Showing papers on "Reionization published in 2003"

First year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Preliminary maps and basic results

Abstract: We present full sky microwave maps in five frequency bands (23 to 94 GHz) from the WMAP first year sky survey. Calibration errors are less than 0.5% and the low systematic error level is well specified. The cosmic microwave background (CMB) is separated from the foregrounds using multifrequency data. The sky maps are consistent with the 7 in. full-width at half-maximum (FWHM) Cosmic Background Explorer (COBE) maps. We report more precise, but consistent, dipole and quadrupole values. The CMB anisotropy obeys Gaussian statistics with -58 less than f(sub NL) less than 134 (95% CL). The 2 less than or = l less than or = 900 anisotropy power spectrum is cosmic variance limited for l less than 354 with a signal-to-noise ratio greater than 1 per mode to l = 658. The temperature-polarization cross-power spectrum reveals both acoustic features and a large angle correlation from reionization. The optical depth of reionization is tau = 0.17 +/- 0.04, which implies a reionization epoch of t(sub r) = 180(sup +220, sub -80) Myr (95% CL) after the Big Bang at a redshift of z(sub r) = 20(sup +10, sub -9) (95% CL) for a range of ionization scenarios. This early reionization is incompatible with the presence of a significant warm dark matter density. A best-fit cosmological model to the CMB and other measures of large scale structure works remarkably well with only a few parameters. The age of the best-fit universe is t(sub 0) = 13.7 +/- 0.2 Gyr old. Decoupling was t(sub dec) = 379(sup +8, sub -7)kyr after the Big Bang at a redshift of z(sub dec) = 1089 +/- 1. The thickness of the decoupling surface was Delta(sub z(sub dec)) = 195 +/- 2. The matter density of the universe is Omega(sub m)h(sup 2) = 0.135(sup +0.008, sub -0.009) the baryon density is Omega(sub b)h(sup 2) = 0.0224 +/- 0.0009, and the total mass-energy of the universe is Omega(sub tot) = 1.02 +/- 0.02. There is progressively less fluctuation power on smaller scales, from WMAP to fine scale CMB measurements to galaxies and finally to the Ly-alpha forest. This is accounted for with a running spectral index, significant at the approx. 2(sigma) level. The spectral index of scalar fluctuations is fit as n(sub s) = 0.93 +/-0.03 at wavenumber k(sub o) = 0.05/Mpc ((sub eff) approx. = 700), with a slope of dn(sub s)/d I(sub nk) = -0.031(sup + 0.016, sub -0.018) in the best-fit model.

First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Preliminary Maps and Basic Results

Abstract: We present full sky microwave maps in five bands (23 to 94 GHz) from the WMAP first year sky survey. Calibration errors are 1 per mode to l=658. The temperature-polarization cross-power spectrum reveals both acoustic features and a large angle correlation from reionization. The optical depth of reionization is 0.17 +/- 0.04, which implies a reionization epoch of 180+220-80 Myr (95% CL) after the Big Bang at a redshift of 20+10-9 (95% CL) for a range of ionization scenarios. This early reionization is incompatible with the presence of a significant warm dark matter density. The age of the best-fit universe is 13.7 +/- 0.2 Gyr old. Decoupling was 379+8-7 kyr after the Big Bang at a redshift of 1089 +/- 1. The thickness of the decoupling surface was dz=195 +/- 2. The matter density is Omega_m h^2 = 0.135 +0.008 -0.009, the baryon density is Omega_b h^2 = 0.0224 +/- 0.0009, and the total mass-energy of the universe is Omega_tot = 1.02 +/- 0.02. The spectral index of scalar fluctuations is fit as n_s = 0.93 +/- 0.03 at wavenumber k_0 = 0.05 Mpc^-1, with a running index slope of dn_s/d ln k = -0.031 +0.016 -0.018 in the best-fit model. This flat universe model is composed of 4.4% baryons, 22% dark matter and 73% dark energy. The dark energy equation of state is limited to w<-0.78 (95% CL). Inflation theory is supported with n_s~1, Omega_tot~1, Gaussian random phases of the CMB anisotropy, and superhorizon fluctuations. An admixture of isocurvature modes does not improve the fit. The tensor-to-scalar ratio is r(k_0=0.002 Mpc^-1)<0.90 (95% CL).

First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature-Polarization Correlation

Abstract: The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped the full sky in Stokes I, Q, and U parameters at frequencies of 23, 33, 41, 61, and 94 GHz. We detect correlations between the temperature and polarization maps significant at more than 10 σ. The correlations are inconsistent with instrument noise and are significantly larger than the upper limits established for potential systematic errors. The correlations are present in all WMAP frequency bands with similar amplitude from 23 to 94 GHz and are consistent with a superposition of a cosmic microwave background (CMB) signal with a weak foreground. The fitted CMB component is robust against different data combinations and fitting techniques. On small angular scales (θ 20 agree well with the signal predicted solely from the temperature power spectra, with no additional free parameters. We detect excess power on large angular scales (θ > 10°) compared to predictions based on the temperature power spectra alone. The excess power is well described by reionization at redshift 11 < zr < 30 at 95% confidence, depending on the ionization history. A model-independent fit to reionization optical depth yields results consistent with the best-fit Λ-dominated cold dark matter model, with best-fit value τ = 0.17 ± 0.04 at 68% confidence, including systematic and foreground uncertainties. This value is larger than expected given the detection of a Gunn-Peterson trough in the absorption spectra of distant quasars and implies that the universe has a complex ionization history: WMAP has detected the signal from an early epoch of reionization.

