Abstract
We use hydrodynamic simulations post-processed with the radiative-transfer
code RADAMESH to assess recent claims that the low HeII opacity observed in z>3
quasar spectra may be incompatible with models of HeII reionization driven by
the observed population of active galactic nuclei (AGNs). In particular,
building upon our previous work, we consider an early population of sources and
start the radiative-transfer calculation at redshifts z>=5. Our model
faithfully reproduces the emissivity of optically selected AGNs as inferred
from measurements of their luminosity function. We find that HeII reionization
is very extended in redshift (Delta z>=2) and highly spatially inhomogeneous.
In fact, mock spectra extracted from the simulations show a large variability
in the evolution of the HeII effective optical depth within chunks of size
Delta z=0.04. Regions with low opacity (\tau^eff_HeII<3) can be found at
high redshift, in agreement with the most recent observations of
UV-transmitting quasars. At the highest redshift currently probed by
observations (z~3.4), our updated model predicts a much lower HeII effective
optical depth than previous simulations in the literature relieving most of the
tension with the current data, that, however, still persists at about the
(Gaussian) 2\sigma level. Given the very small number of observed lines of
sight, our analysis indicates that current data cannot rule out a purely
AGN-driven scenario with high statistical significance.
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