We present new synthesis models of the extragalactic background light (EBL) from far infra-red (FIR) to TeV γ-rays, with an emphasis on the extreme ultraviolet (UV) background which is responsible for the observed ionization and thermal state of the intergalactic medium across the cosmic time. Our models use updated values of the star formation rate density and dust attenuation in galaxies, QSO emissivity, and the distribution of H I gas in the IGM. Two of the most uncertain parameters in these models, the escape fraction of H I ionizing photons from galaxies and the spectral energy distribution (SED) of QSOs, are determined to be consistent with the latest measurements of H I and He II photoionization rates, the He II Lyman-α effective optical depths, various constraints on H I and He II reionization history and many measurements of the local EBL from soft X-rays till γ-rays. We calculate the EBL from FIR to TeV γ-rays by using FIR emissivities from our previous work and constructing an average SED of high-energy emitting QSOs, i.e, type-2 QSOs and blazars. For public use, we also provide the EBL models obtained using different QSO SEDs at extreme-UV energies over a wide range of redshifts. These can be used to quantify uncertainties in the parameters derived from photoionization models and numerical simulations originating from the allowed variations in the UV background radiation.

Using our cosmological radiative transfer code, we study the implications of the updated quasi-stellar object (QSO) emissivity and star formation history for the escape fraction (f esc ) of hydrogen ionizing photons from galaxies. We estimate the f esc that is required to reionize the Universe and to maintain the ionization state of the intergalactic medium in the post-reionization era. At z > 5.5, we show that a constant f esc of 0.14 to 0.22 is sufficient to reionize the Universe. At z < 3.5, consistent with various observations, we find that f esc can have values from 0 to 0.05. However, a steep rise in f esc , of at least a factor of ∼ 3, is required between z = 3.5 to 5.5. It results from a rapidly decreasing QSO emissivity at z > 3 together with a nearly constant measured H i photoionization rates at 3 < z < 5. We show that this requirement of a steep rise in f esc over a very short time can be relaxed if we consider the contribution from a recently found large number density of faint QSOs at z 4. In addition, a simple extrapolation of the contribution of such QSOs to high-z suggests that QSOs alone can reionize the Universe. This implies, at z > 3.5, that either the properties of galaxies should evolve rapidly to increase the f esc or most of the low-mass galaxies should host massive black holes and sustain accretion over a prolonged period. These results motivate a careful investigation of theoretical predictions of these alternate scenarios that can be distinguished using future observations. Moreover, it is also very important to revisit the measurements of H i photoionization rates that are crucial to the analysis presented here.

We constrain the H i photoionization rate (Γ HI ) at z 0.45 by comparing the flux probability distribution function and power spectrum of the Lyα forest data along 82 QSO sightlines obtained using Cosmic Origins Spectrograph with models generated from smoothed particle hydrodynamic simulations. We have developed a module named "Code for Ionization and Temperature Evolution (cite)" for calculating the intergalactic medium (IGM) temperature evolution from high to low redshifts by post-processing the gadget-2 simulation outputs. Our method, that produces results consistent with other simulations, is computationally less expensive thus allowing us to explore a large parameter space. It also allows rigorous estimation of the error covariance matrix for various statistical quantities of interest. We find that the bestfit Γ HI (z) increases with z and follows (4 ± 0.1) × 10 −14 (1 + z) 4.99±0.12 s −1 . At any given z the typical uncertainties ∆Γ HI /Γ HI are ∼ 25 per cent which contains not only the statistical errors but also those arising from possible degeneracy with the thermal history of the IGM and cosmological parameters and uncertainties in fitting the QSO continuum. These values of Γ HI favour the scenario where only QSOs contribute to the ionizing background at z < 2. Our derived 3σ upper limit on average escape fraction is 0.008, consistent with measurements of low-z galaxies.