When and where did GW150914 form?

Abstract 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, Mgal, the formation redshift of the progenitor system zf and different progenitor metallicities Zp. For Zₚ 0. 1 Z, the signal is dominated by binaries in massive galaxies with zf ≃ 2 while below Zₚ 0. 1 Z most mergers come from binaries formed around zf ≃ 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.