Reionization Of He Research Articles

Cosmic Reionization in the JWST Era: Back to AGNs?

Deep surveys with the James Webb Space Telescope (JWST) have revealed an emergent population of moderate-luminosity, broad-line active galactic nuclei (AGNs) at 4 ≲ z ≲ 13 powered by accretion onto early massive black holes. The high number densities reported, together with the large Lyman-continuum (LyC) production efficiency and leakiness into the intergalactic medium that are typical of UV-selected AGNs, lead us to reassess a scenario where AGNs are the sole drivers of the cosmic hydrogen/helium reionization process. Our approach is based on the assumptions, grounded in recent observations, that (a) the fraction of broad-line AGNs among galaxies is around 10%–15%; (b) the mean escape fraction of hydrogen LyC radiation is high, at ≳80%, in AGN hosts and is negligible otherwise; and (c) internal absorption at 4 ryd or a steep ionizing EUV spectrum delay full reionization of He ii until z ≃ 2.8–3.0, in agreement with observations of the He ii Lyα forest. In our fiducial models, (1) hydrogen reionization is 99% completed by redshift z ≃ 5.3–5.5 and reaches its midpoint at z ≃ 6.5–6.7; (2) the integrated Thomson scattering optical depth to reionization is ≃0.05, consistent with constraints from cosmic microwave background anisotropy data; and (3) the abundant AGN population detected by JWST does not violate constraints on the unresolved X-ray background.

Inferring the Thermal History of the Intergalactic Medium from the Properties of the Hydrogen and Helium Lyα Forest

The filamentary network of intergalactic medium (IGM) gas that gives origin to the Lyα forest in the spectra of distant quasars encodes information on the physics of structure formation and the early thermodynamics of diffuse baryonic material. Here we use a massive suite of more than 400 high-resolution cosmological hydrodynamical simulations run with the Graphics Processing Unit–accelerated code Cholla to study the IGM at high spatial resolution maintained over the entire computational volume. The simulations capture a wide range of possible IGM thermal histories by varying the photoheating and photoionizing background produced by star-forming galaxies and active galactic nuclei. A statistical comparison of synthetic spectra with the observed 1D flux power spectra of hydrogen at redshifts 2.2 ≤ z ≤ 5.0 and with the helium Lyα opacity at redshifts 2.4 < z < 2.9 tightly constrains the photoionization and photoheating history of the IGM. By leveraging the constraining power of the available Lyα forest data to break model degeneracies, we find that the IGM experienced two main reheating events over 1.2 Gyr of cosmic time. For our best-fit model, hydrogen reionization completes by z R ≈ 6.0 with a first IGM temperature peak of T 0 ≃ 1.3 × 104 K and is followed by the reionization of He ii that completes by z R ≈ 3.0 and yields a second temperature peak of T 0 ≃ 1.4 × 104 K. We discuss how our results can be used to obtain information on the timing and the sources of hydrogen and helium reionization.

A New Measurement of the Temperature–density Relation of the IGM from Voigt Profile Fitting

Abstract We decompose the Lyman-α (Lyα) forest of an extensive sample of 75 high signal-to-noise ratio and high-resolution quasar spectra into a collection of Voigt profiles. Absorbers located near caustics in the peculiar velocity field have the smallest Doppler parameters, resulting in a low-b cutoff in the b– distribution set primarily by the thermal state of the intergalactic medium (IGM). We fit this cutoff as a function of redshift over the range , which allows us to measure the evolution of the IGM temperature–density ( ) relation parameters T 0 and γ. We calibrate our measurements against mock Lyα forest data generated using 26 hydrodynamic simulations with different thermal histories from the THERMAL suite, also encompassing different values of the IGM pressure smoothing scale. We adopt a forward-modeling approach and self-consistently apply the same algorithms to both data and simulations, propagating both statistical and modeling uncertainties via Monte Carlo. The redshift evolution of T 0 (γ) shows a suggestive peak (dip) at z = 2.9 (z = 3). Our measured evolution of T 0 and γ is generally in good agreement with previous determinations in the literature. Both the peak in the evolution of T 0 at z = 2.8, as well as the high temperatures that we observe at 2.4 &lt; z &lt; 3.4, strongly suggest that a significant episode of heating occurred after the end of H i reionization, which was most likely the cosmic reionization of He ii.

