Gunn-Peterson Optical Depth Research Articles

EDGES result versus CMB and low-redshift constraints on ionization histories

We examine the results from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), which has recently claimed the detection of a strong absorption in the 21 cm hyperfine transition line of neutral hydrogen, at redshifts demarcating the early stages of star formation. More concretely, we study the compatibility of the shape of the EDGES absorption profile, centered at a redshift of $z \sim 17.2$, with measurements of the reionization optical depth, the Gunn-Peterson optical depth, and Lyman-$\alpha$ emission from star-forming galaxies, for a variety of possible reionization models within the standard $\Lambda$CDM framework (that is, a Universe with a cosmological constant and cold dark matter). When, conservatively, we only try to accommodate the location of the absorption dip, we identify a region in the parameter space of the astrophysical parameters that successfully explains all of the aforementioned observations. However, one of the most abnormal features of the EDGES measurement is the absorption amplitude, which is roughly a factor of two larger than the maximum allowed value in the $\Lambda$CDM framework. We point out that the simple considered astrophysical models that produce the largest absorption amplitudes are unable to explain the depth of the dip and of reproducing the observed shape of the absorption profile.

Warm Dark Matter and Cosmic Reionization

Abstract In models with dark matter made of particles with keV masses, such as a sterile neutrino, small-scale density perturbations are suppressed, delaying the period at which the lowest mass galaxies are formed and therefore shifting the reionization processes to later epochs. In this study, focusing on Warm Dark Matter (WDM) with masses close to its present lower bound, i.e., around the 3 keV region, we derive constraints from galaxy luminosity functions, the ionization history and the Gunn–Peterson effect. We show that even if star formation efficiency in the simulations is adjusted to match the observed UV galaxy luminosity functions in both CDM and WDM models, the full distribution of Gunn–Peterson optical depth retains the strong signature of delayed reionization in the WDM model. However, until the star formation and stellar feedback model used in modern galaxy formation simulations is constrained better, any conclusions on the nature of dark matter derived from reionization observables remain model-dependent.

COSMIC REIONIZATION ON COMPUTERS. I. DESIGN AND CALIBRATION OF SIMULATIONS

Cosmic Reionization On Computers (CROC) is a long-term program of numerical simulations of cosmic reionization. Its goal is to model fully self-consistently (albeit not necessarily from the first principles) all relevant physics, from radiative transfer to gas dynamics and star formation, in simulation volumes of up to 100 comoving Mpc, and with spatial resolution approaching 100 pc in physical units. In this method paper we describe our numerical method, the design of simulations, and the calibration of numerical parameters. Using several sets (ensembles) of simulations in 20 Mpc/h and 40 Mpc/h boxes with spatial resolution reaching 125 pc at z=6, we are able to match the observed galaxy UV luminosity functions at all redshifts between 6 and 10, as well as obtain reasonable agreement with the observational measurements of the Gunn-Peterson optical depth at z<6.

COSMIC REIONIZATION ON COMPUTERS. II. REIONIZATION HISTORY AND ITS BACK-REACTION ON EARLY GALAXIES

We compare the results from several sets of cosmological simulations of cosmic reionization, produced under Cosmic Reionization On Computers (CROC) project, with existing observational data on the high-redshift Ly-alpha forest and the abundance of Ly-alpha emitters. We find good consistency with the observational measurements and the previous simulation work. By virtue of having several independent realizations for each set of numerical parameters, we are able to explore the effect of cosmic variance on observable quantities. One unexpected conclusion we are forced into is that cosmic variance is unusually large at z>6, with both our simulations and, most likely, observational measurements are still not fully converged for even such basic quantities as the average Gunn-Peterson optical depth or the volume-weighted neutral fraction. We also find that reionization has little effect on the early galaxies or on global cosmic star formation history, because galaxies whose gas content is affected by photoionization contain no molecular (i.e. star-forming) gas in the first place. In particular, measurements of the faint end of the galaxy luminosity function by JWST are unlikely to provide a useful constraint on reionization.