First year Wilkinson Microwave Anisotropy Probe (WMAP) observations: The Angular power spectrum

Abstract: We present the angular power spectrum derived from the first-year Wilkinson Microwave Anisotropy Probe (WMAP) sky maps We study a variety of power spectrum estimation methods and data combinations and demonstrate that the results are robust The data are modestly contaminated by diffuse Galactic foreground emission, but we show that a simple Galactic template model is sufficient to remove the signal Point sources produce a modest contamination in the low frequency data After masking approximately 700 known bright sources from the maps, we estimate residual sources contribute approximately 3500 mu sq Kappa at 41 GHz, and approximately 130 mu sq Kappa at 94 GHz, to the power spectrum [iota(iota + 1)C(sub iota)/2pi] at iota = 1000 Systematic errors are negligible compared to the (modest) level of foreground emission Our best estimate of the power spectrum is derived from 28 cross-power spectra of statistically independent channels The final spectrum is essentially independent of the noise properties of an individual radiometer The resulting spectrum provides a definitive measurement of the CMB power spectrum, with uncertainties limited by cosmic variance, up to iota approximately 350 The spectrum clearly exhibits a first acoustic peak at iota = 220 and a second acoustic peak at iota approximately 540, and it provides strong support for adiabatic initial conditions Researchers have analyzed the CT(sup Epsilon) power spectrum, and present evidence for a relatively high optical depth, and an early period of cosmic reionization Among other things, this implies that the temperature power spectrum has been suppressed by approximately 30% on degree angular scales, due to secondary scattering

Wilkinson Microwave Anisotropy Probe (WMAP) First Year Observations: TE Polarization

Abstract: The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped the full sky in Stokes I, Q, and U parameters at frequencies 23, 33, 41, 61, and 94 GHz. We detect correlations between the temperature and polarization maps significant at more than 10 standard deviations. The correlations are present in all WMAP frequency bands with similar amplitude from 23 to 94 GHz, and are consistent with a superposition of a CMB signal with a weak foreground. The fitted CMB component is robust against different data combinations and fitting techniques. On small angular scales theta 20 agree well with the signal predicted solely from the temperature power spectra, with no additional free parameters. We detect excess power on large angular scales (theta > 10 deg) compared to predictions based on the temperature power spectra alone. The excess power is well described by reionization at redshift 11 < z_r < 30 at 95% confidence, depending on the ionization history. A model-independent fit to reionization optical depth yields results consistent with the best-fit LambdaCDM model, with best fit value tau = 0.17 +- 0.04 at 68% confidence, including systematic and foreground uncertainties. This value is larger than expected given the detection of a Gunn-Peterson trough in the absorption spectra of distant quasars, and implies that the universe has a complex ionization history: WMAP has detected the signal from an early epoch of reionization.

The history of star formation in a Λ cold dark matter universe

Abstract: Employing hydrodynamic simulations of structure formation in a A cold dark matter cosmology, we study the history of cosmic star formation from the 'dark ages' at redshift z ∼20 to the present. In addition to gravity and ordinary hydrodynamics, our model includes radiative heating and cooling of gas, star formation, supernova feedback and galactic winds. By making use of a comprehensive set of simulations on interlocking scales and epochs, we demonstrate numerical convergence of our results on all relevant halo mass scales, ranging from 10 8 to 10 1 5 h - 1 M O .. The predicted density of cosmic star formation, ρ * (z), is broadly consistent with measurements, given the observational uncertainty. From the present epoch, ρ * (z) gradually rises by approximately a factor of 10 to a peak at z ∼5-6, which is beyond the redshift range where it has been estimated observationally. In our model, fully 50 per cent of the stars are predicted to have formed by redshift z ≃ 2.14, and are thus older than 10.4 Gyr, while only 25 per cent form at redshifts lower than z ≃ 1. The mean age of all stars at the present is approximately 9 Gyr. Our model predicts a total stellar density at z = 0 of Ω * = 0.004, corresponding to approximately 10 per cent of all baryons being locked up in long-lived stars, in agreement with recent determinations of the luminosity density of the Universe. We determine the 'multiplicity function of cosmic star formation' as a function of redshift; i.e. the distribution of star formation with respect to halo mass. At redshifts around z ≃ 10, star formation occurs preferentially in haloes of mass 10 8 -10 1 0 h - 1 M O ., while at lower redshifts, the dominant contribution to ρ * (z) comes from progressively more massive haloes. Integrating over time, we find that approximately 50 per cent of all stars formed in haloes less massive than 10 1 1 . 5 h - 1 M O ., with nearly equal contributions per logarithmic mass interval in the range 10 1 0 -10 1 3 . 5 h - 1 M O ., making up ∼70 per cent of the total. We also briefly examine possible implications of our predicted star formation history for reionization of hydrogen in the Universe. According to our model, the stellar contribution to the ionizing background is expected to rise for redshifts z > 3, at least up to redshift z ∼5, in accord with estimates from simultaneous measurements of the H and He opacities of the Lyman-a forest. This suggests that the ultraviolet background will be dominated by stars for z > 4, provided that there are not significantly more quasars at high z than are presently known. We measure the clumping factor of the gas from the simulations and estimate the growth of cosmic H II regions, assuming a range of escape fractions for ionizing photons. We find that the star formation rate predicted by the simulations is sufficient to account for hydrogen reionization by z ∼6, but only if a high escape fraction close to unity is assumed.