The reionization of He II and the temperature evolution of the intergalactic medium

A number of observations suggest that He II in the intergalactic medium (IGM) was fully ionized at z ∼ 3, probably by quasi-stellar objects (QSOs). Here we construct a simple model of a QSO to study the reionization of He II and the corresponding thermal evolution of the IGM. We assume that QSOs are triggered by major mergers of dark matter halos, and the luminosity evolution of individual QSOs is described by an initial accretion stage with a constant Eddington ratio and then a power-law decay driven by long term disk evolution or fueling. Once a QSO is triggered, it immediately ionizes its surrounding area as an ionized bubble. The resulting changes in size and volume of the bubble are determined by the luminosity evolution of the central QSO. With the emergence of more and more bubbles, they eventually overlap each other and finally permeate the whole universe. During the He II reionization, the IGM temperature increases due to the photoheating by the ionization processes. Applying the bubble model and considering various heating and cooling mechanisms, we trace the thermal evolution of the IGM and obtain the average IGM temperature as a function of redshift, which is very consistent with observations. The increase in IGM temperature due to the reionization of He II may be determined more accurately in the future, which may put robust constraints on the QSO model and the physics of He II reionization.

The effect of fluctuations on the helium-ionizing background

Interpretation of He II Ly{\alpha} absorption spectra after the epoch of He II reionization requires knowledge of the He II ionizing background. While past work has modelled the evolution of the average background, the standard cosmological radiative transfer technique assumes a uniform radiation field despite the discrete nature of the (rare) bright quasars that dominate the background. We implement a cosmological radiative transfer model that includes the most recent constraints on the ionizing spectra and luminosity function of quasars and the distribution of IGM absorbers. We also estimate, for the first time, the effects of fluctuations on the evolving continuum opacity in two ways: by incorporating the complete distribution of ionizing background amplitudes into the standard approach, and by explicitly treating the quasars as discrete -- but isolated -- sources. Our model results in a He II ionization rate that evolves steeply with redshift, increasing by a factor ~2 from z=3.0 to z=2.5. This causes rapid evolution in the mean He II Ly{\alpha} optical depth -- as recently observed -- without appealing to the reionization of He II. The observed behaviour could instead result from rapid evolution in the mean free path of ionizing photons as the helium in higher H I column density absorbers becomes fully ionized.

Improved measurements of the intergalactic medium temperature around quasars: possible evidence for the initial stages of He ii reionization at z ≃ 6

We present measurements of the intergalactic medium (IGM) temperature within ∼5 proper Mpc of seven luminous quasars at z ≃ 6. The constraints are obtained from the Doppler widths of Lyα absorption lines in the quasar near zones and build upon our previous measurement for the z = 6.02 quasar SDSS J0818+1722. The expanded data set, combined with an improved treatment of systematic uncertainties, yields an average temperature at the mean density of (⁠⁠) at 68 (95) per cent confidence for a flat prior distribution over 3.2 ≤ log (T0/K) ≤ 4.8. In comparison, temperatures measured from the general IGM at z ≃ 5 are ∼0.3 dex cooler, implying an additional source of heating around these quasars which is not yet present in the general IGM at slightly lower redshift. This heating is most likely due to the recent reionization of He ii in vicinity of these quasars, which have hard and non-thermal ionizing spectra. The elevated temperatures may therefore represent evidence for the earliest stages of He ii reionization in the most biased regions of the high-redshift Universe. The temperature as a function of distance from the quasars is consistent with being constant, log (T0/K) ≃ 4.2, with no evidence for a line-of-sight thermal proximity effect. However, the limited extent of the quasar near zones prevents the detection of He iii regions larger than ∼5 proper Mpc. Under the assumption that the quasars have reionized the He ii in their vicinity, we infer that the data are consistent with an average optically bright phase of duration in excess of 106.5 yr. These measurements represent the highest redshift IGM temperature constraints to date, and thus provide a valuable data set for confronting models of H i reionization.

