The SN Ia rate normalized to the galaxy stellar mass as a function of time as predicted by our best models (M_inf = 10⁹ M_⊙). The solid red, dash-dotted black, long-dashed magenta and short-dashed blue lines are for models M9b1, M9b2, M9b3, and M9b4, respectively. As a comparison, the shaded area shows the observed range of normalized SN Ia rate in the Irr galaxies, per century and per 10¹⁰ M_⊙ (Mannucci et al. 2005).