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

We determine the thermal evolution of the intergalactic medium (IGM) over $3\, \mathrm{Gyr}$ of cosmic time $1.8reionization of He II, as well as the subsequent cooling of the IGM due to the expansion of the Universe after all reionization events are complete. Our results are broadly consistent with previous measurements of thermal evolution based on a variety of approaches, but the sensitivity of the power spectrum, the combination of high precision BOSS measurements of large-scale modes ($k\lesssim 0.02\, \mathrm{s/km}$) 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 $z\geq5$ 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.