Merging black hole binaries: the effects of progenitor's metallicity,
  mass-loss rate and Eddington factor
  The first four gravitational wave events detected by LIGO were all
interpreted as merging black hole binaries (BHBs), opening a new perspective on
the study of such systems. Here we use our new population-synthesis code MOBSE,
an upgraded version of BSE (Hurley et al. 2002), to investigate the demography
of merging BHBs. MOBSE includes metallicity-dependent prescriptions for mass
loss of massive hot stars. It also accounts for the impact of the
electron-scattering Eddington factor on mass loss. We perform >10^8 simulations
of isolated massive binaries, with 12 different metallicities, to study the
impact of mass loss, core-collapse supernovae and common envelope on merging
BHBs. Accounting for the dependence of stellar winds on the Eddington factor
leads to the formation of black holes (BHs) with mass up to 65 Msun at
metallicity Z~0.0002. However, most BHs in merging BHBs have masses <40 Msun.
We find merging BHBs with mass ratios in the 0.1 - 1.0 range, even if mass
ratios >0.6 are more likely. We predict that systems like GW150914, GW170814
and GW170104 can form only from progenitors with metallicity Z<=0.006, Z<=0.008
and Z<=0.012, respectively. Most merging BHBs have gone through a common
envelope phase, but up to ~17 per cent merging BHBs at low metallicity did not
undergo any common envelope phase. We find a much higher number of mergers from
metal-poor progenitors than from metal-rich ones: the number of BHB mergers per
unit mass is ~10^-4 Msun^-1 at low metallicity (Z = 0.0002 - 0.002) and drops
to ~10^-7 Msun^-1 at high metallicity (Z ~ 0.02).