First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters

Abstract: WMAP precision data enables accurate testing of cosmological models. We find that the emerging standard model of cosmology, a flat Lambda-dominated universe seeded by nearly scale-invariant adiabatic Gaussian fluctuations, fits the WMAP data. With parameters fixed only by WMAP data, we can fit finer scale CMB measurements and measurements of large scle structure (galaxy surveys and the Lyman alpha forest). This simple model is also consistent with a host of other astronomical measurements. We then fit the model parameters to a combination of WMAP data with other finer scale CMB experiments (ACBAR and CBI), 2dFGRS measurements and Lyman alpha forest data to find the model's best fit cosmological parameters: h=0.71+0.04-0.03, Omega_b h^2=0.0224+-0.0009, Omega_m h^2=0.135+0.008-0.009, tau=0.17+-0.06, n_s(0.05/Mpc)=0.93+-0.03, and sigma_8=0.84+-0.04. WMAP's best determination of tau=0.17+-0.04 arises directly from the TE data and not from this model fit, but they are consistent. These parameters imply that the age of the universe is 13.7+-0.2 Gyr. The data favors but does not require a slowly varying spectral index. By combining WMAP data with other astronomical data sets, we constrain the geometry of the universe, Omega_tot = 1.02 +- 0.02, the equation of state of the dark energy w = -1), and the energy density in stable neutrinos, Omega_nu h^2 < 0.0076 (95% confidence limit). For 3 degenerate neutrino species, this limit implies that their mass is less than 0.23 eV (95% confidence limit). The WMAP detection of early reionization rules out warm dark matter.

Molecular gas in the host galaxy of a quasar at redshift z = 6.42

Abstract: Observations of molecular hydrogen in quasar host galaxies at high redshifts provide fundamental constraints on galaxy evolution, because it is out of this molecular gas that stars form. Molecular hydrogen is traced by emission from the carbon monoxide molecule, CO; cold H2 itself is generally not observable. Carbon monoxide has been detected in about ten quasar host galaxies with redshifts z > 2; the record-holder is at z = 4.69 (refs 1–3). Here we report CO emission from the quasar SDSS J114816.64 + 525150.3 (refs 5, 6) at z = 6.42. At that redshift, the Universe was only 1/16 of its present age, and the era of cosmic reionization was just ending. The presence of about 2 × 1010 M⊙ of H2 in an object at this time demonstrates that molecular gas enriched with heavy elements can be generated rapidly in the youngest galaxies.

The Universe Was Reionized Twice

Abstract: We show that the universe was reionized twice, first at z ~ 15-16 and again at z ~ 6. Such an outcome appears inevitable when normalizing to two well-determined observational measurements, namely, the epoch of the final cosmological reionization at z ~ 6 and the density fluctuations at z ~ 6, which in turn are tightly constrained by Lyα forest observations at z ~ 3. These two observations most importantly fix the product of star formation efficiency and the ionizing photon escape fraction from galaxies at high redshift. The only major assumption made is that the initial mass function of metal-free, Population III stars is top-heavy. To the extent that the relative star formation efficiencies in gaseous minihalos with H2 cooling and large halos with atomic cooling at high redshift are unknown, the primary source for the first reionization is still uncertain. If star formation efficiency in minihalos is at least 10% of that in large halos, then Population III stars in the minihalos may be largely responsible for the first reionization; otherwise, the first reionization will be attributable largely to Population III stars in large halos. In the former case, H2 cooling in minihalos is necessarily efficient. We show that gas in minihalos can be cooled efficiently by H2 molecules and that star formation can continue to take place largely unimpeded throughout the first reionization period, as long as gas is able to accumulate in them. This comes about thanks to two new mechanisms for generating a high X-ray background during the Population III era put forth here, namely, X-ray emission from the cooling energy of Population III supernova blast waves and that from miniquasars powered by Population III black holes. Consequently, H2 formation in the cores of minihalos is significantly induced to be able to counteract the destruction by Lyman-Werner photons produced by the same Population III stars. In addition, an important process for producing a large number of H2 molecules in relic H II regions of high-redshift galaxies, first pointed out by Ricotti, Gnedin, & Shull in 2001, is quantified here for Population III galaxies. It is shown that H2 molecules produced by this process may overwhelm the dissociating effects of the Lyman-Werner photons produced by stars in the same Population III galaxies. As a result, the Lyman-Werner background may not build up in the first place during the Population III era. The long cosmological reionization and reheating history is complex. From z ~ 30, Population III stars gradually heat up and ionize the intergalactic medium, completing the first reionization at z ~ 15-16, followed by a brief period of Δz ~ 1, during which the intergalactic medium stays completely ionized because of sustained ionizing photon emission from concomitant Population III galaxies. The transition from Population III stars to Population II stars at z ~ 13 suddenly reduces, by a factor of ~10, the ionizing photon emission rate, causing hydrogen to rapidly recombine, marking the second cosmological recombination. From z ~ 13 to 6, Compton cooling by the cosmic microwave background and photoheating by the stars self-regulate the Jeans mass and the star formation rate, giving rise to a mean temperature of the intergalactic medium maintained nearly at a constant of ~104 K. Meanwhile, recombination and photoionization balance one another such that the intergalactic medium stays largely ionized during this stage, with n/nH ≥ 0.6. Most of the star formation in this period occurs in large halos with dominant atomic line cooling. We discuss a wide range of implications and possible tests for this new reionization picture. In particular, the Thomson scattering optical depth is increased to 0.10 ± 0.03, compared to 0.027 for the case of only one rapid reionization at z = 6. Upcoming Wilkinson Microwave Anisotropy Probe observations of the polarization of the cosmic microwave background should be able to distinguish between these two scenarios. In addition, properties of minihalos at high redshift (z ≥ 6) will be very different from previous expectations; in particular, they will be largely deprived of gas, perhaps alleviating the cosmological overcooling problem.

