The recent detection of gravitational waves has proven the existence of massive stellar black hole binaries ( BBHs ) , but the formation channels of BBHs are still an open question .
Here , we investigate the demography of BBHs by using our new population-synthesis code MOBSE . MOBSE is an updated version of the widely used binary population-synthesis code BSE [ 6 , 7 ] and includes the key ingredients to determine the fate of massive stars : up-to-date stellar wind prescriptions and supernova models .
With MOBSE , we form BBHs with total mass up to \sim { } 120 M _ { \odot } at low metallicity , but only systems with total mass up to \sim { } 80 M _ { \odot } merge in less than a Hubble time .
Our results show that only massive metal-poor stars ( Z \lesssim 0.002 ) can be the progenitors of gravitational wave events like GW150914 .
Moreover , we predict that merging BBHs form much more efficiently from metal-poor than from metal-rich stars .