Diffuse gas in galaxies sheds new light on the origin of Type Ia supernovae

We measure the strength of He II λ4686 nebular emission in passively evolving (`retired') galaxies, aiming to constrain their populations of hot accreting white dwarfs (WDs) in the context of the single-degenerate (SD) scenario of Type Ia supernovae (SNe Ia). In the SD scenario, as a WD burns hydrogen-rich material accreted from a companion star, it becomes a powerful source of ionizing ultraviolet emission. If significant populations of such sources exist in galaxies, strong emission in the recombination lines of He II should be expected from the interstellar medium. To explore this conjecture, we select from the Sloan Digital Sky Survey ∼11 500 emission-line galaxies with stellar ages >1 Gyr showing no signs of active galactic nuclei activity and co-add their spectra in bins of stellar population age. For the first time, we detect He II λ4686 nebular emission in retired galaxies and find it to be significantly weaker than that expected in the SD scenario, especially in the youngest age bin (1-4 Gyr) where the SN Ia rate is the highest. Instead, the strength of the observed He II λ4686 nebular emission is consistent with post-asymptotic giant branch stars being the sole ionizing source in all age bins. These results limit populations of accreting WDs with photospheric temperatures (T_eff) in the range ∼(1.5-6) × 10⁵ K to the level at which they can account for no more than ∼5-10 per cent of the observed SN Ia rate. Conversely, should all WD progenitors of SN Ia go through the phase of steady nuclear burning with T_eff ∼ (1.5-6) × 10⁵ K, they do not increase their mass by more than ∼0.03 M_⊙ in this regime.