Showing papers on "Reionization published in 2002"

Evolution of the ionizing background and the epoch of reionization from the spectra of z ∼ 6 quasars

Abstract: We study the process of cosmic reionization and estimate the ionizing background in the intergalactic medium (IGM) using the Lyman series absorption in the spectra of the four quasars at 5.7 3) are still mostly neutral, and the comoving mean free path of ionizing photons is shorter than 8 Mpc. Comparison with simulations of cosmological reionization shows that the observed properties of the IGM at z ~ 6 are typical of those in the era at the end of the overlap stage of reionization when the individual H II regions merge. Thus z ~ 6 marks the end of the reionization epoch. The redshift of reionization constrains the small-scale power of the mass-density fluctuations and the star-forming efficiency of the first generation of objects.

Early Metal Enrichment by Pregalactic Outflows. II. Three-dimensional Simulations of Blow-Away

Abstract: Supernova (SN)-driven pregalactic outflows may be an efficient mechanism for distributing the product of stellar nucleosynthesis over large cosmological volumes prior to the reionization epoch. Here, we present results from three-dimensional numerical simulations of the dynamics of SN-driven bubbles as they propagate through and escape the grasp of subgalactic halos with masses M = 108 h-1 M☉ at redshift z = 9. Halos in this mass range are characterized by very short dynamical timescales (and even shorter gas cooling times) and may therefore form stars in a rapid but intense burst before SN "feedback" quenches further star formation. The hydrodynamic simulations use a nested grid method to follow the evolution of explosive multi-SN events operating on the characteristic timescale of a few times 107 yr, the lifetime of massive stars. The results confirm that if the star formation efficiency of subgalactic halos is 10%, a significant fraction of the halo gas will be lifted out of the potential well ("blow-away"), shock the intergalactic medium, and pollute it with metal-enriched material, a scenario recently advocated by Madau, Ferrara, & Rees. The volume filling factor of the ejecta is of order unity. Depending on the stellar distribution, we find that less than 30% of the available SN energy gets converted into kinetic energy of the blown-away material, the remainder being radiated away. It appears that mechanical feedback is less efficient than expected from simple energetic arguments, as off-nuclear SN explosions drive inward-propagating shocks that tend to collect and pile up cold gas in the central regions of the host halo. Low-mass galaxies at early epochs may survive multiple SN events and continue forming stars.

Radio Foregrounds for the 21 Centimeter Tomography of the Neutral Intergalactic Medium at High Redshifts

Abstract: Absorption or emission against the cosmic microwave background (CMB) radiation may be observed in the redshifted 21 cm line if the spin temperature of the neutral intergalactic medium (IGM) prior to reionization differs from the CMB temperature. This so-called 21 cm tomography should reveal important information on the physical state of the IGM at high redshifts. The fluctuations in the redshifted 21 cm line, due to gas density inhomogeneities at early times, should be observed at meter wavelengths by the next-generation radio telescopes such as the proposed Square Kilometer Array (SKA). Here we show that the extragalactic radio sources provide a serious contamination to the brightness temperature fluctuations expected in the redshifted 21 cm emission from the IGM at high redshifts. Unless the radio source population cuts off at flux levels above the planned sensitivity of SKA, its clustering noise component will dominate the angular fluctuations in the 21 cm signal. The integrated foreground signal is smooth in frequency space, and it should nonetheless be possible to identify the sharp spectral feature arising from the nonuniformities in the neutral hydrogen density during the epoch when the first UV sources reionize the IGM.

