Binaries drive high Type Ia supernova rates in dwarf galaxies

The scaling of the specific Type Ia supernova (SN Ia) rate with host galaxy stellar mass (N) over dot(Ia)/M-star similar to M-star(-0.3) as measured in ASAS-SN and DES strongly suggests that the number of SNe Ia produced by a stellar population depends inversely on its metallicity. We estimate the strength of the required metallicity dependence by combining the average star formation histories (SFHs) of galaxies as a function of their stellar mass with the mass-metallicity relation (MZR) for galaxies and common parametrizations for the SN Ia delay-time distribution. The differences in SFHs can account for only similar to 30 per cent of the increase in the specific SN Ia rate between stellar masses of M-star = 10(10) and 10(7.2) M-circle dot. We find that an additional metallicity dependence of approximately similar to Z(-0.5) is required to explain the observed scaling. This scaling matches the metallicity dependence of the close binary fraction observed in APOGEE, suggesting that the enhanced SN Ia rate in low-mass galaxies can be explained by a combination of their more extended SFHs and a higher binary fraction due to their lower metallicities. Due to the shape of the MZR, only galaxies below M-star approximate to 3 x 10(9) M-circle dot are significantly affected by the metallicity-dependent SN Ia rates. The (N) over dot(Ia)/M-star similar to M-star(-0.3) scaling becomes shallower with increasing redshift, dropping by factor of similar to 2 at 10(7.2) M-circle dot between z = 0 and 1 with our similar to Z(-0.5) scaling. With metallicity-independent rates, this decrease is a factor of similar to 3. We discuss the implications of metallicity-dependent SN Ia rates for one-zone models of galactic chemical evolution.