Common Envelope*. In this model we allow HG stars to be CE donors. When the donor initiates the CE phase, the CE outcome is determined via energy balance. The remaining physics is identical to the Standard model.

*3) Delayed SN*. This model utilizes the “Delayed” SN engine instead of the Rapid one. The former is also a convection driven, neutrino enhanced engine, but is sourced from the standing accretion shock instability (SASI), and can produce an explosion as late as $1\,\mbox{s}$ after bounce. The Delayed engine produces a continuous mass spectrum of compact objects, ranging from NSs through light BHs to massive BHs [@massgap].

*4) High BH kicks*. In this model the BHs receive full natal kicks, i.e. we set $f_{\rm fb}=0$ in Eq. (\[vkick\]). Otherwise this model is identical to the Standard model.

Metallicity evolution {#sec:metallicity}
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In this paper we employ two distinct metallicity evolution scenarios: “high-end” and “low-end”. These are identical to those in our previous study (Paper 2), and a detailed description can be found therein. Employing such calibrations allows us to explore and bracket uncertainties in the chemical evolution of the Universe. In both cases the average metallicity decreases with increasing redshift.

The high-end metallicity 