Spectroscopic Confirmation of Three Redshift 5.7 Lyman-alpha Emitters from the Large Area Lyman Alpha Survey

Abstract: Narrow-band searches for Lyman alpha emission are an efficient way of identifying star-forming galaxies at high redshifts. We present Keck telescope spectra confirming redshifts z = 5.7 for three objects discovered in the Large Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory. All three spectra show strong, narrow emission lines with the asymmetric profile that is characteristically produced in high redshift Lyman alpha emitters by preferential HI absorption in the blue wing of the line. These objects are undetected in deep Bw, V, R, and 6600A narrow-band images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyman alpha forest absorption at redshift z = 5.7. All three objects show large equivalent widths (>= 150A in the rest-frame), suggesting at least one of the following: a top-heavy initial mass function, very low stellar metallicity, or the presence of an active nucleus. We consider the case for an active nucleus to be weak in all three objects due to the limited width of the Lyman alpha emission line (< 500 km/s) and the absence of any other indicator of quasar activity. The three confirmed high redshift objects were among four spectroscopically observed targets drawn from the sample of 18 candidates presented by Rhoads and Malhotra (2001). Thus, these spectra support the Lyman alpha emitter population statistics from our earlier photometric study, which imply little evolution in number density from z=5.7 to z=4.5 and provide strong evidence that the reionization redshift is greater than 5.7.

The contribution of the first stars to the cosmic infrared background

Abstract: We calculate the contribution to the cosmic infrared background from very massive metal-free stars at high redshift. We explore two plausible star formation models and two limiting cases for the reprocessing of the ionizing stellar emission. We find that Population III stars may contribute significantly to the cosmic near-infrared background if the following conditions are met. (i) The first stars were massive, with M 100 M� . (ii) Molecular hydrogen can cool baryons in low-mass haloes. (iii) Population III star formation is ongoing, and not shut off through negative feedback effects. (iv) Virialized haloes form stars at ∼40 per cent efficiency up to the redshift of reionization, z ∼ 7. (v) The escape fraction of the ionizing radiation into the intergalactic medium is small. (vi) Nearly all of the stars end up in massive black holes without contributing to the metal enrichment of the Universe.

Low-Frequency Gravitational Waves from Massive Black Hole Binaries: Predictions for LISA and Pulsar Timing Arrays

Abstract: The coalescence of massive black hole (BH) binaries due to galaxy mergers provides a primary source of low-frequency gravitational radiation detectable by pulsar timing measurements and by the proposed LISA (Laser Interferometry Space Antenna) observatory. We compute the expected gravitational radiation signal from sources at all redshifts by combining the predicted merger rate of galactic halos with recent measurements of the relation between BH mass, M_bh, and the velocity dispersion of its host galaxy, sigma. Our main findings are as follows: (i) the nHz frequency background is dominated by BH binaries at redshifts z 7; (iii) the number of LISA sources per unit redshift per year should drop substantially after reionization as long as BH formation is triggered by gas cooling in galaxies. Studies of the highest redshift sources among the few hundred detectable events per year, will provide unique information about the physics and history of black hole growth in galaxies.

The 2dF Galaxy Redshift Survey: The Amplitudes of fluctuations in the 2dFGRS and the CMB, and implications for galaxy biasing

Abstract: We compare the amplitudes of fluctuations probed by the 2dF Galaxy Redshift Survey (2dFGRS) and by the latest measurements of the cosmic microwave background (CMB) anisotropies. By combining the 2dFGRS and CMB data, we find the linear-theory rms mass fluctuations in 8 h (-1) Mpc spheres to be sigma (8m) =0.73+/-0.05 (after marginalization over the matter density parameter Omega(m) and three other free parameters). This normalization is lower than the COBE normalization and previous estimates from cluster abundance, but it is in agreement with some revised cluster abundance determinations. We also estimate the scale-independent bias parameter of present-epoch L (s) =1.9L * APM-selected galaxies to be b (L (s) ,z =0)=1.10+/-0.08 on comoving scales of 0.02

On the Direct Detectability of the Cosmic Dark Ages: 21 Centimeter Emission from Minihalos

