He ii recombination lines as a test of the nature of SN Ia progenitors in elliptical galaxies

To date, the question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, with the single and double degenerate scenarios remaining the leading contenders. The former implies a large population of hot accreting white dwarfs with photospheric temperatures of T similar to 10(5)-10(6) K during some part of their accretion history. We show that in early-type galaxies, a population of accreting white dwarfs large enough to reproduce the SN Ia rate would contribute significantly to the ionizing ultraviolet (UV) radiation expected from the stellar population. For mean stellar ages less than or similar to 5 Gyr, single degenerate progenitors would dominate the ionizing background produced by stars, increasing the continuum beyond the He ii-ionizing limit more than 10-fold. This opens a new avenue for constraining the progenitors of SNe Ia, through consideration of the spatially extended low-ionization emission-line regions now found in many early-type galaxies. Modelling the expected emission, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in E/S0 galaxies from upper limits on the luminosity of He ii recombination lines in the optical and far-ultraviolet (FUV). We discuss future directions, as well as possible implications for the evolution of SNe Ia in old stellar populations.