Temperature fluctuations in the intergalactic medium

The temperature of the low‐density intergalactic medium (IGM) is set by the balance between adiabatic cooling resulting from the expansion of the Universe, and photoheating by the ultraviolet (UV) background. We have analysed the Ly α forest of 11 high‐resolution quasar spectra using wavelets, and find strong evidence of a marked jump in the temperature at the mean density, T 0 , of 60±14 per cent over the redshift interval z =[3.5,3.1] , which we attribute to reionization of He  ii . 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 photoheating by a UV background from quasi‐stellar objects (QSOs) and galaxies. We find no evidence of thermal fluctuations on scales 5000 km s −1 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/ T 0 , and calibrate the relation with hydrodynamical simulations. We find T 0 1.2×10 4  K at z 3.6 . Such high temperatures suggest that H  i reionization occurred relatively recently.