Abstract: In the standard cold dark matter (CDM) theory of structure formation, virialized minihalos (with Tvir 104 K) form in abundance at high redshift (z > 6), during the cosmic "dark ages." The hydrogen in these minihalos, the first nonlinear baryonic structures to form in the universe, is mostly neutral and sufficiently hot and dense to emit strongly at the 21 cm line. We calculate the emission from individual minihalos and the radiation background contributed by their combined effect. Minihalos create a "21 cm forest" of emission lines. We predict that the angular fluctuations in this 21 cm background should be detectable with the planned Low Frequency Array (LOFAR) and Square Kilometer Array (SKA), thus providing a direct probe of structure formation during the dark ages. Such a detection will serve to confirm the basic CDM paradigm while constraining the shape of the power spectrum of primordial density fluctuations down to much smaller scales than have previously been constrained, the onset and duration of the reionization epoch, and the conditions that led to the first stars and quasars. We present results here for the currently favored, flat ΛCDM model, for different tilts of the primordial power spectrum.

Constraints on reionization from the thermal history of the intergalactic medium

Abstract: 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.

Constraints on Ωm and σ8 from Weak Lensing in Red-Sequence Cluster Survey Fields

Abstract: We have analyzed 53 deg2 of RC-band imaging data from the Red-Sequence Cluster Survey (RCS) and measured the excess correlations in the shapes of galaxies on scales out to ~15. We separate the signal into an "E" (lensing) and "B" (systematics) mode, which allows us to study the contribution from residual systematics and intrinsic alignments. On scales larger than 10', we find no B mode, suggesting that the signal at those scales is caused solely by gravitational lensing. On smaller scales we find a small but significant B mode. This signal is also present when we select a sample of bright (20 0.1 + 0.16Ωm (95% confidence). Comparison of the RCS results with three other recent cosmic shear measurements shows excellent agreement. The current weak-lensing results are also in good agreement with CMB measurements, when we allow the reionization optical depth τ and the spectral index ns to vary. We present a simple demonstration of how the weak-lensing results can be used as a prior in the parameter estimation from CMB measurements to derive constraints on the reionization optical depth τ.

The Detectability of High-Redshift Lyα Emission Lines prior to the Reionization of the Universe

Abstract: For a source of Lyα radiation embedded in a neutral intergalactic medium (IGM) prior to the reionization epoch, the Lyα emission line is strongly suppressed by the intervening IGM. The damping wing of the so-called Gunn-Peterson trough can extend to the red side of the emission line and erase a significant fraction of the total line flux. However, the transmitted fraction increases with the size of the local cosmological H II region surrounding the source, and therefore with the ionizing luminosity and age of the source. Motivated by the recent discovery of a Lyα-emitting galaxy at a redshift z = 6.56 (Hu et al.), possibly prior to the reionization of the IGM, we revisit the effects of a neutral IGM on the Lyα emission line. We show that for faint sources with little or even no ionizing continuum, a sufficiently broad (Δv 300 km s-1) emission line can still remain observable. In particular, the line detected by Hu et al. is consistent with a source embedded in a neutral IGM. We provide characterizations of the asymmetry and total transmitted flux of the Lyα line as functions of the ionizing emissivity of its source. We argue that a statistical sample of Lyα emitters extending beyond the reionization redshift can be a useful probe of reionization.

Constraints on Omega_m and sigma_8 from weak lensing in RCS fields

Abstract: We have analysed 53 square degrees of imaging data from the Red-Sequence Cluster Survey (RCS), and measured the excess correlations in the shapes of galaxies on scales out to ~1.5 degrees. We separate the signal into an ``E''- (lensing) and ``B''-mode (systematics), which allows us to study residual systematics. On scales larger than 10 arcminutes, we find no ``B''-mode. On smaller scales we find a small, but significant ``B''-mode. This signal is also present when we select a sample of bright galaxies. These galaxies are rather insensitive to observational distortions, and we therefore conclude that the oberved ``B''-mode is likely to be caused by intrinsic alignments. We therefore limit the cosmic shear analysis to galaxies with 22 0.1+0.16\Omega_m$ (95% confidence). Comparison of the RCS results with three other recent cosmic shear measurements shows excellent agreement. The current weak lensing results are also in good agreement with CMB measurements, when we allow the reionization optical depth tau and the spectral index n_s to vary. We present a simple demonstration of how the weak lensing results can be used as a prior in the parameter estimation from CMB measurements to derive constraints on the reionization optical depth tau. (abridged)

