The thermal history of the intergalactic medium

At redshifts z2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Lyα forest. This photoheated gas follows a tight temperature-density relation which introduces a cut-off in the distribution of widths of the Lyα absorption lines (b-parameters) as a function of column density. We have measured this cut-off in a sample of nine high resolution, high signal-to-noise quasar spectra, and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At redshift z ∼ 3, the temperature at the mean density shows a prominent peak and there is evidence that the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He ii. We also find that current models of the ionizing background from quasars predict too much absorption at z4, suggesting that galaxies might contribute significantly to the UV-background at these redshifts.