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The high redshift Lyman-alpha forest, in particular the Gunn-Peterson trough,is the most unambiguous signature of the neutral to ionized transition of theintergalactic medium (IGM) taking place during the Epoch of Reionization (EoR).Recent studies, e.g. Kulkarni et al. (2019a) and Keating et al. (2019), showedthat reproducing the observed Lyman-alpha opacities after overlap required anon-monotonous evolution of cosmic emissivity: rising, peaking at z=6, and thendecreasing onwards to z=4. Such an evolution is puzzling considering galaxybuild-up and the cosmic star formation rate are still continously on the riseat these epochs. Here, we use new RAMSES-CUDATON simulations to show that sucha peaked evolution may occur naturally in a fully coupledradiation-hydrodynamical framework. In our fiducial run, cosmic emissivity atz>6 is dominated by a low mass (M$_{\rm DM}<2.10^9$ M$_{\odot}$), high escapefraction halo population, driving reionization, up to overlap. Approaching z=6,this population is radiatively suppressed due to the rising ionizing UVbackground, and its emissivity drops. In the meantime, the high mass halopopulation builds up and its emissivity rises, but not fast enough tocompensate the dimming of the low mass haloes, because of low escape fractions.The combined ionizing emissivity of these two populations therefore naturallyresults in a rise and fall of the cosmic emissivity, from z=12 to z=4, with apeak at z=6. An alternative run, which features higher escape fractions for thehigh mass haloes and later suppression at low mass, leads to overshooting theionizing rate, over-ionizing the IGM and therefore too low Lyman-alphaopacities.

Lyman-alpha opacities at z=4-6 require low mass, radiatively-suppressed galaxies to drive cosmic reionization