Inferring the Thermal History of the Intergalactic Medium from the Properties of the Hydrogen and Helium Lyman-alpha Forest

Abstract

The filamentary network of intergalactic medium (IGM) gas that gives origin to the Lyman-α forest in the spectra of distant quasars encodes information on the physics of structure formation and the early thermodynamics of diffuse baryonic material. Here, we use a massive suite of more than 400 highresolution cosmological hydrodynamical simulations run with the Graphics Processing Unit-accelerated code Cholla to study the IGM at high spatial resolution maintained over the entire computational volume. The simulations capture a wide range of possible thermal histories of intergalactic gas by varying the amplitude and timing of the photoheating and photoionizing background produced by star-forming galaxies and active galactic nuclei. A statistical comparison of synthetic spectra with the observed 1D flux power spectra of hydrogen in 14 redshift bins over the full range 2.2 ≤ z ≤ 5.0 and with the Lyman-α opacity of helium in 5 redshift bins over the range 2.4 < z < 2.9 tightly constrains the photoionization and photoheating history of the IGM. By leveraging the constraining power of the available Lyman-α forest data to break model degeneracies, we find that the IGM experienced two main reheating events from the non-equilibrium ionization of hydrogen and helium over 1.2 Gyr of cosmic time. For our best-fit model, hydrogen reionization completes by z R ≈ 6.1 with a first IGM temperature peak T 0 1.3 × 10 4 K, and is followed by the reionization of He II that completes by z R ≈ 3.0 and yields a second temperature peak of T 0 1.4 × 10 4 K. We discuss how our results can be used to obtain information on the timing and the sources of hydrogen and helium reionization.

Limits on the Light Dark Matter–Proton Cross Section from Cosmic Large-Scale Structure

“…[65]. The thermal and ionization state of the IGM over which we marginalize is statistically consistent with previous results [28,39,58,59] and fiducial models [42,48,60]. We find that the IGM at z ¼ 4.2 to z ¼ 5 is at a plateau between the end of hydrogen reionization and the start of helium reionization (although IGM bounds are weak since we marginalize dark matter parameters).…”

New limits on light dark matter - proton cross section from the cosmic large-scale structure

“…Table I gives the cross section bound for some fixed dark matter masses, as well as marginalized nuisance IGM and cosmological parameter constraints: e. g., a 95% credible lower limit σ < 6 × 10 −30 cm 2 , for m = 100 keV. The thermal and ionization state of the IGM over which we marginalize is statistically consistent with previous results [21,43,49,50] and fiducial models [31,36,51]. We find that the IGM at z = 4.2 to z = 5 is at a plateau between the end of hydrogen reionization and the start of helium reionization (although IGM bounds are weak since we marginalize dark matter parameters).…”

Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra

“…At z < 6, therefore, the IGM in this model is fully ionized and the gas is hydrodynamically relaxed. A homogeneous-UVB model that produced a later reionization (e.g., Puchwein et al 2019;Villasenor et al 2021) would mainly alter the temperature and pressure smoothing at z < 6. These are small-scale effects, however, that should only minimally impact our measurements.…”