The Detectability of High Redshift Lyman Alpha Emission Lines Prior to the Reionization of the Universe

Abstract: For a source of Ly alpha radiation embedded in a neutral intergalactic medium (IGM) prior to the reionization epoch, the emission line is strongly suppressed by the intervening IGM. The damping wing of the so-called Gunn-Peterson trough can extend to the red side of the emission line, and erase a significant fraction of the total line flux. However, the transmitted fraction increases with the size of the local cosmological HII region surrounding the source, and therefore with the ionizing luminosity and age of the source. Motivated by the recent discovery of a Ly alpha emitting galaxy at a redshift z=6.56 (Hu et al. 2002), possibly prior to the reionization of the IGM, we revisit the effects of a neutral IGM on the Ly alpha emission line. We show that even for faint sources with little ionizing continuum, the emission line can remain observable. In particular, the line detected by Hu et al. is consistent with a source embedded in a neutral IGM. We provide characterizations of the asymmetry and total transmitted flux of the Ly alpha line as functions of the ionizing emissivity of its source. A statistical sample of Ly alpha emitters extending beyond the reionization redshift can be a useful probe of reionization.

On the Direct Detectability of the Cosmic Dark Ages: 21-cm Emission from Minihalos

Abstract: In the standard Cold Dark Matter (CDM) theory of structure formation, virialized minihalos (with T_vir} 6), during the cosmic ``dark ages.'' The hydrogen in these minihalos, the first nonlinear baryonic structures to form in the universe, is mostly neutral and sufficiently hot and dense to emit strongly at the 21-cm line. We calculate the emission from individual minihalos and the radiation background contributed by their combined effect. Minihalos create a ``21-cm forest'' of emission lines. We predict that the angular fluctuations in this 21-cm background should be detectable with the planned LOFAR and SKA radio arrays, thus providing a direct probe of structure formation during the ``dark ages.'' Such a detection will serve to confirm the basic CDM paradigm while constraining the shape of the power-spectrum of primordial density fluctuations down to much smaller scales than have previously been constrained, the onset and duration of the reionization epoch, and the conditions which led to the first stars and quasars. We present results here for the currently-favored, flat $\Lambda$CDM model, for different tilts of the primordial power spectrum.

H I 21 Centimeter Absorption beyond the Epoch of Reionization

Abstract: We explore the possibility of detecting H I 21 cm absorption by the neutral intergalactic medium (IGM) toward very high redshift radio sources. The epoch considered is between the time when the first ionizing sources form and when the bulk of the neutral IGM becomes ionized. Because of the extreme Lyα opacities of the neutral IGM, objects within this "gray age" can be observed only at wavelengths longer than about 1 μm. We use the latest simulations of the evolution of the IGM in the context of Λ cold dark matter structure formation models constrained by observations of the highest redshift QSOs to predict the optical depth as a function of frequency of the neutral IGM due to the H I 21 cm line. We then simulate radio spectra assuming observational parameters for future large-area radio telescopes. These spectra show that H I 21 cm absorption studies could be a powerful probe of the rich structure of the neutral IGM prior to the epoch of reionization, including ~1% absorption by the mean neutral IGM, plus deeper, narrow lines (≥5% and a few kilometers per second). Most of the variations in transmissivity are due to mild density inhomogeneities with typical values of the cosmic overdensity δ ~ 10, precisely the structures that at later times give rise to the Lyα forest. We also consider sensitivity limits and the evolution of radio source populations and conclude that it is reasonable to hypothesize the existence of an adequate number of high-z radio sources against which such absorption studies could be performed.

