A Measurement of the Temperature‐Density Relation in the Intergalactic Medium Using a New Lyα Absorption‐Line Fitting Method

The evolution of the temperature in the intergalactic medium is related tothe reionization of hydrogen and helium, and has important consequences for ourunderstanding of the Lya forest and of galaxy formation in gravitational modelsof large-scale structure. We measure the temperature-density relation ofintergalactic gas from Lya forest observations of eight quasar spectra withhigh resolution and signal-to-noise ratio, using a new line fitting techniqueto obtain a lower cutoff of the distribution of line widths from which thetemperature is derived. We carefully test the accuracy of this technique torecover the gas temperature with a hydrodynamic simulation. The temperature atredshift z=(3.9, 3.0, 2.4) is best determined at densities slightly above themean: T_star=(20200\pm2700, 20200\pm1300, 22600\pm1900)K (statistical errorbars) for gas density (in units of the mean density) Delta_star=(1.42\pm0.08,1.37\pm0.11, 1.66\pm0.11). The power-law index of the temperature-densityrelation, defined by T=T_star(Delta/Delta_star)^{gamma-1}, is gamma-1=(0.43\pm0.45, 0.29\pm0.30, 0.52\pm0.14) for the same three redshifts. Thetemperature at the fixed over-density Delta=1.4 is T_1.4=(20100\pm2800,20300\pm1400, 20700\pm1900)K. These temperatures are higher than expected forphotoionized gas in ionization equilibrium with a cosmic background, and can beexplained by a gradual additional heating due to on-going HeII reionization.The measurement of the temperature reduces one source of uncertainty in thelower limit to the baryon density implied by the observed mean flux decrement.We find that the temperature cannot be reliably measured for under-dense gas,because the velocities due to expansion always dominate the widths of thecorresponding weak lines.Comment: submitted to Ap