to have the
reported masses. At this metallicity the progenitor merger times can be close
to ~10Gyr and rotationally-mixed stars evolving through quasi-homogeneous
evolution, due to mass transfer in a binary, dominate the rate. The masses
inferred for the black holes in the binary progenitor of GW\,150914 are amongst
the most massive expected at anything but the lowest metallicities in our
models. We discuss the implications of our analysis for the electromagnetic
follow-up of future LIGO event detections.