Detection of extended He ii reionization in the temperature evolution of the intergalactic medium★

We present new measurements of the temperature of the intergalactic medium (IGM) derived from the Lyman-alpha forest over 2.0 < z < 4.8. The small-scale structure in the forest of 61 high-resolution QSO spectra is quantified using a new statistic, the curvature, and the conversion to temperature calibrated using a suite of hydrodynamic simulations. At each redshift we focus on obtaining the temperature at an optimal overdensity probed by the Lyman-alpha forest, T(Delta), where the temperature is nearly a one-to-one function of the curvature regardless of the slope of the temperature-density relation. The median 2-sigma statistical uncertainty in these measurements is 8 per cent, though there may be comparable systematic errors due to the unknown amount of Jeans smoothing in the IGM. We use our T(Delta) results to infer the temperature at the mean density, T0. Even for a maximally steep temperature-density relation, T0 must increase from ~8000 K at z ~ 4.4 to >~12000 K at z ~ 2.8. This increase is not consistent with the monotonic decline in T0 expected in the absence of He II reionization. We therefore interpret the observed rise in temperature as evidence of He II reionization beginning at z >~ 4.4. The evolution of T0 is consistent with an end to He II reionization at z ~ 3, as suggested by opacity measurements of the He II Lyman-alpha forest, although the redshift at which T0 peaks will depend somewhat on the evolution of the temperature-density relation. These new temperature measurements suggest that the heat input due to the reionization of He II dominates the thermal balance of the IGM over an extended period with Delta_z >~ 1.

Photoheating and the fate of hard photons during the reionization of He ii by quasars

We use a combination of analytic and numerical arguments to consider the impact of quasar photoheating during He ii reionization on the thermal evolution of the intergalactic medium (IGM). We demonstrate that rapid (Δz < 0.1–0.2), strong (ΔT > 104 K) photoheating is difficult to achieve across the entire IGM unless quasar spectra are significantly harder than implied by current observational constraints. Although filtering of intrinsic quasar radiation through dense regions in the IGM does increase the mean excess energy per He ii photoionization, it also weakens the radiation intensity and lowers the photoionization rate, preventing rapid heating over time intervals shorter than the local photoionization time-scale. Moreover, the hard photons responsible for the strongest heating are more likely to deposit their energy inside dense clumps, which cool rapidly and are furthermore invisible to most observational probes of the IGM temperature. The abundance of such clumps is, however, uncertain and model dependent, leading to a fairly large uncertainty in the photoheating rates. Nevertheless, although some of the IGM may be exposed to a hardened and weakened ionizing background for long periods, most of the IGM must instead be reionized by the more abundant, softer photons and with accordingly modest heating rates (ΔT≲ 104 K), although localized patches of much higher temperature are still possible. The repeated ionization of fossil quasar He iii regions does not increase the net heating because the recombination times in these regions typically exceed the IGM cooling times and the average time lag between successive rounds of quasar activity. Detailed line-of-sight radiative transfer simulations confirm these expectations and predict a rich thermal structure in the IGM during He ii reionization. The resulting complex relationship between temperature and density may help resolve discrepancies between optically thin simulations of the Lyα forest and recent observations.

Lyα leaks and reionization

Lya absorption spectra of QSOs at redshifts z ≃ 6 show complete Gunn-Peterson absorption troughs (dark gaps) separated by tiny leaks. The dark gaps are from the intergalactic medium (IGM) where the density of neutral hydrogen are high enough to produce almost saturated absorptions, however, where the transmitted leaks come from is still unclear so far. We demonstrate that leaking can originate from the lowest density voids in the IGM as well as the ionized apatches around ionizing sources using semi-analytical simulations. If leaks are produced in lowest density voids, the IGM must already be highly ionized, and the ionizing background should be almost uniform; in contrast, if leaks come from ionized patches, the neutral fraction of IGM should be still high, and the ionizing background is significantly inhomogeneous. Therefore, the origin of leaking is crucial to determining the epoch of inhomogeneous-to-uniform transition of the ionizing photon background. We show that the origin could be studied with the statistical features of leaks. Actually, Lya leaks can be well defined and described by the equivalent width W and the full width of half-area W H , both of which are less contaminated by instrumental resolution and noise. It is found that the distributions of Wand W H of Lyα leaks are sensitive to the modelling of the ionizing background. We consider four representative models: uniform ionizing background (model 0), the photoionization rate of neutral hydrogen Γ HI and the density of IGM are either linearly correlated (model I), or anticorrelated (model II), and Γ HI is correlated with high-density peaks containing ionizing sources (model III). Although all of these models can match to the mean of the observed effective optical depth of the IGM at z ≃ 6, the distributions of W and W H are very different from each other. Consequently, the leak statistics provides an effective tool to probe the evolutionary history of reionization at z ≃ 5-6.5. Similar statistics will also be applicable to the reionization of He II at z ≃ 3.