Lyα Emitting Galaxies as a Probe of Reionisation

AbstractThe Epoch of Reionization (EoR) represents a milestone in the evolution of our Universe. Star-forming galaxies that existed during the EoR likely emitted a significant fraction ( ~ 5 − 40%) of their bolometric luminosity as Lyα line emission. However, neutral intergalactic gas that existed during the EoR was opaque to Lyα emission that escaped from galaxies during this epoch, which makes it difficult to observe. The neutral intergalactic medium (IGM) may thus reveal itself by suppressing the Lyα flux from background galaxies. Interestingly, a ‘sudden’ reduction in the observed Lyα flux has now been observed in galaxies at z &gt; 6. This review contains a detailed summary of Lyα radiative processes: I describe (i) the main Lyα emission processes, including collisional-excitation &amp; recombination (and derive the origin of the famous factor ‘0.68’), and (ii) basic radiative transfer concepts, including e.g. partially coherent scattering, frequency diffusion, resonant versus wing scattering, optically thick versus ‘extremely’ optically thick (static/outflowing/collapsing) media, and multiphase media. Following this review, I derive expressions for the Gunn-Peterson optical depth of the IGM during (inhomogeneous) reionisation and post-reionisation. I then describe why current observations appear to require a very rapid evolution of volume-averaged neutral fraction of hydrogen in the context of realistic inhomogeneous reionisation models, and discuss uncertainties in this interpretation. Finally, I describe how existing &amp; futures surveys and instruments can help reduce these uncertainties, and allow us to fully exploit Lyα emitting galaxies as a probe of the EoR.

Reionization constraints using principal component analysis

Using a semi-analytical model developed by Choudhury & Ferrara (2005) we study the observational constraints on reionization via a principal component analysis (PCA). Assuming that reionization at z>6 is primarily driven by stellar sources, we decompose the unknown function N_{ion}(z), representing the number of photons in the IGM per baryon in collapsed objects, into its principal components and constrain the latter using the photoionization rate obtained from Ly-alpha forest Gunn-Peterson optical depth, the WMAP7 electron scattering optical depth and the redshift distribution of Lyman-limit systems at z \sim 3.5. The main findings of our analysis are: (i) It is sufficient to model N_{ion}(z) over the redshift range 2<z<14 using 5 parameters to extract the maximum information contained within the data. (ii) All quantities related to reionization can be severely constrained for z<6 because of a large number of data points whereas constraints at z>6 are relatively loose. (iii) The weak constraints on N_{ion}(z) at z>6 do not allow to disentangle different feedback models with present data. There is a clear indication that N_{ion}(z) must increase at z>6, thus ruling out reionization by a single stellar population with non-evolving IMF, and/or star-forming efficiency, and/or photon escape fraction. The data allows for non-monotonic N_{ion}(z) which may contain sharp features around z \sim 7. (iv) The PCA implies that reionization must be 99% completed between 5.8<z<10.3 (95% confidence level) and is expected to be 50% complete at z \approx 9.5-12. With future data sets, like those obtained by Planck, the z>6 constraints will be significantly improved.

The He i 584 Å forest as a diagnostic of helium reionization

We discuss the potential of using the HeI 584 A forest to detect and study HeII reionization. Significant 584 A absorption is expected from intergalactic HeII regions, whereas there should be no detectable absorption from low density gas in HeIII regions. Unlike HeII Ly-alpha absorption (the subject of much recent study), the difficulty with using this transition to study HeII reionization is not saturation but rather that the absorption is weak. The Gunn-Peterson optical depth for this transition is tau ~ 0.1 x_{HeII} Delta^2 [(1+z)/5]^{9/2}, where x_{HeII} is the fraction of helium in HeII and Delta is the density in units of the cosmic mean. In addition, HeI 584 A absorption is contaminated by lower redshift HI Ly-alpha absorption with a comparable flux decrement. We estimate the requirements for a definitive detection of redshifted HeI absorption from low density gas (Delta ~ 1), which would indicate that HeII reionization was occurring. We find that this objective can be accomplished (using coeval HI Ly-alpha absorption to mask dense regions and in cross correlation) with a spectral resolution of 10^4 and a signal-to-noise ratio per resolution element of ~ 10. Such specifications may be achievable on a few known z ~ 3.5 quasar sightlines with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We also discuss how HeI absorption can be used to measure the hardness of the ionizing background above 13.6 eV.

