Inferring the Thermal History of the Intergalactic Medium from the Properties of the Hydrogen and Helium Lyα Forest

The filamentary network of intergalactic medium (IGM) gas that gives origin to the Ly alpha 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 high-resolution 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 IGM thermal histories by varying 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 at redshifts 2.2 <= z <= 5.0 and with the helium Ly alpha opacity at redshifts 2.4 < z < 2.9 tightly constrains the photoionization and photoheating history of the IGM. By leveraging the constraining power of the available Ly alpha forest data to break model degeneracies, we find that the IGM experienced two main reheating events over 1.2 Gyr of cosmic time. For our best-fit model, hydrogen reionization completes by z ( R ) approximate to 6.0 with a first IGM temperature peak of T (0) similar or equal to 1.3 x 10(4) K and is followed by the reionization of He ii that completes by z (R) approximate to 3.0 and yields a second temperature peak of T (0) similar or equal to 1.4 x 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.

Automated summary

Using a suite of high-resolution simulations, researchers have discovered that the filamentary network of intergalactic medium contains valuable information about structure formation and early thermodynamics. By comparing synthetic spectra with observed data, the study effectively constrains the photoionization and photoheating history of the intergalactic medium. The findings provide insights into the timing and sources of hydrogen and helium reionization.