New Constraints on IGM Thermal Evolution from the Lyα Forest Power Spectrum

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 alpha forest power spectrum to a suite of similar to 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 T-0 rises steadily from T-0 similar to 6000 K at z = 5.4, peaks at 14,000 K for z similar to 3.4, and decreases at lower redshift, reaching T-0 similar to 7000 K by z similar to 1.8. 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 less than or similar to 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 well-known degeneracy between the temperature at mean density T-0 and the slope of the temperature-density relation gamma that has plagued previous analyses. At the highest redshifts, z >= 5, we infer lower temperatures than expected from the standard picture of IGM thermal evolution leaving little room for additional smoothing of the Ly alpha forest by free streaming of warm dark matter.