Seeing through the trough: outflows and the detectability of Lyα emission from the first galaxies

The next generation of telescopes aim to directly observe the first generation of galaxies that initiated the reionization process in our Universe. The Lyman Alpha (Lya) emission line is robustly predicted to be the most prominent intrinsic spectral feature of these galaxies, making it an ideal target to search for and study high redshift galaxies. Unfortunately the large Gunn-Peterson optical depth of the surrounding neutral intergalactic medium (IGM) is thought to render this line extremely difficult to detect prior to reionization. In this paper we demonstrate that the radiative transfer effects in the interstellar medium (ISM), which cause Lya flux to emerge from galaxies at frequencies where the Gunn-Peterson optical depth is reduced, can substantially enhance the prospects for detection of the Lya line at high redshift. In particular, scattering off outflows of interstellar HI gas can modify the Lya spectral line shape such that >5% of the emitted Lya radiation is transmitted directly to the observer, even through a fully neutral IGM. It may therefore be possible to directly observe `strong' Lya emission lines (EW > 50 Angstrom rest frame) from the highest redshift galaxies that reside in the smallest HII `bubbles' early in the reionization era with JWST. In addition, we show that outflows can boost the fraction of Lya radiation that is transmitted through the IGM during the latter stages of reionization, and even post-reionization. Coupled with the fact that the first generation of galaxies are thought to have very large intrinsic equivalent Lya equivalent widths, our results suggest that the search for galaxies in their redshifted Lya emission line can be competitive with the drop-out technique out to the highest redshifts that can be probed in the JWST era.

Log-Poisson non-Gaussianity of Lyα transmitted flux fluctuations at high redshift

We investigate the non-Gaussian features of the IGM at redshift $z\sim 5 - 6$ using Ly$\alpha$ transmitted flux of quasar absorption spectra and cosmological hydrodynamic simulation of the concordance $\Lambda$CDM universe. We show that the neutral hydrogen mass density field and Ly$\alpha$ transmitted flux fluctuations possess all the non-Gaussian features predicted by the log-Poisson hierarchy, which depends only on two dimensionless parameters $\beta$ and $\gamma$, describing, respectively, the intermittence and singularity of the random fields. We find that the non-Gaussianity of the Ly$\alpha$ transmitted flux of quasars from $z=4.9$ to $z=6.3$ can be well reconstructed by the hydrodynamical simulation samples. Although the Gunn-Peterson optical depth and its variance underwent a significant evolution in the redshift range of $5 - 6$, the intermittency measured by $\beta$ is almost redshift-independent in this range. More interesting, the intermittency of quasar's absorption spectra on physical scales $0.1-1$ h$^{-1}$Mpc in redshift $5 - 6$ are found to be about the same as that on physical scales $1-10$ h$^{-1}$Mpc at redshifts $2 - 4$. Considering the Jeans length is less than 0.1 h$^{-1}$Mpc at $z\sim 5$, and $1$ h$^{-1}$Mpc at $z\sim 2$, these results imply that the nonlinear evolution in high and low redshifts will lead the cosmic baryon fluid to a state similar to fully developed turbulence. The log-Poisson high order behavior of current high redshift data of quasar's spectrum can be explained by uniform UV background in the redshift range considered. We also studied the log-Poisson non-Gaussianity by considering inhomogeneous background. With several simplified models of inhomogeneous background, we found the effect of the inhomogeneous background on the log-Poisson non-Gaussianity is not larger than 1-sigma.

