Abstract
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, due to radiative suppression of star
formation. In our best matching run, cosmic emissivity at z>6 is dominated by a
low mass (M$_DM<2.10^9$ M$_ødot$), 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, low escape fraction,
halo population builds up and its emissivity rises, but not fast enough to
compensate the dimming of the low mass haloes. 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 a peak at z=6. An alternative
simulation, which features a later suppression and higher escape fractions for
the high mass haloes, leads to overshooting the ionizing rate, over-ionizing
the IGM and therefore too low Lyman-alpha opacities.
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