Globular cluster formation efficiencies from black-hole X-ray binary
  feedback
  We investigate a scenario in which feedback from black-hole X-ray binaries
(BHXBs) sometimes begins inside young star clusters before strong supernova
feedback. Those BHXBs could reduce the gas fraction inside embedded young
clusters whilst maintaining virial equilibrium, which may help globular
clusters (GCs) to stay bound when supernova-driven gas ejection subsequently
occurs. Adopting a simple toy model with parameters guided by BHXB population
models, we produce GC formation efficiencies consistent with
empirically-inferred values. The metallicity dependence of BHXB formation could
naturally explain why GC formation efficiency is higher at lower metallicity.
For reasonable assumptions about that metallicity dependence, our toy model can
produce a GC metallicity bimodality in some galaxies without a bimodality in
the field-star metallicity distribution.