Ionization state of cosmic hydrogen by early stars and quasars

Cosmic hydrogen is reionized and maintained in its highly ionized state by the ultraviolet emission attributed to an early generation of stars and quasars. The Lyα opacity observed in absorption spectra of high-redshift quasars permits more stringent constraints on the ionization state of cosmic hydrogen. Based on density perturbation and structure formation theory, we develop an analytic model to trace the evolution of the ionization state in the post-overlap epoch of reionization, in which the bias factor is taken into account. For quasars, we represent an improved luminosity function by utilizing a hybrid approach for the halo formation rate that is in reasonable agreement with the published measurements at 2 ≲ z ≲ 6. Comparison with the classic Press–Schechter mass function of dark matter halos, we demonstrate that the biased mass distribution indeed enhances star formation efficiency in the overdense environment by more than 25 per cent following the overlap of ionized bubbles. In addition, an alternative way is introduced to derive robust estimates of the mean free path for ionizing photons. In our model, star-forming galaxies are likely to dominate the ionizing background radiation beyond z = 3, and quasars contribute equally above a redshift of z ∼ 2.5. From 5 ≤ z ≤ 6, the lack of evolution in photoionization rate can thus be explained by the relatively flat evolution in star formation efficiency, although the mean free path of ionizing photons increases rapidly. Moreover, in the redshift interval z ∼ 2–6, the expected mean free path and Gunn–Peterson optical depth obviously evolve by a factor of ∼ 500 and ∼ 50 respectively. We find that the relative values of critical overdensities for hydrogen ionization and collapse could be 430% at z ≈ 2 and 2% at z ≈ 6, suggesting a rapid overlap process in the overdense regions around instant quasars following reionization. We further illustrate that the absolute estimates of the fraction of neutral hydrogen computed from theoretical models may not be important because of comparable uncertainties in the computation.

Four Quasars above Redshift 6 Discovered by the Canada-France High-zQuasar Survey

The Canada-France High-z Quasar Survey (CFHQS) is an optical survey designed to locate quasars during the epoch of reionization. In this paper we present the discovery of the first four CFHQS quasars at redshifts greater than 6, including the most distant known quasar, CFHQS J2329-0301 at z = 6.43. We describe the observational method used to identify the quasars and present optical, infrared, and millimeter photometry and optical and near-infrared spectroscopy. We investigate the dust properties of these quasars, finding an unusual dust extinction curve for one quasar and a high far-infrared luminosity due to dust emission for another. The mean millimeter continuum flux for CFHQS quasars is substantially lower than that for SDSS quasars at the same redshift, likely due to a correlation with quasar UV luminosity. For two quasars with sufficiently high signal-to-noise ratio optical spectra, we use the spectra to investigate the ionization state of hydrogen at z > 5. For CFHQS J1509-1749 at z = 6.12 we find significant evolution (beyond a simple extrapolation of lower redshift data) in the Gunn-Peterson optical depth at z > 5.4. The line of sight to this quasar has one of the highest known optical depths at z ≈ 5.8. An analysis of the sizes of the highly ionized near-zones in the spectra of two quasars at z = 6.12 and 6.43 suggest that the intergalactic medium surrounding these quasars was substantially ionized before these quasars turned on. Together, these observations point toward an extended reionization process, but we caution that cosmic variance is still a major limitation in z > 6 quasar observations.

Cosmic Reionization Redux

We show that numerical simulations of reionization that resolve the Lyman limit systems (and, thus, correctly count absorptions of ionizing photons) have converged to about a 10% level for 5 < z < 6.2 and are in reasonable agreement (within 10%) with the SDSS data in this redshift interval. The SDSS data thus constrain the redshift of overlap of cosmic H II regions to zOVL = 6.1 ± 0.15. At higher redshifts, the simulations are far from convergence on the mean Gunn-Peterson optical depth but achieve good convergence for the mean neutral hydrogen fraction. The simulations that fit the SDSS data, however, do not have nearly enough resolution to resolve the earliest episodes of star formation and are very far from converging on the precise value of the optical depth to Thomson scattering—any value between 6% and 10% is possible, depending on the convergence rate of the simulations and the fractional contribution of Population III stars. This is generally consistent with the third-year WMAP results, but much higher resolution simulations are required to come up with the sufficiently precise value for the Thomson optical depth that can be statistically compared with the WMAP data.

