Lawrence Berkeley National Laboratory

We determine the thermal evolution of the intergalactic medium (IGM) over 3 Gyr of cosmic time 1.8 < z < 5.4 by comparing measurements of the Ly forest power spectrum to a suite of 70 hydrodynamical simulations. We conduct Bayesian inference of IGM thermal parameters using an end-to-end forward modeling framework whereby mock spectra generated from our simulation grid are used to build a custom emulator that interpolates the power spectrum between thermal grid points. The temperature at mean density T0 rises steadily from T0 ∼ 6000 K at z=5.4, peaks at 14,000 K for z3.4, and decreases at lower redshift, reaching T07000 K by z1.8. This evolution provides conclusive evidence for photoionization heating resulting from the reionization of He II, as well as the subsequent cooling of the IGM due to the expansion of the universe after all reionization events are complete. Our results are broadly consistent with previous measurements of thermal evolution based on a variety of approaches, but the sensitivity of the power spectrum, the combination of high-precision measurements of largescale modes (k 0.02 s km-1) from the Baryon Oscillation Spectroscopic Survey with our recent determination of the small-scale power, our large grid of models, and our careful statistical analysis allow us to break the wellknown degeneracy between the temperature at mean density T0 and the slope of the temperature-density relation that has plagued previous analyses. At the highest redshifts, z5, we infer lower temperatures than expected from the standard picture of IGM thermal evolution leaving little room for additional smoothing of the Ly forest by free streaming of warm dark matter.