upper panel: predicted EW(Hα) (Å) as a function of time for a solar-metallicity stellar population with a Salpeter IMF forming instantaneously (black), continuously at a constant SFR (yellow), and with an exponentially decreasing SFR with e-folding times t_e of 1 Gyr and 3 Gyr (green and red curve, respectively). The light gray shaded area in the lower right part of the diagram encompasses the range in EW(Hα) predicted from pAGB photoionization for ages ≥0.1 Gyr, and the narrow dark strip depicts the corresponding prediction (EW(Hα) ≃ 1 Å ≡ ⟨ EW_⋆ ⟩) for the typical age of ETGs (≥6 Gyr). The lower panel shows the time evolution of the Ly_c photon rate expected by the instantaneous and exponentially decreasing SFR models, and the mass fraction in percent of stars older than 100 Myr (referred to as pAGB) is displayed separately in the lower right diagram.