Constraining the reionization history with QSO absorption spectra

We use a semi-analytical approach to simulate absorption spectra of QSOs at high redshifts with the aim of constraining the cosmic reionization history. We consider two physically motivated and detailed reionization histories: (i) an early reionization model (ERM) in which the intergalactic medium is reionized by Pop III stars at z≈ 14, and (ii) a more standard late reionization model (LRM) in which overlapping, induced by QSOs and normal galaxies, occurs at z≈ 6. From the analysis of current Lyα forest data at z < 6, we conclude that it is impossible to disentangle the two scenarios, which fit equally well the observed Gunn–Peterson optical depth, flux probability distribution function and dark gap width distribution. At z > 6, however, clear differences start to emerge which are best quantified by the dark gap and peak width distributions. We find that 35 (0) per cent of the lines of sight (LOS) within 5.7 < z < 6.3 show dark gaps of widths >50 Å in the rest frame of the QSO if reionization is not (is) complete at z≳ 6. Similarly, the ERM predicts peaks of width ∼1 Å in 40 per cent of the LOS in the redshift range 6.0–6.6; in the same range, LRM predicts no peaks of width >0.8 Å. We conclude that the dark gap and peak width statistics represent superb probes of cosmic reionization if about ten QSOs can be found at z > 6. We finally discuss strengths and limitations of our method.

Simulating cosmic reionization at large scales - I. The geometry of reionization

We present the first large-scale radiative transfer simulations of cosmic reionization, in a simulation volume of (100 h−1 Mpc)3. This is more than a two orders of magnitude improvement over previous simulations. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and efficient code for 3D radiative transfer, c2-ray, which we have recently developed. These simulations allow us to do the first numerical studies of the large-scale structure of reionization which at the same time, and crucially, properly take account of the dwarf galaxy ionizing sources which are primarily responsible for reionization. In our realization, reionization starts around z∼ 21, and final overlap occurs by z∼ 11. The resulting electron-scattering optical depth is in good agreement with the first-year Wilkinson Microwave Anisotropy Probe (WMAP) polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earlier, on average, than the voids. Ionization histories of smaller-size (5–10 comoving Mpc) subregions exabit a large scatter about the mean and do not describe the global reionization history well. This is true even when these subregions are at the mean density of the universe, which shows that small-box simulations of reionization have little predictive power for the evolution of the mean ionized fraction. The minimum reliable volume size for such predictions is ∼30 Mpc. We derive the power spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcmin and larger scales. We find two populations of H ii regions according to their size, numerous, mid-sized (∼10-Mpc) regions and a few, rare, very large regions tens of Mpc in size. Thus, local overlap on fairly large scales of tens of Mpc is reached by z∼ 13, when our volume is only about 50 per cent ionized, and well before the global overlap. We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly non-Gaussian. All these quantities are critical for predicting and interpreting the observational signals from reionization from a variety of observations like 21-cm emission, Lyα emitter statistics, Gunn–Peterson optical depth and small-scale cosmic microwave background secondary anisotropies due to patchy reionization.

Cosmic reionization constraints on the nature of cosmological perturbations

We study the reionization history of the Universe in cosmological models with non-Gaussian density fluctuations, taking them to have a renormalized Χ 2 probability distribution function parametrized by the number of degrees of freedom, v. We compute the ionization history using a simple semi-analytical model, considering various possibilities for the astrophysics of reionization. In all our models we require that reionization is completed prior to z = 6, as required by the measurement of the Gunn-Peterson optical depth from the spectra of high-redshift quasars. We confirm previous results demonstrating that such a non-Gaussian distribution leads to a slower reionization as compared to the Gaussian case. We further show that the recent WMAP three-year measurement of the optical depth due to electron scattering, τ = 0.09 ± 0.03, weakly constrains the allowed deviations from Gaussianity on the small scales relevant to reionization if a constant spectral index is assumed. We also confirm the need for a significant suppression of star formation in minihaloes, which increases dramatically as we decrease v.

