The Goldilocks problem of the quasar contribution to reionization
  The detection of an unexpectedly large number of faint candidate quasars
(QSO) at $z \gtrsim 4$ has motivated Madau and Haardt to investigate a
reionization scenario in which active galactic nuclei dominate the photon
budget at all times. Their analytical study reveals that this picture is in
agreement with the evolution of the HI volume fraction in the intergalactic
medium (IGM) and the optical depth to Thomson scattering of the cosmic
microwave background. We employ a suite of hydrodynamical simulations
post-processed with a radiative transfer code to further investigate the
properties of such a reionization history. In particular, we generate synthetic
absorption-line spectra for HI and HeII that we compare with observational
data. Although we confirm the analytical results mentioned above, we also find
that the evolution of the IGM temperature and of the HeII optical depth are at
odds with current observational constraints. Nevertheless, the QSO-dominated
scenario presents the attractive feature that it naturally generates an
inhomogeneous IGM and thus produces an extended tail at high values in the
distribution function of the HI effective optical depth, in agreement with
recent observations. We find tentative evidence that considering some QSO
emission at redshift $z \gtrsim 5$ could reconcile such observations with
numerical simulations that have so far failed to reproduce this feature.
Finally, we show that collecting more HeII-absorption spectra at $z \gtrsim 3$
and studying their distinctive characteristics will be key to precisely
constraining the QSO contribution to reionization.
