roughly takes a $\Delta t^{-1}=(t-t^\prime)^{-1}$ form (e.g., @Totani:2008by [@Maoz:2010qm; @Maoz:2010dw]). The physics behind this relation remains unclear, although such a distribution may naturally result from binary mergers (see, e.g., @Ruiter:2009dk) or a single-degenerate scenario [@Hachisu:2008zw].

@Gallazzi:2005df also derive $r$-band light-weighted galaxy ages, which vary from $\sim 10^9$ yr at $10^9\,M_\odot$ to $\sim 10^{10}$ yr at $10^{12}\,M_\odot$ (see their Fig. 8), using galactic models with an exponentially declining star formation history from a time $t_{\rm form}$ with subsequent random bursts. Ideally, one would have at hand the detailed history of star formation in every galaxy. This is understandably difficult to achieve with any certainty. Attempts have been made in this direction (e.g., @Brandt:2010jn [@Maoz:2010dw]); however, using what amounts to an average over the galaxy population should be suitable for comparison with global rates. We will use both of the @Gallazzi:2005df relations in estimating the expected SN Ia rate in galaxies of varying mass.

A Simple Galactic Rate Model {#section:iii}
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We first attempt to explain the rate variations in galaxies of different mass as due to an age effect alone. If the @Gallazzi:2005df ages corresponded to a single-age stellar population at a given galactic mass, deriving the expected SN Ia rate 