A New Measurement of the Temperature–density Relation of the IGM from Voigt Profile Fitting

We decompose the Lyman- α (Ly α ) forest of an extensive sample of 75 high signal-to-noise ratio and high-resolution quasar spectra into a collection of Voigt profiles. Absorbers located near caustics in the peculiar velocity field have the smallest Doppler parameters, resulting in a low- b cutoff in the b – distribution set primarily by the thermal state of the intergalactic medium (IGM). We fit this cutoff as a function of redshift over the range , which allows us to measure the evolution of the IGM temperature–density ( ) relation parameters T 0 and γ . We calibrate our measurements against mock Ly α forest data generated using 26 hydrodynamic simulations with different thermal histories from the THERMAL suite, also encompassing different values of the IGM pressure smoothing scale. We adopt a forward-modeling approach and self-consistently apply the same algorithms to both data and simulations, propagating both statistical and modeling uncertainties via Monte Carlo. The redshift evolution of T 0 ( γ ) shows a suggestive peak (dip) at z = 2.9 ( z = 3). Our measured evolution of T 0 and γ is generally in good agreement with previous determinations in the literature. Both the peak in the evolution of T 0 at z = 2.8, as well as the high temperatures that we observe at 2.4 < z < 3.4, strongly suggest that a significant episode of heating occurred after the end of H i reionization, which was most likely the cosmic reionization of He ii .