Constraining the reionization and thermal history of the Universe using a semi-numerical photon-conserving code SCRIPT

Abstract

Given that the reionization history of cosmic hydrogen is yet to be stringently constrained, it is worth checking the prospects of doing so using physically motivated models and available observational data. For this purpose, we use an extended version of the explicitly photon-conserving semi-numerical model of reionization, SCRIPT, which also includes thermal evolution of the intergalactic medium (IGM). The model incorporates the effects of inhomogeneous recombination and radiative feedback selfconsistently and is characterized by five free parameters (two for the redshift-dependent ionization efficiency, two for the ionizing escape fraction, and another for reionization temperature increment). We constrain these free parameters by simultaneously matching with various observational probes, e.g., estimates of the ionized hydrogen fraction, the CMB scattering optical depth and the galaxy UV luminosity function. In addition, we include the low-density IGM temperature measurements obtained from Lyman-𝛼 absorption spectra at 𝑧 ∼ 5.5, a probe not commonly used for Bayesian analysis of reionization parameters. We find that the interplay of the various data sets, particularly inclusion of the temperature data, leads to tightening of the parameter constraints. Our default models prefer a late end of reionization (at 𝑧 6), in agreement with other recent studies. We can also derive constraints on the duration of reionization, Δ𝑧 = 1.81 +0.51 −0.67 and the midpoint of reionization, 𝑧 mid = 7.0 +0.30 −0.40 . The constraints can be further tightened by including other available and upcoming data sets.

Modelling the star-formation activity and ionizing properties of high-redshift galaxies

“…This would need to be counterbalanced by also decreasing the normalization ε esc,10 to remain consistent with data. The mass-dependency of f esc suggested by our analysis is in qualitative agreement with that found in different hydrodynamic simulations [138][139][140][141] and other semi-numerical models [23,84,142] of reionization. The typical behavior of the favored reionization histories can be understood from figure 4 where we have plotted the evolution of the globally averaged neutral fraction of 200 random samples from the MCMC chains.…”

“…This helps our models to explain the observed flatness and mild evolution of the bright end of the z ≥ 10 UVLFs. The value of the power-law index obtained for z ≤ 9 from our analysis agrees qualitatively with other semi-analytical or semi-numerical studies that used a power-law index of ∼ 0.4 − 0.5 to match the UVLF observations from the HST [23,68,84,103,131]. Whatever the physical reasoning may be, these findings clearly reveal that processes regulating star formation and the production of UV photons in the earliest galaxies may be significantly different from those seen in galaxies at lower redshifts.…”

“…It is worthwhile to emphasize that while the ionization history is sensitive to the product of the star-formation efficiency (f * ) and the escape fraction (f esc ) of ionizing photons, the UVLF for faint and intermediate magnitudes can be determined from the knowledge of f * . Therefore, combining LF observations with reionization observations holds great promise in unraveling the characteristics of high-redshift galaxies, especially in constraining the unknown fraction of ionizing photons that escape from the interstellar medium of these galaxies into the IGM [23,83,84]. In this work, our main aim is to use this flexible semi-analytical model for computing different observables and place constraints on the astrophysical properties of high-redshift galaxies by comparing the model predictions against a wide variety of recent observations simultaneously.…”