The 21 Centimeter Forest: Radio Absorption Spectra as Probes of Minihalos before Reionization

Abstract: We study the absorption along lines of sight toward high-z radio sources caused by the 21 cm transition of neutral hydrogen in the intergalactic medium (IGM) before reionization. Using semianalytic methods, we compute the number density of observable features caused by both "minihalos" (bound objects that are unable to cool efficiently because of their small virial temperatures) and protogalactic disks. We show that both sets of features should be observable by the next generation of low-frequency radio telescopes, including the Low Frequency Array and the Square Kilometer Array, provided that sufficiently bright background sources exist. The statistics of minihalo absorption features seen along lines of sight to radio-loud quasars offer a way to measure the evolution of the radiation background and the IGM temperature with cosmic time. Intersections with disks are much less common but cause significantly deeper absorption features that would be visible in the spectra of both radio-loud quasars and gamma-ray bursts (GRBs). The absorption feature caused by H I in the host galaxy of a GRB should be observable, offering a route to determine spectroscopically the burst redshift.

Approaching Reionization: The Evolution of the Lyman Alpha Forest from Redshifts Four to Six

Abstract: We analyze the Ly alpha forest properties of a sample of 15 high redshift quasars lying between z = 4.42 and z = 5.75, using high signal-to-noise spectra obtained with ESI on the Keck II 10 m telescope. The distribution of transmissions in the Ly alpha region in this redshift range is shown to be consistent with that found in lambda cold dark matter simulations with a smoothly evolving ionization rate as a function of redshift. The extrapolation of the ionizing flux to z = 6.05 lies a factor of two higher than a 2 sigma upper limit placed by Cen & McDonald (2001) at this redshift, based on the Becker et al. (2001) spectra of the z = 6.28 quasar SDSS 1030+0524. However, the data are also consistent with models in which there is substantial variation of the ionization parameter about the mean value, and in this case, dark gaps such as those seen by Becker et al. become much more likely. We conclude that further data are needed before we can be sure that we have seen the epoch of reionization. We also summarize the damped Ly alpha systems seen in these quasar lines of sight and measure their metallicities and kinematic properties. We argue that the mean DLA metallicity has dropped substantially by z = 5 compared with its value at z < 4.

HI 21cm absorption beyond the epoch of re-ionization

Abstract: We explore the possibility of detecting HI 21cm absorption by the neutral intergalactic medium (IGM) toward very high redshift radio sources. The epoch considered is between the time when the first ionizing sources form and when the bulk of the neutral IGM becomes ionized. Due to the extreme Ly-alpha opacities of the neutral IGM, objects within this 'gray age' can only be observed at wavelengths longer than about 1micron. We use the latest simulations of the evolution of the IGM in the context of LambdaCDM structure formation models constrained by observations of the highest redshift QSOs to predict the optical depth as a function of frequency of the neutral IGM due to the HI 21cm line. We then simulate radio spectra assuming observational parameters for future large area radio telescopes. These spectra show that HI 21cm absorption studies could be a powerful probe of the rich structure of the neutral IGM prior to the epoch of reionization, including 1% absorption by the mean neutral IGM, plus deeper, narrow lines (5% and a few km/s). Most of the variations in transmissivity are due to the mild density inhomogeneities with typical values of the cosmic overdensity of about 10, precisely the structures that at later times give rise to the Ly-alpha forest. We also consider sensitivity limits and the evolution of radio source populations, and conclude that it is reasonable to hypothesize the existence of an adequate number of high-z radio sources against which such absorption studies could be performed.

