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Register a new userHeating of the Intergalactic Medium by Hydrogen Reionization
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During reionization, the intergalactic medium is heated impulsively by supersonic ionization fronts (I-fronts). The peak gas temperatures behind the I-fronts, $T_mathrm{reion}$, 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 $T_mathrm{reion}$. We show that $T_mathrm{reion}$ 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 $T_mathrm{reion}$ 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 $T_mathrm{reion} = 17,000-22,000$ K are typical during the first half of reionization, but $T_mathrm{reion} = 25,000 - 30,000$ K may be achieved near the end of this process if I-front speeds reach $sim10^4$ km/s as found in our simulations. Shorter reionization epochs lead to hotter $T_mathrm{reion}$. We discuss implications for $z>5$ Ly$alpha$ 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 $T_mathrm{reion}$ on the I-front speed.
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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 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.
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.
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.
The intergalactic medium is expected to be at its coldest point before the formation of the first stars in the universe. Motivated by recent results from the EDGES experiment, we revisit the standard calculation of the kinetic temperature of the neutral gas through this period. When the first ultraviolet (UV) sources turn on, photons redshift into the Lyman lines of neutral hydrogen and repeatedly scatter within the Lyman-$alpha$ line. They heat the gas via atomic recoils, and, through the Wouthuysen-Field effect, set the spin temperature of the 21-cm hyperfine (spin-flip) line of atomic hydrogen in competition with the resonant cosmic microwave background (CMB) photons. We show that the Lyman-$alpha$ photons also mediate energy transfer between the CMB photons and the thermal motions of the hydrogen atoms. In the absence of X-ray heating, this new mechanism is the major correction to the temperature of the adiabatically cooling gas ($sim 10 %$ at $z=17$), and is several times the size of the heating rate found in previous calculations. We also find that the effect is more dramatic in non-standard scenarios that either enhance the radio background above the CMB or invoke new physics to cool the gas in order to explain the EDGES results. The coupling with the radio background can reduce the depth of the 21-cm absorption feature by almost a factor of two relative to the case with no sources of heating, and prevent the feature from developing a flattened bottom. As an inevitable consequence of the UV background that generates the absorption feature, this heating should be accounted for in any theoretical prediction.
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