to, but not above, the threshold for explosion, so that the rate of massive mergers is further enhanced. The end state of the secondary resulting in a massive CO WD could then lead to a merger with total mass upwards of $\sim 2.5\,M_\odot$.

If, contrary to @Umeda:1998ij, the CO WD mass limit actually increases modestly for lower metallicities, then the rate of such extreme super-Chandrasekhar mergers rises dramatically at lower metallicities without a need to turn to binary evolution for a solution. This is due to the presence of the threshold in reaching the requisite total merger mass, which would lead to a large relative difference between low/high-$Z$ galaxies. The stars giving rise to these massive white dwarfs would also evolve more rapidly and could thus lead to “prompter” explosions in low-$Z$ environments. Whether this scenario occurs is a question left for stellar evolutionary modeling and observations of host galaxies.

Additionally, observations of Milky Way dwarf spheroidal galaxies have revealed decreasing values of \[$\alpha$/Fe\] with increasing \[Fe/H\], indicating the influence of Type Ia supernovae down to metallicities of \[Fe/H\] $\approx -2.5$ [@Kirby:2010dc]. Since model fits to these measurements are naturally sensitive to the SN Ia rate over a range of 