We present a new investigation of the thermal history of the intergalactic medium (IGM) during and after reionization using the Lyman-α forest flux power spectrum at 4.0 z 5.2. Using a sample of 15 highresolution spectra, we measure the flux power down to the smallest scales ever probed at these redshifts (−1 log(k/km −1 s) −0.7). These scales are highly sensitive to both the instantaneous temperature of the IGM and the total energy injected per unit mass during and after reionization. We measure temperatures at the mean density of T 0 ∼ 7000-8000 K, consistent with no significant temperature evolution for redshifts 4.2 z 5.0. We also present the first observational constraints on the integrated IGM thermal history, finding that the total energy input per unit mass increases from u 0 ∼ 4.6 eV m −1 p to 7.3 eV m −1 p from z ∼ 6 to 4.2 assuming a Λ-CDM cosmology. We show how these results can be used simultaneously to obtain information on the timing and the sources of the reionization process. Our first proof of concept using simplistic models of instantaneous reionization produces results comparable to and consistent with the recent Planck constraints, favoring models with z rei ∼ 8.5 +1.1 −0.8 .

According to the photo-heating model of the intergalactic medium (IGM), He II reionization is expected to affect its thermal evolution. Evidence for additional energy injection into the IGM has been found at 3 z 4, though the evidence for the subsequent fall-off below z ∼ 2.8 is weaker and depends on the slope of the temperature-density relation, γ. Here we present, for the first time, an extension of the IGM temperature measurements down to the atmospheric cut-off of the H I Lyman-α forest at z ≃ 1.5. Applying the curvature method on a sample of 60 UVES spectra we investigated the thermal history of the IGM at z < 3 with precision comparable to the higher redshift results. We find that the temperature of the cosmic gas traced by the Ly-α forest [T (∆)] increases for increasing overdensity from T (∆) ∼ 22670 K to 33740 K in the redshift range z ∼ 2.8 − 1.6. Under the assumption of two reasonable values for γ, the temperature at the mean density (T 0 ) shows a tendency to flatten at z 2.8. In the case of γ ∼ 1.5, our results are consistent with previous ones which indicate a falling T 0 for redshifts z 2.8. Finally, our T (∆) values show reasonable agreement with moderate blazar heating models.

We compare a sample of five high-resolution, high S/N Lyα forest spectra of bright 6 < z <∼ 6.5 QSOs aimed at spectrally resolving the last remaining transmission spikes at z > 5 with those obtained from mock absorption spectra from the Sherwood and Sherwood-Relics suites of hydrodynamical simulations of the intergalactic medium (IGM). We use a profile fitting procedure for the inverted transmitted flux, 1−F , similar to the widely used Voigt profile fitting of the transmitted flux F at lower redshifts, to characterise the transmission spikes that probe predominately underdense regions of the IGM. We are able to reproduce the width and height distributions of the transmission spikes, both with optically thin simulations of the post-reionization Universe using a homogeneous UV background and full radiative transfer simulations of a late reionization model. We find that the width of the fitted components of the simulated transmission spikes is very sensitive to the instantaneous temperature of the reionized IGM. The internal structures of the spikes are more prominent in low temperature models of the IGM. The width distribution of the observed transmission spikes, which require high spectral resolution (≤ 8 km s −1 ) to be resolved, is reproduced for optically thin simulations with a temperature at mean density of T 0 = (11000 ± 1600, 10500 ± 2100, 12000 ± 2200) K at z = (5.4, 5.6, 5.8). This is weakly dependent on the slope of the temperature-density relation, which is favored to be moderately steeper than isothermal. In the inhomogeneous, late reionization, full radiative transfer simulations where islands of neutral hydrogen persist to z ∼ 5.3, the width distribution of the observed transmission spikes is consistent with the range of T 0 caused by spatial fluctuations in the temperature-density relation.