The Impact of Metallicity on the Rate of Type Ia Supernovae

The metallicity of a star strongly affects both its evolution and the properties of the stellar remnant that results from its demise. It is generally accepted that stars with initial masses below ∼ 8M⊙ leave behind white dwarfs and that some sub-population of these lead to Type Ia supernovae. However, it is often tacitly assumed that metallicity has no effect on the rate of SNe Ia. We propose that a consequence of the effects of metallicity is to significantly increase the SN Ia rate in lower-metallicity galaxies, in contrast to previous expectations. This is because lower-metallicity stars leave behind higher-mass white dwarfs, which should be easier to bring to explosion. We first model SN Ia rates in relation to galaxy masses and ages alone, finding that the elevation in the rate of SNe Ia in lower-mass galaxies measured by LOSS is readily explained. However, we then see that models incorporating this effect of metallicity agree just as well. Using the same parameters to estimate the cosmic SN Ia rate, we again find good agreement with data up to z ≈ 2. We suggest that this degeneracy warrants more detailed examination of host galaxy metallicities. We discuss additional implications, including for hosts of high-z SNe Ia, the SN Ia delay time distribution, super-Chandrasekhar SNe, and cosmology. Subject headings: binaries: close — white dwarfs — supernovae: general