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 $\dot{\text{N}}_\text{Ia} / \text{M}_\star \sim \text{M}_\star ^{-0.3}$ 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 ~30 per cent of the increase in the specific SN Ia rate between stellar masses of M_⋆ = 10¹⁰ and 10^7.2 M_⊙. We find that an additional metallicity dependence of approximately ~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_⋆ ≈ 3 × 10⁹ M_⊙ are significantly affected by the metallicity-dependent SN Ia rates. The $\dot{\text{N}}_\text{Ia} / \text{M}_\star \sim \text{M}_\star ^{-0.3}$ scaling becomes shallower with increasing redshift, dropping by factor of ~2 at 10^7.2 M_⊙ between z = 0 and 1 with our ~Z^-0.5 scaling. With metallicity-independent rates, this decrease is a factor of ~3. We discuss the implications of metallicity-dependent SN Ia rates for one-zone models of galactic chemical evolution.