Temperature fluctuations in the intergalactic medium ⋆ †

The temperature of the low-density intergalactic medium is set by the balance between adiabatic cooling resulting from the expansion of the universe, and photo-heating by the UV-background. We have analysed the Lyα forest of eleven high-resolution quasar spectra using wavelets, and find strong evidence of a marked jump in the temperature at the mean density, T0, of 60 ± 14 per cent over the redshift interval z = [3.5, 3.1], which we attribute to reionization of Heii . The jump can be seen in all three of our spectra that straddle redshift 3.3, at a significance of ≥ 99 per cent. Below z ∼ 3.1, our results are consistent with a smooth cooling down of the universe, as expected when adiabatic expansion dominates over photo-heating by a UV-background from QSOs and galaxies. We find no evidence of thermal fluctuations on scales ≥ 5000 km s larger than 50 per cent, which could be detected by our method, suggesting that the IGM follows a reasonably well-defined temperature-density relation. We demonstrate that the mean wavelet amplitude 〈A〉 ∝ 1/T0 and calibrate the relation with hydrodynamical simulations. We find T0 ≥ 1.2 × 10 K at z ≥ 3.6. Such high temperature suggest that Hi reionization occured relatively recent.