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Register a new userModeling the Intergalactic Medium during the Epoch of Reionization
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Adam Lidz
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A major goal of observational and theoretical cosmology is to observe the largely unexplored time period in the history of our universe when the first galaxies form, and to interpret these measurements. Early galaxies dramatically impacted the gas around them in the surrounding intergalactic medium (IGM) by photoionzing the gas during the Epoch of Reionization (EoR). This epoch likely spanned an extended stretch in cosmic time: ionized regions formed and grew around early generations of galaxies, gradually filling a larger and larger fraction of the volume of the universe. At some time -- thus far uncertain, but within the first billion years or so after the big bang -- essentially the entire volume of the universe became filled with ionized gas. The properties of the IGM provide valuable information regarding the formation time and nature of early galaxy populations, and many approaches for studying the first luminous sources are hence based on measurements of the surrounding intergalactic gas. The prospects for improved reionization-era observations of the IGM and early galaxy populations over the next decade are outstanding. Motivated by this, we review the current state of models of the IGM during reionization. We focus on a few key aspects of reionization-era phenomenology and describe: the redshift evolution of the volume-averaged ionization fraction, the properties of the sources and sinks of ionizing photons, along with models describing the spatial variations in the ionization fraction, the ultraviolet radiation field, the temperature of the IGM, and the gas density distribution.
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The diffuse soft X-ray emissivity from galactic winds is computed during the Epoch of Reionization (EoR). We consider two analytic models, a pressure-driven wind and a superbubble model, and a 3D cosmological simulation including gas dynamics from the First Billion Years (FiBY) project. The analytic models are normalized to match the diffuse X-ray emissivity of star-forming galaxies in the nearby Universe. The cosmological simulation uses physically motivated star formation and wind prescriptions, and includes radiative transfer corrections. The models and the simulation all are found to produce sufficient heating of the Intergalactic Medium to be detectable by current and planned radio facilities through 21 cm measurements during the EoR. While the analytic models predict a 21 cm emission signal relative to the Cosmic Microwave Background sets in by $z_{rm trans} simeq 8 - 10$, the predicted signal in the FiBY simulation remains in absorption until reionization completes. The 21 cm absorption differential brightness temperature reaches a minimum of $Delta T simeq -130$ to $-200$ mK, depending on model. Allowing for additional heat from high mass X-ray binaries pushes the transition to emission to $z_{rm trans} simeq 10 - 12$, with shallower absorption signatures having a minimum of $Delta T simeq -110$ to $-140$ mK. The 21 cm signal may be a means of distinguishing between the wind models, with the superbubble model favouring earlier reheating. While an early transition to emission may indicate X-ray binaries dominate the reheating, a transition to emission as early as $z_{rm trans} > 12$ would suggest the presence of additional heat sources.
The Murchison Widefield Array (MWA) team has derived new upper limits on the spherically averaged power spectrum of the 21-cm signal at six redshifts in the range $z approx 6.5-8.7$. We use these upper limits and a Bayesian inference framework to derive constraints on the ionization and thermal state of the intergalactic medium (IGM) as well as on the strength of a possible additional radio background. We do not find any constraints on the state of the IGM for $zgtrsim 7.8$ if no additional radio background is present. In the presence of such a radio background, the 95 per cent credible intervals of the disfavoured models at redshift $gtrsim 6.5 $ correspond to an IGM with a volume averaged fraction of ionized regions below 0.6 and an average gas temperature $lesssim 10^3$ K. In these models, the heated regions are characterised by a temperature larger than that of the radio background, and by a distribution with characteristic size $lesssim 10$ $h^{-1}$ Mpc and a full width at half maximum (FWHM) of $lesssim 30$ $h^{-1}$ Mpc. Within the same credible interval limits, we exclude an additional radio background of at least $0.008%$ of the CMB at 1.42 GHz.
We derive constraints on the thermal and ionization states of the intergalactic medium (IGM) at redshift $approx$ 9.1 using new upper limits on the 21-cm power spectrum measured by the LOFAR radio-telescope and a prior on the ionized fraction at that redshift estimated from recent cosmic microwave background (CMB) observations. We have used results from the reionization simulation code GRIZZLY and a Bayesian inference framework to constrain the parameters which describe the physical state of the IGM. We find that, if the gas heating remains negligible, an IGM with ionized fraction $gtrsim 0.13$ and a distribution of the ionized regions with a characteristic size $gtrsim 8 ~h^{-1}$ comoving megaparsec (Mpc) and a full width at the half maximum (FWHM) $gtrsim 16 ~h^{-1}$ Mpc is ruled out. For an IGM with a uniform spin temperature $T_{rm S} gtrsim 3$ K, no constraints on the ionized component can be computed. If the large-scale fluctuations of the signal are driven by spin temperature fluctuations, an IGM with a volume fraction $lesssim 0.34$ of heated regions with a temperature larger than CMB, average gas temperature 7-160 K and a distribution of the heated regions with characteristic size 3.5-70 $h^{-1}$ Mpc and FWHM of $lesssim 110$ $h^{-1}$ Mpc is ruled out. These constraints are within the 95 per cent credible intervals. With more stringent future upper limits from LOFAR at multiple redshifts, the constraints will become tighter and will exclude an increasingly large region of the parameter space.
The neutral hydrogen (HI) and its 21 cm line are promising probes to the reionization process of the intergalactic medium (IGM). To use this probe effectively, it is imperative to have a good understanding on how the neutral hydrogen traces the underlying matter distribution. Here we study this problem using semi-numerical modeling by combining the HI in the IGM and the HI from halos during the epoch of reionization (EoR), and investigate the evolution and the scale-dependence of the neutral fraction bias as well as the 21 cm line bias. We find that the neutral fraction bias on large scales is negative during reionization, and its absolute value on large scales increases during the early stage of reionization and then decreases during the late stage. During the late stage of reionization, there is a transition scale at which the HI bias transits from negative on large scales to positive on small scales, and this scale increases as the reionization proceeds to the end.
The intergalactic medium is expected to clump on scales down to $10^4-10^8$ M$_{odot}$ before the onset of reionization. The impact of these small-scale structures on reionization is poorly understood despite the modern understanding that gas clumpiness limits the growth of H II regions. We use a suite of radiation-hydrodynamics simulations that capture the $sim 10^4$ $M_odot$ Jeans mass of unheated gas to study density fluctuations during reionization. Our simulations track the complex ionization and hydrodynamical response of gas in the wake of ionization fronts. The clumping factor of ionized gas (proportional to the recombination rate) rises to a peak value of $5-20$ approximately $Delta t = 10$ Myr after ionization front passage, depending on the incident intensity, redshift, and degree to which the gas had been pre-heated by the first X-ray sources. The clumping factor reaches its relaxed value of $approx 3$ by $Delta t = 300$ Myr. The mean free path of Lyman-limit photons evolves in unison, being up to several times shorter in un-relaxed, recently reionized regions compared to those that were reionized much earlier. Assessing the impact of this response on the global reionizaton process, we find that un-relaxed gaseous structures boost the total number of recombinations by $approx 50$ % and lead to spatial fluctuations in the mean free path that persist appreciably for several hundred million years after the completion of reionization.
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