The Hell Lyman alpha forest and the thermal state of the IGM

Abstract

Aims. Recent analyses of the intergalactic UV background by means of the He II Lyα forest assume that He II and H I absorption features have the same line widths. We omit this assumption to investigate possible effects of thermal line broadening on the inferred He II/H I ratio η and to explore the potential of intergalactic He II observations to constrain the thermal state of the intergalactic medium. Methods. Deriving a simple relation between the column density and the temperature of an absorber based on the temperature-density relation T = T0(1 + δ)γ-1 we develop a procedure to fit T0, γ, and η simultaneously by modeling the observed spectra with Doppler profiles. In an alternative approach the temperature T of an absorber, the He II/H I ratio η, and the redshift scale of η variations are estimated simultaneously by optimizing the Doppler parameters of He II. Results. Testing our procedure with artificial data shows that well-constrained results can be obtained only if the signal-to-noise ratio in the He II forest is S/N ≳ 20. Additionally, ambiguities in the line profile decomposition may result in significant systematic errors. Thus, it is impossible to give an estimate of the temperature-density relation with the Hell data available at present (S/N ∼ 5). However, we find that only 45% of the lines in our sample favor turbulent line widths while the remaining lines are probably affected by thermal broadening. Furthermore, the inferred η values are on average about 0.05 dex larger if a thermal component is taken into account, and their distribution is 46% narrower in comparison to a purely turbulent fit. Therefore, variations of η on a 10% level may be related to the presence of thermal line broadening. The apparent correlation between the strength of the H I absorption and the η value, which has been found in former studies assuming turbulent line broadening, essentially disappears if thermal broadening is taken into account. In the redshift range 2.58 < z < 2.74 towards the quasars HE 2347-4342 and HS 1700+6416 we obtain η ≈ 100 and slightly larger. In the same redshift range the far-UV spectrum of HS 1700+6416 is best and we estimate a mean value of log η = 2.11 ± 0.32 taking into account combined thermal and turbulent broadening. © ESO 2007.