Radiation Hydrodynamical Evolution of Primordial H II Regions

Abstract: We simulate the ionization environment of z ~ 20 luminous objects formed within the framework of the current CDM cosmology and compute their UV escape fraction. These objects are likely single very massive stars that are copious UV emitters. We present analytical estimates as well as one--dimensional radiation hydrodynamical calculations of the evolution of these first HII regions in the universe. The initially D--type ionization front evolves to become R--type within $\lesssim 10^5$ yrs at a distance $\sim1$ pc. This ionization front then completely overruns the halo, accelerating an expanding shell of gas outward to velocities in excess of 30 km s$^{-1}$, about ten times the escape velocity of the confining dark matter halo. We find that the evolution of the HII region depends only weakly on the assumed stellar ionizing luminosities. Consequently, most of the gas surrounding the first stars will leave the dark halo whether or not the stars produce supernovae. If they form the first massive seed black holes these are unlikely to accrete within a Hubble time after they formed until they are incorporated into larger dark matter halos that contain more gas. Because these I--fronts exit the halo on timescales much shorter than the stars' main sequence lifetimes their host halos have UV escape fractions of $\gtrsim 0.95$, fixing an important parameter for theoretical studies of cosmological hydrogen reionization.

Reionization of Hydrogen and Helium by Early Stars and Quasars

Abstract: We compute the reionization histories of hydrogen and helium caused by the ionizing radiation fields produced by stars and quasars. For the quasars we use a model based on halo-merger rates that reproduces all known properties of the quasar luminosity function at high redshifts. The less constrained properties of the ionizing radiation produced by stars are modeled with two free parameters: (i) a transition redshift, ztran, above which the stellar population is dominated by massive, zero-metallicity stars and below which it is dominated by a Scalo mass function; and (ii) the product of the escape fraction of stellar ionizing photons from their host galaxies and the star formation efficiency, fescf*. We constrain the allowed range of these free parameters at high redshifts on the basis of the lack of the H I Gunn-Peterson trough at z 6 and the upper limit on the total intergalactic optical depth for electron scattering, τes < 0.18, from recent cosmic microwave background (CMB) experiments. We find that quasars ionize helium by a redshift z ~ 4, but cannot reionize hydrogen by themselves before z ~ 6. A major fraction of the allowed combinations of fescf* and ztran leads to an early peak in the ionized fraction because of the presence of metal-free stars at high redshifts. This sometimes results in two reionization epochs, namely, an early H II or He III overlap phase followed by recombination and a second overlap phase. Even if early overlap is not achieved, the peak in the visibility function for scattering of the CMB often coincides with the early ionization phase rather than with the actual reionization epoch. Consequently, τes does not correspond directly to the reionization redshift. We generically find values of τes 7%, which should be detectable by the MAP satellite.

Photoevaporation of Cosmological Minihalos during Reionization

Abstract: We present the first gas dynamical simulations of the photoevaporation of cosmological minihalos overtaken by the ionization fronts which swept through the IGM during reionization in a LCDM universe, including the effects of radiative transfer. We demonstrate the phenomenon of I-front trapping inside minihalos, in which the weak, R-type fronts which traveled supersonically across the IGM decelerated when they encountered the dense, neutral gas inside minihalos, becoming D-type I-fronts, preceded by shock waves. For a minihalo with virial temperature T_vir < 10^4 K, the I-front gradually burned its way through the minihalo which trapped it, removing all of its baryonic gas by causing a supersonic, evaporative wind to blow backwards into the IGM, away from the exposed layers of minihalo gas just behind the advancing I-front. Such hitherto neglected feedback effects were widespread during reionization. N-body simulations and analytical estimates of halo formation suggest that sub-kpc minihalos such as these, with T_vir < 10^4 K, were so common as to cover the sky around larger-mass source halos and possibly dominate the absorption of ionizing photons. This means that previous estimates of the number of ionizing photons per H atom required to complete reionization which neglected this effect may be too low. Regardless of their effect on the progress of reionization, however, the minihalos were so abundant that random lines of sight thru the high-z universe should encounter many of them, which suggests that it may be possible to observe the processes described here in the absorption spectra of distant sources.

The Cosmic Dispersion Measure from Gamma-Ray Burst Afterglows: Probing the Reionization History and the Burst Environment

Abstract: We show a possible way to measure the column density of free electrons along the light path, the so-called dispersion measure (DM), from the early [~415(ν/1 GHz)-2(DM/105 pc cm-3) s] radio afterglows of the gamma-ray bursts. We find that the proposed Square Kilometer Array can detect bright radio afterglows around the time ~103(ν/160 MHz)-2 s to measure the intergalactic DM (6000 pc cm-3 at redshift z > 6) up to z ~ 30, from which we can determine the reionization history of the universe and identify the missing warm-hot baryons if many DMs can be measured. At low z, the DM in the host galaxy may reach ~105 pc cm-3 depending on the burst environment, which may be probed by the current detectors. Free-free absorption and diffractive scattering may also affect the radio emission at a high density.