The 21 cm Forest: Radio Absorption Spectra as Probes of Minihalos Before Reionization

Abstract: We study the absorption along lines of sight toward high-z radio sources caused by the 21 cm transition of neutral hydrogen in the intergalactic medium (IGM) before reionization. Using semi-analytic methods, we compute the number density of observable features caused by both ``minihalos'' (bound objects that are unable to cool efficiently because of their small virial temperatures) and protogalactic disks. We show that both sets of features should be observable by the next generation of low-frequency radio telescopes, including the Low Frequency Array and the Square Kilometer Array, provided that sufficiently bright background sources exist. The statistics of minihalo absorption features seen along lines of sight to radio-loud quasars offer a way to measure the evolution of the radiation background and the IGM temperature with cosmic time. Intersections with disks are much less common, but also cause much deeper absorption features that would be visible in the spectra of both radio-loud quasars and gamma-ray bursts (GRBs). The absorption feature caused by HI in the host galaxy of a GRB should be observable, offering a route to determine spectroscopically the burst redshift.

Approaching Reionization: The Evolution of the Lyα Forest from z = 4 to z = 6

Abstract: We analyze the Lyα forest properties of a sample of 15 high-redshift quasars lying between z = 4.42 and z = 5.75, using high signal-to-noise spectra obtained with the Echellette Spectrograph and Imager on the Keck II 10 m telescope. The distribution of transmissions in the Lyα region in this redshift range is shown to be consistent with that found in lambda cold dark matter simulations with a smoothly evolving ionization rate as a function of redshift. The extrapolation of the ionizing flux to z = 6.05 lies a factor of 2 higher than a 2 σ upper limit placed by Cen & McDonald at this redshift, based on the Becker et al. spectra of the z = 6.28 quasar SDSS 1030+0524. However, the data are also consistent with models in which there is substantial variation of the ionization parameter about the mean value, and in this case, dark gaps such as those seen by Becker et al. become much more likely. We conclude that further data are needed before we can be sure that we have seen the epoch of reionization. We also summarize the damped Lyα systems (DLAs) seen in these quasar lines of sight and measure their metallicities and kinematic properties. We argue that the mean DLA metallicity has dropped substantially by z = 5, compared with its value at z < 4.

Hubble Space Telescope Space Telescope Imaging System Observations of the He II Gunn-Peterson Effect toward HE 2347-4342

Abstract: We present an HST Space Telescope Imaging System (STIS) spectrum of the He II Gunn-Peterson effect toward HE 2347-4342. Compared to the previous HST Goddard High Resolution Spectrograph data obtained by Reimers et al. the STIS spectrum has a much improved resolution. The two-dimensional detector also allows us to better characterize the sky and dark background. We confirm the presence of two spectral ranges of much reduced opacity, the opacity gaps, and provide improved lower limits on the He II Gunn-Peterson opacity τ in the high-opacity regions. We use the STIS spectrum together with a Keck-HIRES spectrum covering the corresponding H I Lyα forest to calculate a one-dimensional map of the softness S of the ionization radiation along the line of sight toward HE 2347-4342, where S is the ratio of the H I to He II photoionization rates. We find that S is generally large but presents important variations, from ~30 in the opacity gaps to a 1 σ lower limit of 2300 at z 2.86, in a region that shows an extremely low H I opacity over a 6.5 A spectral range. We note that a large softness parameter naturally accounts for most of the large Si IV/C IV ratios seen in other quasar absorption line spectra. We present a simple model that reproduces the shape of the opacity gaps in absence of large individual absorption lines. We extend the model described in Heap et al. to account for the presence of sources close to the line of sight of the background quasar. As an alternative to the delayed reionization model suggested by Reimers et al., we propose that the large softness observed at z 2.86 is due to the presence of bright soft sources close to the line of sight, i.e., for which the ratio between the number of H I to He II ionizing photons reaching the intergalactic medium is large. We discuss these two models and suggest ways to discriminate between them.

