As in Fig. 4, but the color bar refers to the merger timescale of the two compact objects due to gravitational wave emissions. Black triangles refer to the systems whose merger timescales are longer than the present age of the Universe (~13.8 Gyr). z_formation refers to the redshift of the formation of the binary compact object, assuming that the merger took place at redshift ~0 and adopting the standard cosmological parameters (Planck Collaboration XIII 2016).