in comparing these results, which examine the galaxy population as a whole, to those that distinguish between “passive” and “star-forming” galaxies (e.g., @Sullivan:2006ah).

The above is essentially based on an assumption of a single-degenerate scenario. While the full effects of metallicity on a double-degenerate scenario are likely more complicated, if we consider binaries with a uniform mass ratio distribution, the number that will exceed the Chandrasekhar limit depends on metallicity as $$N_{\rm Ia} \propto Z^{(x-1)\,b/a} \sim Z^{-0.4}\,,
\label{binz}$$ where $x=2.35$ is the slope of the IMF, $a=0.5$ approximates the slope of the white dwarf initial-final mass relation of @Kalirai:2007tq, and $b \simeq -0.08$ is the dependence of the final mass on metallicity, estimated from @Umeda:1998ij. The magnitude of the effect is very similar to the case already considered, so we do not repeat the calculations. This model does not include any effect of the higher implied masses on the rate of binary evolution or possible effects in triple systems based on the Kozai mechanism [@Thompson:2010dp].

To illustrate the above effect, we begin with the three known double WD binaries in which each component has a mass of at least $0.5\,M_\odot$ from @Nelemans:2005qb. These have primary/secondary masses of $0.71+0.55\, M_\odot$, $0.58+0.58\, 