Cosmological Hydrogen Reionization with Three-dimensional Radiative Transfer

Abstract: We present new calculations of the inhomogeneous process of cosmological reionization by carefully following the radiative transfer in precomputed hydrodynamic simulations of galaxy formation. These new computations represent an important step on the way toward fully self-consistent and adaptive calculations, which will eventually cover the enormous range of scales from sizes of individual minihalos to the mean free path of ionizing photons in the postoverlap universe. The goal of such simulations is to include enough realistic physics to accurately model the formation of early structures and the end of the "dark ages." Our new calculations demonstrate that the process by which the ionized regions percolate the universe is complex and that the idea of voids being ionized before overdense regions is too simplistic. It seems that observational information pertaining to the reionization epoch may now be in our grasp, through the detection of Gunn-Peterson troughs at z ~ 6. If so, then the comparison of information from many lines of sight with simulations such as ours may allow us to disentangle details of the ionization history and trace the early formation of structure.

Temperature fluctuations in the intergalactic medium

Abstract: 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.

Nonlinear clustering during the cosmic Dark Ages and its effect on the 21-cm background from minihalos

Abstract: Hydrogen atoms inside virialized minihalos generate a radiation background from redshifted 21-cm line emission whose angular fluctuations reflect clustering during before and during reionization. We have shown elsewhere that this emission may be detectable with the planned Low Frequency Array (LOFAR) and Square Kilometer Array (SKA) in a flat LambdaCDM. This is a direct probe of structure during the ``Dark Ages'' at redshifts z>6 and down to smaller scales than have previously been constrained. In our original calculation, we used a standard approximation known as the ``linear bias'' [e.g. Mo & White (1996)]. Here we improve upon that treatment by considering the effect of nonlinear clustering. To accomplish this, we develop a new analytical method for calculating the nonlinear Eulerian bias of halos, which should be useful for other applications as well. Predictions of this method are compared with the results of LambdaCDM N-body simulations, showing significantly better agreement than the standard linear bias approximation. When applied to the 21-cm background from minihalos, our formalism predicts fluctuations that differ from our original predictions by up to 30% at low frequencies and small scales. However, within the range of frequencies and angular scales at which the signal could be observable by LOFAR and SKA as currently planned, the differences are small and our original predictions prove robust. Our results indicate that while a smaller frequency bandwidth of observation leads to a higher signal that is more sensitive to nonlinear effects, this effect is counteracted by the lowered sensitivity of the radio arrays. We calculate the best frequency bandwidth for these observations to be Delta nu~2 MHz.(abbreviated)

A feature at z ~ 3.2 in the evolution of the Ly-alpha forest optical depth

Abstract: The effective optical depth in the Ly-alpha 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, tau_eff ~ (1+z)^{3.8 pm 0.2}, at z ~ 3.2 the effective optical depth decreases suddenly, by about ten percent 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 which 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.

Evolution of the Ionizing Radiation Background and Star Formation in the Aftermath of Cosmological Reionization

Abstract: The temporal evolution of the ionizing UV background radiation field at high redshift provides a probe of the evolution of the early star formation rate. By comparing the observed levels of absorption in the highest redshift quasar spectra to the predictions of a hydrodynamic simulation, we determine the evolution of the photoionization rate (Γ) for neutral hydrogen in the intergalactic medium over the redshift range 4.9 z 6.1. After accounting for sampling variance, we infer a sharp increase in Γ from z 6.1 to z 5.8, probably implying reionization at this redshift. We find a decrease in Γ from z 5.6 to 5.2 at 3 σ significance. This feature is a generic signature in the aftermath of reionization, entirely consistent with the cosmological reionization process being completed at z ~ 6.1. This generic feature is a result of a significant change in the star formation rate subsequent to the cosmological reionization. There is an abrupt rise of the temperature of the intergalactic medium due to photoheating when it is reionized. This translates to a correspondingly sudden jump in the Jeans mass and a dramatic suppression of gas accretion onto the most abundant (subgalactic) halos at the epochs of interest. The star formation rate suffers a temporary setback in the aftermath of reionization, resulting in a temporary decrease in the amplitude of the ionizing radiation field.

