A consistent and robust measurement of the thermal state of the IGM at 2 ≤ z ≤ 4 from a large sample of  Ly α forest spectra: evidence for late and rapid He ii reionization

ABSTRACT We characterize the thermal state of the intergalactic medium (IGM) in 10 redshift bins in the range 2 ≤ z ≤ 4 with a sample of 103 high-resolution, high S/N Ly α forest spectra using four different flux distribution statistics. Our measurements are calibrated with mock spectra from a large suite of hydrodynamical simulations post-processed with our thermal IGM evolution code cite, finely sampling amplitude, and slope of the expected temperature–density relation. The thermal parameters inferred from our measurements of the flux power spectrum, Doppler parameter distribution, as well as wavelet and curvature statistics agree well within their respective errors and all clearly show the peak in temperature and minimum in slope of the temperature density relation expected from He ii reionization. Combining our measurements from the different flux statistics gives T0 = (14 750 ± 1322) K for the peak temperature at mean density and a corresponding minimum slope γ = 1.225 ± 0.120. The peak in the temperature evolution occurs around z ≈ 3, in agreement with previous measurements that had suggested the presence of such a peak, albeit with a large scatter. Using cite, we also calculate the thermal state of the IGM predicted by five widely used (spatially homogeneous) UV-background models. The rather rapid thermal evolution inferred by our measurements is well reproduced by two of the models, if we assume (physically well motivated) non-equilibrium evolution with photoheating rates that are reduced by a moderate factor of ∼0.7–0.8. The other three models predict He ii reionization to be more extended with a higher temperature peak occurring somewhat earlier than our measurements suggest.