The Lyman alpha forest flux probability distribution at z>3
  We present a measurement of the Lyman alpha flux probability distribution
function (PDF) measured from a set of eight high resolution quasar spectra with
emission redshifts at 3.3 < z < 3.8. We carefully study the effect of metal
absorption lines on the shape of the PDF. Metals have a larger impact on the
PDF measurements at lower redshift, where there are fewer Lyman alpha
absorption lines. This may be explained by an increase in the number of metal
lines which are blended with Lyman alpha absorption lines toward higher
redshift, but may also be due to the presence of fewer metals in the
intergalactic medium with increasing lookback time. We also provide a new
measurement of the redshift evolution of the effective optical depth, tau_eff,
at 2.8 < z < 3.6, and find no evidence for a deviation from a power law
evolution in the log(tau_eff)-log(1+z) plane. The flux PDF measurements are
furthermore of interest for studies of the thermal state of the intergalactic
medium (IGM) at z ~ 3 . By comparing the PDF to state-of-the-art cosmological
hydrodynamical simulations, we place constraints on the temperature of the IGM
and compare our results with previous measurements of the PDF at lower
redshift. At redshift z=3, our new PDF measurements are consistent with an
isothermal temperature-density relation, T=T_0 Delta^{gamma-1}, with a
temperature at the mean density of T_0 = 19250 +/- 4800 K and a slope
gamma=0.90+/-0.21 (1 sigma uncertainties). In comparison, joint constraints
with previous PDF measurements at z<3 favour an inverted (gamma<1)
temperature-density relation with T_0=17900 +/- 3500 K and gamma=0.70 +/- 0.12,
in broad agreement with previous analyses.