Wouthuysen-Field coupling strength and application to high-redshift 21-cm radiation

The first ultraviolet sources in the universe are expected to have coupled the H I spin temperature to the gas kinetic temperature via scattering in the Lyα resonance (the 'Wouthuysen-Field effect'). By establishing an Hi spin temperature different from the temperature of the cosmic microwave background, the Wouthuysen-Field effect should allow observations of H I during the reionization epoch in the redshifted 21-cm hyperfine line. This paper investigates four mechanisms that can affect the strength of the Wouthuysen-Field effect that were not previously considered. (1) Photons redshifting into the Hi Lyman resonances may excite an H atom and result in a radiative cascade terminating in two-photon 2s 1/2 → 1s 1/2 emission, rather than always degrading to Lyα as usually assumed. (2) The fine structure of the Lyα resonance alters the photon frequency distribution and leads to a suppression of the scattering rate. (3) The spin-flip scatterings change the frequency of the photon and cause the photon spectrum to relax not to the kinetic temperature of the gas but to a temperature between the kinetic and spin temperatures, effectively reducing the strength of the Wouthuysen-Field coupling. (4) Near line centre, a photon can change its frequency by several times the line width in a single scattering event, thus potentially invalidating the usual calculation of the Lyα spectral distortion based on the diffusion approximation. It is shown that (1) suppresses the Wouthuysen-Field coupling strength by a factor of up to ∼2, while (2) and (3) are important only at low kinetic temperatures. Effect (4) has a ≤3 per cent effect for kinetic temperatures T k ≥ 2 K. In particular, if the pre-reionization intergalactic medium was efficiently heated by X-rays, only effect (1) is important. Fitting formulae for the Wouthuysen-Field coupling strength are provided for the range of T k ≥ 2 K and Gunn-Peterson optical depth 10 5 < TOP < 10 7 so that all of these effects can be easily incorporated into 21-cm codes.

Experimental constraints on self-consistent reionization models

A self-consistent formalism to jointly study cosmic reionization and thermal history of the intergalactic medium (IGM) in a ΛCDM cosmology is presented. The model implements most of the relevant physics governing these processes, such as the inhomogeneous IGM density distribution, three different classes of ionizing photon sources [massive Population III (PopIII) stars, Population II (PopII) stars and quasi-stellar objects (QSOs)], and radiative feedback inhibiting star formation in low-mass galaxies. By constraining the model free parameters with available data on redshift evolution of Lyman-limit absorption systems, Gunn–Peterson and electron scattering optical depths, near-infrared background and cosmic star formation history, we select a fiducial model, whose main predictions are as follows. (i) Hydrogen was completely reionized at z≈ 15, while He ii must have been reionized by z≈ 12, allowing for the uncertainties in the ionizing photon efficiencies of stars. At z≈ 7, He iii suffered an almost complete recombination as a result of the extinction of PopIII stars, as required by the interpretation of the NIRB. (ii) A QSO-induced complete He ii reionization occurs at z= 3.5; a similar double H reionization does not take place due to the large number of photons with energies >13.6 eV from PopII stars and QSOs, even after all PopIII stars have disappeared. (iii) Following reionization, the temperature of the IGM corresponding to the mean gas density, T0, is boosted to 1.5 × 104 K; following that it decreases with a relatively flat trend. Observations of T0 are consistent with the fact that He is singly ionized at z≳ 3.5, while they are consistent with He being doubly ionized at z≲ 3.5. This might be interpreted as a signature of (second) He ii reionization. (iv) Only 0.3 per cent of the stars produced by z= 2 need to be PopIII stars in order to achieve the first hydrogen reionization. In addition, we get useful constraints on the ionizing photon efficiencies (which are a combination of the star-forming efficiency and the escape fraction of ionizing photons from collapsed haloes) of PopII and PopIII stars, namely, ePopII < 0.01, 0.002 < ePopIII < 0.03. Varying the efficiencies in these two ranges does not affect the scenario described above. Such a model not only relieves the tension between the Gunn–Peterson optical depth and WMAP observations, but also accounts self-consistently for all known observational constraints. We discuss how the results compare with recent numerical reionization studies and other theoretical arguments.