Early reionization by miniquasars

Abstract: Motivated by the recent detection by WMAP of a large optical depth to Thomson scattering -- implying a very early reionization epoch -- we assess a scenario where the universe was reionized by `miniquasars' powered by intermediate-mass black holes (IMBHs), the remnants of the first generation of massive stars. Pregalactic IMBHs form within minihalos above the cosmological Jeans mass collapsing at z=24, get incorporated through mergers into larger and larger systems, sink to the center owing to dynamical friction, and accrete cold material. The merger history of dark halos and associated IMBHs is followed by Monte Carlo realizations of the merger hierarchy in a LCDM cosmology. While seed IMBHs that are as rare as the 3.5-sigma peaks of the primordial density field evolve largely in isolation, a significant number of black hole binary systems will form if IMBHs populate the more numerous 3-sigma peaks instead. In the case of rapid binary coalescence a fraction of IMBHs will be displaced from galaxy centers and ejected into the IGM by the `gravitational rocket' effect, rather than accrete and shine as miniquasars. We show that, under a number of plausible assumptions for the amount of gas accreted onto IMBHs and their emission spectrum, miniquasars powered by IMBHs may be responsible for cosmological reionization at z~15. Reionization by miniquasars with a hard spectrum may be more `economical' than stellar reionization, as soft X-rays escape more easily from the dense sites of star formation and travel further than EUV radiation. Energetic photons will make the diffuse IGM warm and weakly ionized prior to the epoch of reionization breakthrough, set an entropy floor, and reduce gas clumping. Future 21 cm observations may detect a preheated, weakly-ionized IGM in emission against the CMB. (abridged)

Fossil H II regions: self-limiting star formation at high redshift

Abstract: Recent results from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite suggest that the intergalactic medium (IGM) was significantly reionized at redshifts as high as z ∼ 17. At this early epoch, the first ionizingsources probably appeared in the shallow potential wells of mini-haloes with virial temperatures T v i r < 10 4 K. Once such an ionizing source turns off, its surrounding H II region Compton cools and recombines. None the less, we show that the 'fossil' H ii regions left behind remain at high adiabats, prohibiting gas accretion and cooling in subsequent generations of mini-haloes. This greatly amplifies feedback effects explored in previous studies, and early star formation is self-limiting. We quantify this effect to show that star formation in mini-haloes cannot account for the bulk of the electron scattering opacity measured by WMAP. which must be due to more massive objects. We argue that gas entropy, rather than IGM metallicity, regulates the evolution of the global ionizing emissivity and impedes full reionization until lower redshifts. We discuss several important consequences of this early entropy floor for reionization. It reduces gas clumping, curtailing the required photon budget for reionization. An entropy floor also prevents H 2 formation and cooling, due to reduced gas densities: it greatly enhances feedback from ultraviolet photodissociation of H 2 . An early X-ray background would also furnish an entropy floor to the entire IGM; thus, X-rays impede rather than enhance H 2 formation. Future 21-cm observations may probe the topology of fossil H II regions.

The Reionization History at High Redshifts. I. Physical Models and New Constraints from Cosmic Microwave Background Polarization

Abstract: The recent discovery of a high optical depth τ to Thomson scattering from the Wilkinson Microwave Anisotropy Probe (WMAP) data implies that significant reionization took place at redshifts z > 6. This discovery has important implications for the sources of reionization and allows, for the first time, constraints to be placed on physical reionization scenarios out to redshift z ~ 20. Using a new suite of semianalytic reionization models, we show that the high value of τ requires a surprisingly high efficiency of the first generation of UV sources for injecting ionizing photons into the intergalactic medium. We find that no simple reionization model can be consistent with the combination of the WMAP result with data from the z 6.5 universe. Satisfying both constraints requires either of the following: (1) H2 molecules form efficiently at z ~ 20, survive feedback processes, and allow UV sources in halos with virial temperatures Tvir 104 K decreased by a factor of 30 between z ~ 20 and z ~ 6. We discuss the relevant physical issues to produce either scenario and argue that both options are viable and allowed by current data. In detailed models of the reionization history, we find that the evolution of the ionized fractions in the two scenarios has distinctive features that Planck can distinguish at 3 σ significance. At the high WMAP value for τ, Planck will also be able to provide tight statistical constraints on reionization model parameters and elucidate much of the physics at the end of the dark ages. The sources responsible for the high optical depth discovered by WMAP should be directly detectable out to z ~ 15 by the James Webb Space Telescope.

Cosmic Dispersion Measure from Gamma-Ray Burst Afterglows: Probing the Reionization History and the Burst Environment

Abstract: We show a possible way to measure the column density of free electrons along the light path, the so-called Dispersion Measure (DM), from the early $[\sim 415 ( u/1 {\rm GHz})^{-2} ({\rm DM}/10^{5} {\rm pc} {\rm cm}^{-3}) {\rm s}]$ radio afterglows of the gamma-ray bursts. We find that the proposed Square Kilometer Array can detect bright radio afterglows around the time $\sim 10^{3}( u/160 {\rm MHz})^{-2}$ s to measure the intergalactic DM ($\simg 6000$ pc cm$^{-3}$ at redshift $z>6$) up to $z\sim 30$, from which we can determine the reionization history of the universe and identify the missing warm-hot baryons. At low $z$, DM in the host galaxy may reach $\sim 10^{5}$ pc cm$^{-3}$ depending on the burst environment, which may be probed by the current detectors. Free-free absorption and diffractive scattering may also affect the radio emission in a high density.

