Detection of extended He ii reionization in the temperature evolution of the intergalactic medium★

Abstract

We present new measurements of the temperature of the intergalactic medium\n(IGM) derived from the Lyman-alpha forest over 2.0 < z < 4.8. The small-scale\nstructure in the forest of 61 high-resolution QSO spectra is quantified using a\nnew statistic, the curvature, and the conversion to temperature calibrated\nusing a suite of hydrodynamic simulations. At each redshift we focus on\nobtaining the temperature at an optimal overdensity probed by the Lyman-alpha\nforest, T(Delta), where the temperature is nearly a one-to-one function of the\ncurvature regardless of the slope of the temperature-density relation. The\nmedian 2-sigma statistical uncertainty in these measurements is 8 per cent,\nthough there may be comparable systematic errors due to the unknown amount of\nJeans smoothing in the IGM. We use our T(Delta) results to infer the\ntemperature at the mean density, T0. Even for a maximally steep\ntemperature-density relation, T0 must increase from ~8000 K at z ~ 4.4 to\n>~12000 K at z ~ 2.8. This increase is not consistent with the monotonic\ndecline in T0 expected in the absence of He II reionization. We therefore\ninterpret the observed rise in temperature as evidence of He II reionization\nbeginning at z >~ 4.4. The evolution of T0 is consistent with an end to He II\nreionization at z ~ 3, as suggested by opacity measurements of the He II\nLyman-alpha forest, although the redshift at which T0 peaks will depend\nsomewhat on the evolution of the temperature-density relation. These new\ntemperature measurements suggest that the heat input due to the reionization of\nHe II dominates the thermal balance of the IGM over an extended period with\nDelta_z >~ 1.\n