When and where did GW150914 form?

The recent LIGO detection of gravitational waves (GW150914), likely originating from the merger of two ∼ 30M_⊙ black holes suggests progenitor stars of low metallicity ([Z/Z_⊙] ≲ 0.3), constraining when and where the progenitor of GW150914 may have formed. We combine estimates of galaxy properties (star-forming gas metallicity, star formation rate and merger rate) across cosmic time to predict the low redshift black hole – black hole merger rate as a function of present day host galaxy mass, M_(gal), the formation redshift of the progenitor system z_f and different progenitor metallicities Z_p. For Z_p ⩾ 0.1Z_⊙, the signal is dominated by binaries in massive galaxies with z_f ≃ 2 while below Z_p ⩽ 0.1Z_⊙ most mergers come from binaries formed around z_f ≃ 0.5 in dwarf galaxies. Additional gravitational wave detections from merging massive black holes will provide constraints on the mass–metallicity relation and massive star formation at high redshifts.