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