The matter power spectrum from the Lyα forest: an optical depth estimate

We measure the matter power spectrum from 31 Lyα spectra spanning the redshift range of 1.6‐3.6. The optical depth, τ , for Lyα absorption of the intergalactic medium is obtained from the flux using the inversion method of Nusser & Haehnelt. The optical depth is converted to density by using a simple power-law relation, τ ∝ (1 + δ) α . The non-linear 1D power spectrum of the gas density is then inferred with a method that makes simultaneous use of the one- and two-point statistics of the flux and compared against theoretical models with a likelihood analysis. A cold dark matter model with standard cosmological parameters fits the data well. The power-spectrum amplitude is measured to be (assuming a flat Universe), σ 8 = (0.92 ± 0.09) × (� m/0.3) −0.3 , with α varying in the range of 1.56‐1.8 with redshift. Enforcing the same cosmological parameters in all four redshift bins, the likelihood analysis suggests some evolution in the temperature‐density relation and the thermal smoothing length of the gas. The inferred evolution is consistent with that expected if reionization of He II occurred at z ∼ 3.2. A joint analysis with the Wilkinson Microwave Anisotropy Probe results together with a prior on the Hubble constant as suggested by the Hubble Space Telescope key project data,

Spatial fluctuations in the spectral shape of the ultraviolet background at 2 &gt; z &gt; 3 and the reionization of helium

The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi- stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H I and He II Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of acold dark matter (�CDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H I and He II ionization rates. We further show that the large spatial variations and the anticorrelation with H I absorber density observed in the ratio of the measured He II to H I column densities can be explained in a model where the H I ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He II ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He II ionization rate. A significant contribution to UV emission at the He II photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He II opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He II Lyα forest spectra at z ∼ 2-3.5 probe the tail end of the reionization of He II by QSOs.

The nature of the ionizing background at z ≈ 2.5–5

Using radiative transfer calculations and cosmological simulations of structure formation, we study constraints that can be placed on the nature of the cosmic ultraviolet (UV) background in the redshift interval 2.5 ≲z≲ 5. Our approach makes use of observational estimates of the opacities of hydrogen and singly ionized helium in the intergalactic medium during this epoch. In particular, we model the reionization of He ii by sources of hard ultraviolet radiation, i.e. quasars, and infer values for our parametrization of this population from observational estimates of the opacity of the He ii Lyman-α forest. Next, we estimate the photoionization rate of H i from these sources and find that their contribution to the ionizing background is insufficient to account for the measured opacity of the H i Lyman-α forest at a redshift z∼ 3. This motivates us to include a soft, stellar component to the ionizing background to boost the hydrogen photoionization rate, but which has a negligible impact on the He ii opacity. In order to simultaneously match observational estimates of the H i and He ii opacities, we find that galaxies and quasars must contribute approximately equally to the ionizing background in H i at z≃ 3. Moreover, our analysis requires the stellar component to rise for z > 3 to compensate for the declining contribution from bright quasars at higher redshift. This inference is consistent with some observational and theoretical estimates of the evolution of the cosmic star formation rate. The increasing dominance of the stellar component towards high redshift leads to a progressive softening of the UV background, as suggested by observations of metal line absorption. In the absence of additional sources of ionizing radiation, such as mini-quasars or weak active galactic nuclei, our results, extrapolated to z > 5, suggest that hydrogen reionization at z∼ 6 mostly probably occurred through the action of stellar radiation.

A Feature at [CLC][ITAL]z[/ITAL][/CLC] ∼ 3.2 in the Evolution of the L[CLC]y[/CLC]α Forest Optical Depth

The effective optical depth in the Ly? forest region of 1061 low-resolution QSO spectra drawn from the SDSS database decreases with decreasing redshift over the range 2.5 ? z ? 4. Although the evolution is relatively smooth, ?eff ? (1 + z)3.8?0.2, at z ~ 3.2 the effective optical depth decreases suddenly by about 10% with respect to this smoother evolution. It climbs back to the original smooth scaling again by z ~ 2.9. We describe two techniques, one of which is new, for quantifying this evolution that give consistent results. A variety of tests show that the feature is not likely to be a consequence of how the QSO sample was selected nor the result of flux calibration or other systematic effects. Other authors have argued that, at this same epoch, the temperature of the IGM also shows a departure from an otherwise smooth decrease with time. These features in the evolution of the temperature and the optical depth are signatures of the reionization of He II.

