  The recent LIGO detection of gravitational waves (GW150914), likely
originating from the merger of two $\sim 30 M_\odot$ black holes suggests
progenitor stars of low metallicity ($[Z/Z_\odot] \lesssim 0.3$), constraining
when and where the progenitor of GW150914 may have formed. We combine estimates
of galaxy properties (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, $M_\mathrm{gal}$, and the formation
redshift of the progenitor system $z_\mathrm{form}$ for different progenitor
metallicities $Z_\mathrm{c}$. At $Z_\mathrm{c}=0.1 Z_\odot$, the signal is
dominated by binaries in massive galaxies with $z_\mathrm{form}\simeq 2$, with
a small contribution from binaries formed around $z_\mathrm{form}\simeq 0.5$ in
dwarf galaxies. For $Z_\mathrm{c}=0.01Z_\odot$, fast mergers are possible and
very recent star formation in dwarfs likely dominates. Additional gravitational
wave detections from merging massive black holes will provide constraints on
the mass-metallicity relation and massive star formation at high redshifts.
