Variation in δμ′ vs. z for simulations that include nebular (line and continuum) emission. As in Fig. 2g, the bulge (panel a) is approximated by a 4 Gyr old SED for a stellar population of solar metallicity that forms with an exponentially decreasing SFR with a τ of 0.5 and 1 Gyr (solid and dashed curve, respectively). As for the disk, we assume SEDs for a continuous SF scenario, one representing the case of moderately strong nebular emission (EW(Hα) ∼ 200 Å) in a 0.6 Gyr old stellar population of Z_⊙/5 (solid curves) and the second one of very intense nebular emission (EW(Hα) ∼ 10³ Å) excited by a 16 Myr old stellar population (dashed curve). These two cases are meant to approximate, respectively, intermediate and very early stages of the evolution of massive star-forming clumps emerging from VDIs (cf. the discussion in Papaderos et al. 2022) in high-z spiral galaxies (cf. Fig. 3). Whereas nebular emission has virtually no effect on the aging bulge, it strongly enhances the surface brightness of the disk at various broad redshift intervals where strong emission lines fall within various filter transmission curves. At z > 0.6 this effect is particularly strong in the NIR. Given the uncertain Lyα escape fraction, predictions in the U and B band should be considered only up to the redshift where the Lyα line (1216 Å) enters the blue edge of these filters (z ∼ 1.51 and ∼2.04, respectively). The layout is the same as in Fig. 2.