We explore scenarios for reionizing the intergalactic medium with low galaxy ionizing photon escape fractions. We combine simulation-based halo-mass dependent escape fractions with an extrapolation of the observed galaxy rest-ultraviolet luminosity functions to solve for the reionization history from z = 20 → 4. We explore the posterior distributions for key unknown quantities, including the limiting halo mass for star-formation, the ionizing photon production efficiency, and a potential contribution from active galactic nuclei (AGN). We marginalize over the allowable parameter space using a Markov Chain Monte Carlo method, finding a solution which satisfies the most model-independent constraints on reionization. Our fiducial model can match observational constraints with an average escape fraction of <5% throughout the bulk of the epoch of reionization if: i ) galaxies form stars down to the atomic cooling limit before reionization and a photosuppression mass of log (M h /M ) ∼ 9 during/after reionization (−13 < M UV,lim < −11); ii ) galaxies become more efficient producers of ionizing photons at higher redshifts and fainter magnitudes, and iii ) there is a significant, but sub-dominant, contribution by AGN at z 7. In this model the faintest galaxies (M UV > −15) dominate the ionizing emissivity, leading to an earlier start to reionization and a smoother evolution of the ionized volume filling fraction than models which assume a single escape fraction at all redshifts and luminosities. The ionizing emissivity from this model is consistent with observations at z=4-5 (and below, when extrapolated), in contrast to some models which assume a single escape fraction. Our predicted ionized volume filling fraction at z = 7 of Q H II = 78% (± 8%) is in modest (∼1-2σ) tension with observations of Lyα emitters at z ∼ 7 and the damping wing analyses of the two known z > 7 quasars, which prefer Q H II ,z=7 ∼ 40-50%.

Recent years have brought more precise temperature measurements of the low-density intergalactic medium (IGM). These new measurements constrain the processes that heated the IGM, such as the reionization of H I and of He II. We present a semi-analytical model for the thermal history of the IGM that follows the photoheating history of primordial gas. Our model adopts a multi-zone approach that, compared to previous models, more accurately captures the inhomogeneous heating and cooling of the IGM during patchy reionization. We compare our model with recent temperature measurements spanning z = 1.6 − 4.8, finding that these measurements are consistent with scenarios in which the He II was reionized at z = 3 − 4 by quasars. Significantly longer duration or higher redshift He II reionization scenarios are ruled out by the measurements. For hydrogen reionization, we find that only low redshift and high temperature scenarios are excluded. For example, a model in which the IGM was heated to 30, 000 K when an ionization front passed, and with hydrogen reionization occurring over 6 < z < 9, is ruled out. Finally, we place constraints on how much heating could owe to TeV blazars, cosmic rays, and other nonstandard mechanisms. We find that by z = 2 a maximum of 1 eV of additional heat could be injected per baryon over standard photoheating-only models, with this limit becoming 0.5 eV at z > 3.

Motivated by the claimed detection of a large population of faint active galactic nuclei (AGN) at high redshift, recent studies have proposed models in which AGN contribute significantly to the z > 4 H I ionizing background. In some models, AGN are even the chief sources of reionization. If correct, these models would make necessary a complete revision to the standard view that galaxies dominated the high-redshift ionizing background. It has been suggested that AGN-dominated models can better account for two recent observations that appear to be in conflict with the standard view: (1) large opacity variations in the z ∼ 5.5 H I Lyα forest, and (2) slow evolution in the mean opacity of the He II Lyα forest. Large spatial fluctuations in the ionizing background from the brightness and rarity of AGN may account for the former, while the earlier onset of He II reionization in these models may account for the latter. Here we show that models in which AGN emissions source 50% of the ionizing background generally provide a better fit to the observed H I Lyα forest opacity variations compared to standard galaxy-dominated models. However, we argue that these AGN-dominated models are in tension with constraints on the thermal history of the intergalactic medium (IGM). Under standard assumptions about the spectra of AGN, we show that the earlier onset of He II reionization heats up the IGM well above recent temperature measurements. We further argue that the slower evolution of the mean opacity of the He II Lyα forest relative to simulations may reflect deficiencies in current simulations rather than favor AGN-dominated models as has been suggested.

During reionization, the intergalactic medium is heated impulsively by supersonic ionization fronts (I-fronts). The peak gas temperatures behind the I-fronts, T reion , are a key uncertainty in models of the thermal history after reionization. Here we use high-resolution radiative transfer simulations to study the parameter space of T reion . We show that T reion is only mildly sensitive to the spectrum of incident radiation over most of the parameter space, with temperatures set primarily by I-front speeds.We also explore what current models of reionization predict for T reion by measuring I-front speeds in cosmological radiative transfer simulations. We find that the post-I-front temperatures evolve toward hotter values as reionization progresses. Temperatures of T reion = 17, 000−22, 000 K are typical during the first half of reionization, but T reion = 25, 000 − 30, 000 K may be achieved near the end of this process if I-front speeds reach ∼ 10 4 km/s as found in our simulations. Shorter reionization epochs lead to hotter T reion . We discuss implications for z > 5 Lyα forest observations, which potentially include sight lines through hot, recently reionized patches of the Universe. Interpolation tables from our parameter space study are made publicly available, along with a simple fit for the dependence of T reion on the I-front speed.

Cosmological simulations of the low-density intergalactic medium exhibit a strikingly tight power-law relation between temperature and density that holds over two decades in density. It is found that this relation should roughly apply ∆z ∼ 1 − 2 after a reionization event, and this limiting behavior has motivated the power-law parameterizations used in most analyses of the Lyα forest. This relation has been explained by using equations linearized in the baryonic overdensity (which does not address why a tight power-law relation holds over two decades in density) or by equating the photoheating rate with the cooling rate from cosmological expansion (which we show is incorrect). Previous explanations also did not address why recombination cooling and Compton cooling off of the cosmic microwave background, which are never negligible, do not alter the character of this relation. We provide an understanding for why a tight power-law relation arises for unshocked gas at all densities for which collisional cooling is unimportant. We also use our results to comment on (1) how quickly fluctuations in temperature redshift away after reionization processes, (2) how much shock heating occurs in the low-density intergalactic medium, and (3) how the temperatures of collapsing gas parcels evolve.