Detection of He II Reionization in the Sloan Digital Sky Survey Quasar Sample

Abstract: The expansion of the universe leads to a rapid drop in the hydrogen Lyα effective optical depth of the intergalactic medium (IGM), eff ∝ (1 + z)3.8, between redshifts 4 and 3. Measurements of the temperature evolution of the IGM and of the He II opacity both suggest that He II reionizes in this redshift range. We use hydrodynamical simulations to show that the observed temperature increase associated with He II reionization leads to a relatively sudden decrease in eff around the reionization epoch of ≈10%. We find clear evidence for such a feature in the evolution of eff determined from a sample of ~1100 quasars obtained from the Sloan Digital Sky Survey. He II reionization starts at redshift ≈3.4 and lasts for Δz ≈ 0.4. The increase in the IGM temperature also explains the widths of hydrogen absorption lines as measured in high-resolution spectra.

Detection of He II reionization in the SDSS quasar sample

Abstract: The expansion of the Universe leads to a rapid drop in the mean hydrogen Ly-alpha opacity tau of the intergalactic medium (IGM), tau (1+z)^{3.8}, between redshifts 4 and 3. Measurements of the temperature evolution of the IGM and of the He II opacity both suggest that He II reionizes in this redshift range. We use hydrodynamical simulations to show that the temperature increase associated with He II reionization leads to a relatively sudden decrease in tau around the reionization epoch of ~ 10 per cent. We find clear evidence for such a feature in the evolution of tau determined from a sample of ~ 1100 quasars obtained from the SDSS. He II reionization starts at redshift ~ 3.4, and lasts for (Delta z) ~ 0.4. The increase in the IGM temperature also explains the widths of hydrogen absorption lines as measured in high-resolution spectra.

Did globular clusters reionize the Universe

Abstract: A problem still unsolved in cosmology is the identification of the sources of radiation able to reionize H I in the intergalactic medium (IGM) by z∼6. Theoretical work and observations seem to indicate that the fraction,(f e s c ), of HI ionizing radiation emitted from galaxies that escapes into the IGM is small in the local Universe ((f e s c )? 10 per cent). At high redshift, galaxies are more compact and probably gas-rich, implying smaller values of (f e s c ) from their discs or spheroids. However, if the sites of star formation are displaced from the disc or spheroid and the star formation efficiency of the proto-clusters is high, then (f e s c ) should be about 1. This star formation scenario is consistent with several models for globular cluster formation. Using simple arguments based on the observed number of globular cluster systems in the local Universe, and assuming that the oldest globular clusters formed before reionization and had (f e s c ) ∼1, I show that they produced enough ionizing photons to reionize the IGM at z 6.

Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization

Abstract: We examine the significance of the first metal-free stars (Pop III) for the cosmological reionization of HI and HeII. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Pop III stellar cluster for HI and HeII being 1.6 and $10^5$ times stronger respectively than those from a Pop II cluster. For the currently favored cosmology, we find that Pop III stars alone can reionize HI and HeII at redshifts, $z$ of about 9 (4.7) and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More realistic scenarios involving combinations of Pop III and Pop II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of HeIII reaches a maximum of about 60 percent at $z$ of about 5.6 if Pop III star formation lasts for $10^9$ yr. Future data on HI reionization can test the amount of small-scale power available to the formation of the first objects, and provide a constraint on values of $\sigma_8$ less than or about 0.7. Since current UV observations indicate an epoch of reionization for HeII at $z$ of about 3, HeII may reionize more than once. Measurements of the HeII Gunn-Peterson effect in the intergalactic medium at redshifts exceeding about 3 may reveal the significance of Pop III stars for HeII reionization, particularly in void regions that may contain relic ionization from early Pop III stellar activity.