The thermal memory of reionization history

Abstract: The recent measurement by WMAP of a large electron-scattering optical depth τe = 0.17 ± 0.04 is consistent with a simple model of reionization in which the intergalactic medium (IGM) is ionized at redshift z ~ 15 and remains highly ionized thereafter. Here we show that existing measurements of the IGM temperature from the Lyα forest at z ~ 2-4 rule out this "vanilla" model. Under reasonable assumptions about the ionizing spectrum, as long as the universe is reionized before z = 10 and remains highly ionized thereafter, the IGM reaches an asymptotic thermal state that is too cold compared to observations. To simultaneously satisfy the cosmic microwave background and Lyα forest constraints, the reionization history must be complex: reionization begins early at z 15, but there must have been significant (order-of-unity) changes in fractions of neutral hydrogen and/or helium at 6 < z < 10 and/or singly ionized helium at 4 < z < 10. We describe a physically motivated reionization model that satisfies all current observations. We also explore the impact of a stochastic reionization history and show that a late epoch of (He → He ) reionization induces a significant scatter in the IGM temperature, but the scatter diminishes with time quickly. Finally, we provide an analytic formula for the thermal asymptote and discuss possible additional heating mechanisms that might evade our constraints.

Spectroscopic Confirmation of Three Redshift z~5.7 Lyα Emitters from the Large-Area Lyman Alpha Survey

Abstract: Narrowband searches for Lyα 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 Lyα emitters by preferential H I absorption in the blue wing of the line. These objects are undetected in deep BW, V, R, and λ ≈ 6600 A narrowband images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyα forest absorption at redshift z ≈ 5.7. All three objects show large equivalent widths (150 A 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 Lyα emission line (less than 500 km s-1) 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 & Malhotra. Thus, these spectra support the Lyα emitter population statistics from our earlier photometric study, which imply little evolution in number density from z = 5.7 to 4.5 and provide strong evidence that the reionization redshift is zr > 5.7.

Early reionization by the first galaxies

Abstract: Large-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering, τ e ∼ 0.17. The reionization of the Universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results (Ω m = 0.3, Ω Λ = 0.7, h = 0.7, Ω b = 0.04, n = 1 and σ 8 = 0.9). Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously published model with ionizing photon production as expected for zero-metallicity stars distributed according to a standard stellar initial mass function (IMF) (10 6 1 photons per unit solar mass of formed stars) and with a moderate photon escape fraction from galaxies (5 per cent), produces τ e = 0.104, which is within 1.0 to 1.5σ of the 'best' WMAP value. Values of up to 0.16 can be produced by taking larger escape fractions or a top-heavy IMF. The data do not require a separate populations of 'miniquasars' or of stars forming in objects with total masses below 10 9 M O .. Reconciling such early reionization with the observed Gunn-Peterson troughs in z > 6 quasars may be challenging. Possible resolutions of this problem are discussed.

Probing beyond the epoch of hydrogen reionization with 21 centimeter radiation

Abstract: We use numerical simulations of hydrogen reionization by stellar sources in the context of ΛCDM cosmogonies to investigate the 21 (1 + z) cm radio signal expected from the diffuse intergalactic medium (IGM) prior to the epoch of reionization breakthrough at redshift zion. Two reionization scenarios are analyzed in detail: an "early reionization" case with zion ≈ 13, consistent with the recent discovery by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite of a large optical depth to Thomson scattering, and a "late reionization" case with zion ≈ 8. It is a generic prediction of these models that the background of Lyα photons produced by the early generation of stars which ultimately ionize the universe will be sufficiently intense to make intergalactic neutral hydrogen visible against the cosmic microwave background during the "gray age," i.e., zion z 20. Depending on the redshift of reionization breakthrough, broad-beam observations at frequencies 150 MHz (below 100 MHz for the early reionization scenario) with the next generation of radio telescopes should reveal angular fluctuations in the sky brightness temperature in the range 5-20 mK (1 σ) on scales below 5'.

Cosmological constraints from archeops

Abstract: We analyze the cosmological constraints that Archeops places on adiabatic cold dark matter models with passive power-law initial fluctuations. Because its angular power spectrum has small bins in l and large l coverage down to COBE scales, Archeops provides a precise determination of the first acoustic peak in terms of position at multipole l_peak=220 +- 6, height and width. An analysis of Archeops data in combination with other CMB datasets constrains the baryon content of the Universe, Omega(b)h^2 = 0.022 (+0.003,-0.004), compatible with Big-Bang nucleosynthesis and with a similar accuracy. Using cosmological priors obtainedfrom recent non-CMB data leads to yet tighter constraints on the total density, e.g. Omega(tot)=1.00 (+0.03,-0.02) using the HST determination of the Hubble constant. An excellent absolute calibration consistency is found between Archeops and other CMB experiments, as well as with the previously quoted best fit model.The spectral index n is measured to be 1.04 (+0.10,-0.12) when the optical depth to reionization, tau, is allowed to vary as a free parameter, and 0.96 (+0.03,-0.04) when tau is fixed to zero, both in good agreement with inflation.

