2012MNRAS.422.3019C

We present a measurement of the Lyman α flux probability distribution function (PDF) obtained from a set of eight high-resolution quasar spectra with emission redshifts in the range 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 relatively fewer Lyman α absorption lines. This may be explained by an increase in the number of metal lines that are blended with Lyman α absorption lines towards higher redshift, but may also be due to the presence of fewer metals in the intergalactic medium (IGM) at earlier times. We also provide a new measurement of the redshift evolution of the effective optical depth, τ_eff, at 2.8 ≤ z ≤ 3.6, and find no evidence for a deviation from a power-law evolution in the log (τ_eff)-log (1 +z) plane. The flux PDF measurements are furthermore of interest for studies of the thermal state of the 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=T₀Δ^γ- 1^, with a temperature at the mean density of T₀= 19 250±4800 K and a slope γ= 0.90±0.21 (1σ uncertainties). In comparison, joint constraints with existing lower redshift PDF measurements at z < 3 favour an inverted temperature-density relation with T₀= 17 900±3500 K and γ= 0.70±0.12, in broad agreement with previous analyses.