\[section:v\]).

The results of using this approach are shown as the solid line in Fig. \[Iarate\], where we have used a $\Delta t_{\rm Gyr}^{-1}$ DTD with $\phi_*=1.4\times 10^{-3}\,(10^{10}\,M_\odot)^{-1}\,$yr$^{-1}$. A parametrization for this model is given in the Appendix. We see that the saturation in age at high masses results in a plateau, which should be a rather robust feature due to the relatively small scatter in estimated ages around $\lesssim 10$ Gyr in this range, while the decrease in age at lower mass results in a rise in the SN Ia rate. Overall, this simplified model agrees rather well with the LOSS data.

Incorporating Metallicity Dependence {#section:iv}
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Historically, studies have focussed upon deriving the DTD without taking into account the possible effects of stellar metallicity on the SN Ia rate in a galaxy, as we have done above. As we next show, this assumption may not be sufficient to determine the actual DTD and to extract its astrophysical origins. We distinguish here between metallicity effects as primary (those involved in the rate of explosions) and secondary (those affecting the detailed properties of individual explosions; e.g., @Timmes:2003xx), with our interest being in the former.

We now examine a plausible scenario 