Effects of Photoionization and Photoheating on Lyα Forest Properties from Cholla Cosmological Simulations

Abstract

The density and temperature properties of the intergalactic medium (IGM) reflect the heating and ionization history during cosmological structure formation, and are primarily probed by the Lyα forest of neutral hydrogen absorption features in the observed spectra of background sources. We present the methodology and initial results from the Cholla IGM Photoheating Simulation (CHIPS) suite performed with the graphics process unit–accelerated Cholla code to study the IGM at high, uniform spatial resolution maintained over large volumes. In this first paper, we examine the IGM structure in CHIPS cosmological simulations that include IGM uniform photoheating and photoionization models where hydrogen reionization is completed early or by redshift z ∼ 6. Comparing with observations of the large- and small-scale Lyα transmitted flux power spectra P(k) at redshifts 2 ≲ z ≲ 5.5, the relative agreement of the models depends on scale, with the self-consistent Puchwein et al. IGM photoheating and photoionization model in good agreement with the flux P(k) at k ≳ 0.01 s km−1 at redshifts 2 ≲ z ≲ 3.5. On larger scales, the P(k) measurements increase in amplitude from z ∼ 4.6 to z ∼ 2.2, faster than the models, and lie in between the model predictions at 2.2 ≲ z ≲ 4.6 for k ≈ 0.002–0.01 s km−1. We argue that the models could improve by changing the He ii photoheating rate associated with active galactic nuclei to reduce the IGM temperature at z ∼ 3. At higher redshifts, z ≳ 4.5, the observed flux P(k) amplitude increases at a rate intermediate between the models, and we argue that for models where hydrogen reionization is completed late (z ∼ 5.5–6), resolving this disagreement will require inhomogeneous or “patchy” reionization. We then use an additional set of simulations to demonstrate that our results have numerically converged and are not strongly affected by varying cosmological parameters.

Galaxy Formation and Reionization: Key Unknowns and Expected Breakthroughs by the James Webb Space Telescope

“…The subsequent thermal and ionized fraction evolution of the IGM has a host of additional observational ramifications beyond the scope of this work (see the review by McQuinn 2016), so, in principle, the evolution of the neutral fraction during reionization and the properties of the IGM after reionization, such as the Lyα-forest forest power spectrum, should be made consistent. Villasenor et al (2021) used hundreds of hydrodynamical cosmological simulations, varying the amplitude and timing of the Puchwein et al ( 2019) photoionization and photoheating rates, and determined a photoionization and photoheating history that matches all the available data on the Lyα-forest at z < 5. These fits then provide a prediction for the neutral fraction evolution xHI(z) fully consistent with the post-reionization properties of the IGM.…”

“…We plot several types of observational constraints on xHI(z) and the model by Villasenor et al (2021) in Figure 5. McGreer et al (2015) quantified the neutral fraction near the end of reionization using dark regions of the Lyα and Lyβ forests in background quasar spectra that transmit zero flux.…”

“…The detailed calculations can follow the histories of individual galaxies, and study how the reionization process affects regions around galaxies as a function of halo mass (Zhu et al 2019) . Also shown is a prediction from cosmological simulations for x HI constrained by the evolution of the Lyman-α forest transmitted flux power spectrum at z < 6, allowing for variations in the range of photo-ionization and photo-heating history (Villasenor et al 2021).…”

“…Also shown is the constraint from Planck Collaboration et al (2020) on the midpoint of reionization z(xHI = 0.5) ∼ 7.82 ± 0.71, which assumes a tanh model for xHI(z) and is therefore not independent of the assumed reionization history. TheVillasenor et al (2021) model is shown for reference, noting that the model is not fit to the data shown in Figure5. Overall,…”

The effect of inhomogeneous reionisation on the Lyman-$α$ forest power spectrum at redshift $z>4$: implications for thermal parameter recovery

“…While these "UVB synthesis" models provide an excellent description of the IGM ionisation state following reionisation, when the mean free path for Lyman continuum photons is 100 cMpc (see e.g. Lukić et al 2015;Bolton et al 2017;Rossi 2020;Villasenor et al 2021), they neglect the large-scale fluctuations in the ionisation and thermal state of the IGM that exist immediately following the completion of reionisation (Becker et al 2015;D'Aloisio et al 2015;Davies & Furlanetto 2016;Chardin et al 2017;Kulkarni et al 2019). It has been suggested that ignoring these fluctuations could weaken existing Ly𝛼 forest constraints on the free streaming length of dark matter, leaving the door firmly open for alternatives to cold dark matter (e.g.…”