The Spin-Kinetic Temperature Coupling and the Heating Rate due to Lyman Alpha Scattering before Reionization: Predictions for 21cm Emission and Absorption

Abstract: We investigate the interaction of Lyman alpha photons produced by the first stars in the universe with the intergalactic medium (IGM) prior to reionization. The background Lyman alpha spectral profile is obtained by solving a Fokker-Planck equation. Accurate values of the heating and scattering rates, and the spin-kinetic temperature coupling coefficient, are presented. We show that the heating rate induced by the Lyman alpha scatterings is much lower than found previously, and is basically negligible. The dominant heating source is most likely the X-rays from the first ionizing sources, which are able to penetrate into the atomic medium. The scattering of Lyman alpha photons couples the hydrogen spin temperature to the kinetic temperature. If the first ionizing sources in the universe did not emit significant X-rays, the spin temperature would be rapidly brought down to the very low gas kinetic temperature, and a 21cm absorption signal against the CMB larger than 100 mK would be predicted. However, we argue that sufficient X-rays are likely to have been emitted by the first stellar population, implying that the gas kinetic temperature should rapidly increase, turning a reduced and brief absorption signal into emission, with a smaller amplitude of about 10 mK. The detection of the 21cm absorption and emission feature would be a hallmark in unravelling the history of the ``dark age'' before reionization.

The Distribution of Metallicity in the IGM at z~2.5: OVI and CIV Absorption in the Spectra of 7 QSOs

Abstract: We present a direct measurement of the metallicity distribution function for the high redshift intergalactic medium. We determine the shape of this function using survival statistics, which account for both detections and non-detections of OVI and CIV associated with HI absorption in quasar spectra. Our OVI sample probes the metal content of ~50% of all baryons at z~2.5. We find a median intergalactic abundance of [O,C/H]=-2.82; the differential abundance distribution is approximately lognormal with mean ~-2.85 and \sigma=0.75 dex. Some 60-70% the Lya forest lines are enriched to observable levels ([O,C/H]>-3.5) while the remaining ~30% of the lines have even lower abundances. Thus we have not detected a universal metallicity floor as has been suggested for some Population III enrichment scenaria. In fact, we argue that the bulk of the intergalactic metals formed later than the first stars that are thought to have triggered reionization. We do not observe a significant trend of decreasing metallicity toward the lower density IGM, at least within regions that would be characterized as filaments in numerical simulations. However, an [O/H] enhancement may be present at somewhat high densities. We estimate that roughly half of all baryons at these redshifts have been enriched to [O/H]>=-3.5. We develop a simple model for the metallicity evolution of the IGM, to estimate the chemical yield of galaxies formed prior to z~2.5. We find that the typical galaxy recycled 0.1-0.4% of its mass back into the IGM as heavy elements in the first 3 Gyr after the Big Bang.

Fossil HII Regions: Self-Limiting Star Formation at High Redshift

Abstract: Recent results by the WMAP satellite suggest that the intergalactic medium was significantly reionized at redshifts as high as z~17 At this early epoch, the first ionizing sources likely appeared in the shallow potential wells of mini-halos with virial temperatures T < 10^4 K Once such an ionizing source turns off, its surrounding HII region Compton cools and recombines Nonetheless, we show that the ``fossil'' HII regions left behind remain at high adiabats, prohibiting gas accretion and cooling in subsequent generations of mini-halos Thus, early star formation is self-limiting We quantify this effect to show that star formation in mini-halos cannot account for the bulk of the electron scattering opacity measured by WMAP, which must be due to more massive objects We argue that gas entropy, rather than IGM metallicity, regulates the evolution of the global ionizing emissivity, and impedes full reionization until lower redshifts We discuss several important consequences of this early entropy floor for reionization It reduces gas clumping, curtailing the required photon budget for reionization An entropy floor also prevents H2 formation and cooling, due to reduced gas densities: it greatly enhances feedback from UV photodissociation of H2 An early X-ray background would also furnish an entropy floor to the entire IGM; thus, X-rays impede rather than enhance H2 formation Future 21cm observations may probe the topology of fossil HII regions

The Reionization History at High Redshifts I: Physical Models and New Constraints from CMB Polarization

Abstract: The recent discovery of a high optical depth tau to Thomson scattering from the WMAP data implies that significant reionization took place at redshifts z~15. This discovery has important implications for the sources of reionization, and allows, for the first time, constraints to be placed on physical reionization scenarios out to redshift z~20. Using a new suite of semi-analytic reionization models, we show that the high value of tau requires a surprisingly high efficiency epsilon of the first generation of UV sources for injecting ionizing photons into the intergalactic medium. We find that no simple reionization model can be consistent with the combination of the WMAP result with data from the z 10^4K decreased by a factor of ~ 30 between (z~20) and (z~6). We discuss the relevant physical issues to produce either scenario, and argue that both options are viable, and allowed by current data. In detailed models of the reionization history, we find that the evolution of the ionized fractions in the two scenarios have distinctive features that Planck can distinguish at 3 sigma significance. At the high WMAP value for tau, Planck will also be able to provide tight statistical constraints on reionization model parameters, and elucidate much of the physics at the end of the Dark Ages. The sources responsible for the high optical depth discovered by WMAP should be directly detectable out to z~15 by the James Webb Space Telescope.

