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

Abstract

The filamentary network of intergalactic medium (IGM) gas that gives origin to the Lyα 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 Lyα 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 Lyα 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 ≈ 6.0 with a first IGM temperature peak of T 0 ≃ 1.3 × 104 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 × 104 K. We discuss how our results can be used to obtain information on the timing and the sources of hydrogen and helium reionization.

On the Coevolution of the AGN and Star-forming Galaxy Ultraviolet Luminosity Functions at 3 < z < 9

“…This implies a significantly stronger contribution to the global ionizing emissivity from AGNs such that, within the uncertainties, star-forming galaxies and AGNs could contribute equally at z ∼ 7. This scenario seems exceedingly unlikely due to intergalactic medium temperature measurements (e.g., Hiss et al 2018;Villasenor et al 2022), as well as the relatively late reionization of He II (e.g., Worseck et al 2016). While exploring such constraints is beyond the scope of this work, the implausibility of this scenario adds additional evidence beyond the DIC results that the starforming galaxy luminosity function is best modeled by a DPL form.…”

Modeling cosmic reionization

“…For illustrative purposes, we have calculated here the mean transmitted flux as a function of redshift using the polynomial fit to the PDF from Bolton and Becker (2009), and assuming constant T 0 ¼ 10 4 K, c ¼ 1:3, and C HI ¼ 10 À12 ; s À1 (cf. Fan et al 2006;Becker and Bolton 2013;Gaikwad et al 2017;Boera et al 2019; Gaikwad et al 2020;Villasenor et al 2022). 10 compares the effective Lyman-a optical depth, À lnhFi, predicted in the redshift range 2.z.6 with the observations.…”

“…More realistically, it is believed that cosmic reionization was an extended process that started around z $ 10, and that the cosmic web actually experienced two main reheating events. Figure 3 shows the thermal history of the IGM at z\6 as recently inferred using a suite of more than 400 high-resolution cosmological hydrodynamics simulations of structure formation that self-consistently evolve a wide range of physically-motivated ionization and thermal histories of the IGM (Villasenor et al 2022). A statistical comparison with the observed transmitted flux power spectrum of the hydrogen forest over the redshift range 2:2 z 5 and with the Lyman-a opacity of intergalactic He II over the redshift range 2:4 z 2:9 produces the best-fit Fig.…”

“…The KCDM simulations used for statistical analysis assume a spatially homogeneous ionizing background. Image adapted from Villasenor et al (2022), copyright by the authors formation (e.g. Oh 2001;Venkatesan et al 2001;Madau et al 2004).…”

“…Modern observations of resonant absorption in the spectra of distant quasars show that reionization was nearly completed at z $ 6 (Fan et al 2006;McGreer et al 2015;Becker et al 2015). A very early onset of reionization is disfavoured by the final fullmission Planck CMB anisotropy analysis, which prefers a late and fast transition from a neutral to an ionized Universe, and by a variety of astrophysical probes (Schroeder et al 2013;Bañados et al 2018;Ouchi et al 2010;Schenker et al 2014;Oñorbe et al 2017;Villasenor et al 2022). Despite a considerable community effort, however, many key aspects of this process, such as the very nature of the first sources of UV radiation, how they interacted with their environment, the back-reaction of reionization on infant galaxies and its imprint on their luminosity function and gas content, the impact of patchy reionization on observables, and the thermodynamics of primordial baryonic gas, all remain highly uncertain.…”