cutoff to be independent of $Z$ (and thus $z$) since the lifetimes of the stars giving the most massive CO WDs should be similar.

We use the same $\phi_*$ and $\Delta t_{\rm Gyr}^{-1}$ DTD as in Section \[section:iii\], with the resulting SN Ia rate history shown in Fig. \[Iahist\] (thick solid line). To compare with prior results (e.g., @Mannucci:2005xh [@Scannapieco:2005uh; @Sullivan:2006ah]), this history is broken down into the components with delay less than 1 Gyr (thin solid lines labelled as “prompt”) and greater than 1 Gyr (labelled as “delayed”). We see that the “prompt” component is subdominant at $z=0$, in agreement with the rates in Fig. \[Iarate\]. Altering either the form of the DTD or $t_c$ can make the “prompt” component relatively more or less important (see, e.g., @Horiuchi for related discussion); however, this would in turn affect the specific SN Ia rate models in Sections \[section:iii\] and \[section:iv\].

Since the universe as whole had a lower metallicity in the past, a relative enhancement should also be effected in the cosmic SN Ia rate. As existing rate measurements average over the entirety of the galaxy population, this effect should not be dramatic at the present epoch, but, as for the 