We compare the event rate density detected by LIGO to the comoving number density of isolated stellar progenitors and find a range for their formation redshift. Our limit depends on the threshold mass for making the black holes (BHs) but only weakly on the metallicity of their progenitor. If $10 %$ of all BHs are in coalescing binaries, then enough progenitors have formed by $ 2< z_f < 9 $.
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