Schematic illustration of a high-z spiral galaxy in the early stages of its evolution. Massive SF clumps (blue) forming in the disk migrate inward and coalesce, promoting the growth of a high-stellar surface density Σ_⋆ protobulge (e.g., Noguchi 1999; Bournaud et al. 2007; Mandelker et al. 2014). The rest-frame EW(Hα) of SF clumps, initially on the order of 10³ Å (cf. Fig. A.7 and discussion in P22), decreases as they move inward, both because of their aging over the theoretically estimated migration timescale (τ_m = 0.6 − 1 Gyr) and because of the increasing line-of-sight dilution by the underlying stellar background. This leads to a radial anticorrelation between EW(Hα) and stellar surface density, similar to the case of the local BCD I Zw 18 (Papaderos et al. 2002, and PO12), as delineated in the right-hand panel.