Cosmic reionization by stellar sources: population III stars

We combine fast radiative transfer calculations with high-resolution hydrodynamical simulations to study an epoch of early hydrogen reionization by primordial stellar sources at redshifts 15 ≤ z ≤ 30. We consider the implications of various local and global feedback mechanisms using a set of models that bracket the severity of these effects to determine, qualitatively, how they may have influenced the global star formation rate and the details of hydrogen reionization. With relatively conservative assumptions, most of our models suggest that population III star formation proceeds in a self-regulated manner both locally and globally and, for a conventional ACDM cosmology, can significantly reionize the intergalactic medium between 15 ≤ z ≤ 20 as long as a large fraction of ionizing photons can escape from these earliest galaxies. We then combine these results with our earlier work focusing on the role of population II stars in galaxies with virial temperatures ≥ 10 4 K at redshifts 5 ≤ z ≤ 20. Hence, we construct a complete reionization history of the Universe that matches the Thomson optical depths as measured by the WMAP satellite as well as the evolution of the Gunn-Peterson optical depth as seen in the absorption spectra of the highest redshift quasars. We find that even with conservative estimates for the impact of negative feedback mechanisms, primordial stellar sources contribute significantly to early reionization. Future observations of a Thomson optical depth of τ e ≥ 0.13 would bolster the claim for the existence of population III stars similar to the ones studied here.

Probing the dark ages with redshift distribution of gamma-ray bursts

In this article, we explore the possibility of using the properties of gamma-ray bursts (GRBs) to probe the physical conditions in the epochs prior to reionization. The redshift distribution of GRBs is modelled using the Press–Schechter formalism with an assumption that they follow the cosmic star formation history. We reproduce the observed star formation rate obtained from galaxies in the redshift range 0 < z < 5, as well as the redshift distribution of the GRBs inferred from the luminosity–variability correlation of the burst light curve. We show that the fraction of GRBs at high redshifts, the afterglows of which cannot be observed in the R and I bands owing to H i Gunn–Peterson optical depth can, at the most, account for one third of the dark GRBs. The observed redshift distribution of GRBs, with much less scatter than the one available today, can put stringent constraints on the epoch of reionization and the nature of gas cooling in the epochs prior to reionization.

The Gunn–Peterson Effect: A Test for a Black-Hole-Induced Photoionization of the Intergalactic Medium

There are many experimental evidences of the presence of an ionizing background radiation flux at large redshifts, whose nature is doubtful. A lot of information about the characteristics of such a background can be obtained both from the study of the Gunn–Peterson effect and from the so-called "proximity effect." In some previous works I suggested the possibility that this ionizing flux comes from the quantum evaporation of primordial black holes (PBH's); here, I discuss the constraints that the experimental measurements put upon the free parameters of this reionization model and I try to verify its reliability. In particular, the radiation intensity of the background at the hydrogen Lyman edge, as inferred from the proximity effect, enables me to determine an upper limit to the PBH's average relic mass; due to our poor knowledge of the ultimate fate of the evaporating black holes, this limit represents an important theoretical information. In the second part of this paper I study the absorption of the ionizing background due to Lyα clouds: in particular, I discuss this phenomenon in the presence of different absorption levels and I calculate the HI Gunn–Peterson optical depth τ GP (z); from a comparison with the experimental data of Giallongo et al.(τ GP, HI &lt; 0.02 ± 0.03) I obtain a constraint on the intergalactic medium density parameter, namely Ω IGM &lt; 0.020. A study of the characteristics of the absorbers is also performed: I determine the hydrogen gas density nH,c and the column density N HI for Lyα clouds; a satisfactory agreement with the available experimental data is obtained in the case of expanding, adiabatically cooled clouds. Finally, the same kind of analysis is performed for He II: in this case, the theoretical optical depth I obtain is smaller than the preliminary experimental lower limit of Jakobsen et al.(τ GP &gt; 1.7).