The thermal history of the intergalactic medium

At redshifts z greater than or similar to2, most of the baryons reside in t he smooth intergalactic medium which is responsible for the low column dens ity Ly alpha forest. This photoheated gas follows a tight temperature-densi ty relation which introduces a cut-off in the distribution of widths of the Ly alpha absorption lines (b-parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high sig nal-to-noise ratio quasar spectra and determined the thermal evolution of t he intergalactic medium in the redshift range 2.0-4.5. At a redshift z simi lar to3, the temperature at the mean density shows a peak and the gas becom es nearly isothermal. We interpret this as evidence for the reionization of He II.