Temperature fluctuations in the intergalactic medium

ABSTRA C T The temperature of the low-density intergalactic medium (IGM) is set by the balance between adiabatic cooling resulting from the expansion of the Universe, and photoheating by the ultraviolet (UV) background. We have analysed the Lya forest of 11 high-resolution quasar spectra using wavelets, and find strong evidence of a marked jump in the temperature at the mean density, T0 ,o f 60^ 14 per cent over the redshift interval zaâ3:5; 3:1a, which we attribute to reionization of He II. The jump can be seen in all three of our spectra that straddle redshift 3.3, at a significance of > 99 per cent. Below z , 3:1, our results are consistent with a smooth cooling down of the universe, as expected when adiabatic expansion dominates over photoheating by a UV background from quasi-stellar objects (QSOs) and galaxies. We find no evidence of thermal fluctuations on scales > 5000 km s 21 larger than 50 per cent, which could be detected by our method, suggesting that the IGM follows a reasonably well-defined temperature ‐ density relation. We demonstrate that the mean wavelet amplitude kAl/ 1/T 0, and calibrate the relation with hydrodynamical simulations. We find T 0 > 1:2 £ 10 4 Ka t z > 3:6. Such high temperatures suggest that H I reionization occurred relatively recently.

Constraints on Reionization from the Thermal History of the Intergalactic Medium

The temperature of the diffuse, photoheated intergalactic medium (IGM) depends on its reionization history because the thermal timescales are long. The widths of the hydrogen Lyα absorption lines seen in the spectra of distant quasars that arise in the IGM can be used to determine its temperature. We use a wavelet analysis of the Lyα forest region of quasar spectra to demonstrate that there is a relatively sudden increase in the line widths between redshifts z ≈ 3.5 and 3.0, which we associate with entropy injection resulting from the reionization of He II. The subsequent falloff in temperature after z ≈ 3.5 is consistent with a thermal evolution dominated by adiabatic expansion. If, as expected, the temperature also drops rapidly after hydrogen reionization, then the high temperatures inferred from the line widths before He II reionization imply that hydrogen reionization occurred below redshift z = 9.

Simulating reionization in numerical cosmology

The incorporation of radiative transfer effects into cosmological hydrodynamical simulations is essential for understanding how the intergalactic medium (IGM) makes the transition from a neutral medium to one that is almost fully ionized. Here, we present an approximate numerical method designed to study in a statistical sense, how a cosmological density field is ionized by a set of discrete point sources. A diffuse background radiation field is also computed self-consistently in our procedure. The method requires relatively few time steps and can be employed with simulations having high resolution. We describe the details of the algorithm and provide a description of how the method can be applied to the output from a pre-existing cosmological simulation to study the systematic reionization of a particular ionic species. As a first application, we compute the reionization of He II by quasars in the range 3≲ z≲6.

The thermal history of the intergalactic medium

At redshifts z ≳ 2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Lyα forest. This photoheated gas follows a tight temperature-density relation which introduces a cut-off in the distribution of widths of the Lyα absorption lines (b-parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He ii.

The Evolution of the Effective Equation of State of the Intergalactic Medium

We develop a method to extract the equation of of the intergalactic medium from the Doppler (b) parameter distribution of the low-density Lyα forest. We test the method on numerical simulations and then apply it to published observations of the Lyα forest at redshifts z 0-4. We find that the effective equation of state is close to isothermal at redshift z ~ 3, indicating that a second reheating of the intergalactic medium took place at z ~ 3. This reheating can plausibly be identified with the reionization of He II observed to occur at z ~ 3.

Elastic scattering and charge exchange in He + ion scattering from alkali metal overlayers

Low energy (1 keV) He + ion scattering data from clean Cu(110) reveal simple binary collision spectra expected from a surface which readily neutralizes the ions by an Auger process. Experiments with sodium, potassium and caesium overlayers show that scattering from these species leads to energy loss features characteristic of reionization by electron promotion; similar features in the copper scattering peak on alkali adsorption are thought to be due to inelastic losses in the overlayer. Sodium adsorption in particular also causes a high background of inelastically scattered ions to be observed, implying reionization of He + ions which penetrate the copper substrate; the mechanism for this reionization is unclear.