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We compare the event rate density detected by LIGO to the comoving numberdensity of isolated stellar progenitors and find a range for their formationredshift. 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 allBHs are in coalescing binaries, then enough progenitors have formed by $ 2< z_f< 9 $.

Formation Redshift of the Massive Black Holes Detected by LIGO