Gaussian Process Reconstruction of Reionization History
  We reconstruct the history of reionization using Gaussian process regression.
Using the UV luminosity data compilation from Hubble Frontiers Fields we
reconstruct the redshift evolution of UV luminosity density and thereby the
evolution of the source term in the ionization equation. This model-independent
reconstruction rules out single power-law evolution of the luminosity density
but supports the logarithmic double power-law parametrization. We obtain
reionization history by integrating ionization equations with the reconstructed
source term. Using optical depth constraint from Planck Cosmic Microwave
Background observation, measurement of UV luminosity function integrated till
truncation magnitude of -17 and -15, and derived ionization fraction from high
redshift quasar, galaxies and gamma-ray burst observations, we constrain the
history of reionization. In the conservative case we find the constraint on the
optical depth as $\tau =0.052\pm0.001\pm0.002$ at 68% and 95% confidence
intervals. We find the redshift duration between 10% and 90% ionization to be
$2.05_{-0.21-0.30}^{+0.11+0.37}$. Longer duration of reionization is supported
if UV luminosity density data with truncation magnitude of -15 is used in the
joint analysis. Our results point out that even in a conservative
reconstruction, a combination of cosmological and astrophysical observations
can provide stringent constraints on the epoch of reionization.
