Radial distribution of the EW(Hα) and the light-weighted mean stellar age (⟨ log t_⋆ ⟩ _L), as predicted by the adopted inside-out galaxy formation scenario for 28 ages between 0 and 13.52 Gyr (cf. color coding on the right side bar).The region where the observed EW(Hα) is consistent with pure post-AGB photoionization (< 2.4 Å) is shown with the shaded gray area (see G15a). The model adopts a constant age gradient ∇t = −0.5 Gyr/kpc and outward propagating SF at v ~ 2 km s^-1. In this particular simulation the inside-out growth ceases when the galaxy reaches a radius of 20 kpc. The observational properties are computed for each zone assuming an exponentially declining SFR with an e-folding timescale of 1 Gyr.