Probing reionization with Lyman-alpha emission lines

Abstract: Lyman-alpha emission from high-redshift galaxies may be a powerful probe of the ionization history of the IGM at z>6: the observed Lyman-alpha emission line is sensitive to the neutral fraction of IGM hydrogen in the range 0.1-1. We present calculations of observed Lyman-alpha emission lines from z>6 galaxies, illustrating the effect of varying the many free parameters associated with the emitting galaxy, its halo, and the IGM around the galaxy. In particular, we use a dynamic model of the IGM that includes the effect of IGM infall toward the emitting galaxy. Galactic winds may play a crucial role in determining observed Lyman-alpha line fluxes. We compare our model predictions with observations of two z=6.5 galaxies and conclude that, if galactic winds are allowed for, existing observations place no constraint on the neutral fraction of the IGM at z=6.5. Future space-based observations will constrain the importance of galactic winds; if winds are unimportant for the observed z=6.5 galaxies, our models suggest that the IGM neutral fraction at z=6.5 is <~0.1.

Lyman break galaxies and the star formation rate of the Universe at z≈ 6

Abstract: We determine the space density of UV-luminous starburst galaxies at z≈ 6 using deep HST ACS SDSS-i′ (F775W) and SDSS-z′ (F850LP) and VLT ISAAC J and Ks band imaging of the Chandra Deep Field South. We find eight galaxies and one star with (i′−z′) > 1.5 to a depth of z′AB= 25.6 (an 8σ detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of ≈15 h−270 M⊙ yr−1 at z= 5.9, equivalent to L* for the Lyman-break population at z= 3–4 (ΩΛ= 0.7, ΩM= 0.3). We are sensitive to star-forming galaxies at 5.6 ≲z≲ 7.0 with an effective comoving volume of ≈1.8 × 105h−370 Mpc3 after accounting for incompleteness at the higher redshifts due to luminosity bias. This volume should encompass the primeval subgalactic-scale fragments of the progenitors of about a thousand L* galaxies at the current epoch. We determine a volume-averaged global star formation rate of (6.7 ± 2.7) × 10−4h70 M⊙ yr−1 Mpc−3 at z∼ 6 from rest-frame UV selected starbursts at the bright end of the luminosity function: this is a lower limit because of dust obscuration and galaxies below our sensitivity limit. This measurement shows that at z∼ 6 the star formation density at the bright end is a factor of ∼6 times less than that determined by Steidel et al. for a comparable sample of UV-selected galaxies at z= 3–4, and so extends our knowledge of the star formation history of the Universe to earlier times than previous work and into the epoch where reionization may have occurred.

Cosmic reionization by stellar sources: Population II stars

Abstract: We study the reionization of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high-resolution hydrodynamical simulations. By supplementing a one-step radiative transfer code specifically designed for following ionization processes with an adaptive ray-tracing algorithm, we are able to speed up the calculations significantly to the point where handling a vast number of sources becomes technically feasible. This allows us to study how dim low-mass sources, excluded in previous investigations owing to computational limitations, affect the morphological evolution of the reionization process. Ionizing fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Analysis of numerically converged results for star formation rates and halo mass functions allows us to assess the consequences of not including low-mass objects and enables us to correct for resolution effects. With these corrections, we are able to reduce the effective mass resolution limit for sources to M∼ 4.0 × 107h−1 M⊙ in a 10 h−1 Mpc comoving box. Our calculations reveal that the process by which ionized regions in the intergalactic medium (IGM) percolate is complex and is especially sensitive to the inclusion of dim sources. Moreover, we find that, given the same level of cosmic star formation, the number of ionizing photons required to reionize the Universe is significantly overestimated if sources with masses below ∼109h−1 M⊙ are excluded. This result stems from the fact that low-mass sources preferentially reside in less clumpy environments than their massive counterparts. Consequently, their exclusion has the net effect of concentrating more of the cosmic ionizing radiation in regions which have higher recombination rates. We present the results of our reionization simulation assuming a range of escape fractions for ionizing photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z∼ 6 derived from the z= 6.28 Sloan Digital Sky Survey (SDSS) quasar of Becker and coworkers. We find that, given the amplitude and form of the underlying star formation predictions, an escape fraction near fesc= 0.10–0.20 is most consistent with the observational results. In these models, reionization is expected to have occurred between z∼ 7–8, although the IGM remains fairly opaque until z≃ 6. Our method is also capable of handling the simultaneous reionization of the helium component in the IGM, allowing us to explore the plausibility of the scenario where sources with harder spectra are primarily responsible for reionization. In this case, we find that if the sources responsible for reionizing hydrogen by z∼ 8 had spectra similar to active galactic nuclei, then the helium component of the IGM should have been reionized by z∼ 6. We find that such an early reionization epoch for helium does not necessarily conflict with observational constraints obtained at z≃ 3, but may be challenged by future observations at higher redshifts. The recent WMAP measurements of the electron scattering optical depth (τe= 0.17 ± 0.04 according to the ‘model independent’ analysis of Kogut et al.) appear to be inconsistent with the relatively late onset of reionization by the normal Population II type stars that we consider. In order to simultaneously match the observations from the z= 6.28 SDSS quasar and the optical depth measurement from WMAP with the sources modelled here, we require a boosting factor for the number of ionizing photons released in the fesc= 0.20 model which evolves from unity at z= 6 to ≳50 by z∼ 18. Such a steep enhancement in the stellar production rate of ionizing photons would be consistent with an IMF that becomes more and more top heavy with increasing redshift.

Does the small CMB quadrupole moment suggest new physics

Abstract: Motivated by WMAP's confirmation of an anomalously low value of the quadrupole moment of the CMB temperature fluctuations, we investigate the effects on the CMB of cutting off the primordial power spectrum P(k) at low wave numbers. This could arise, for example, from a break in the inflaton potential, a prior period of matter or radiation domination, or an oscillating scalar field which couples to the inflaton. We reanalyse the full WMAP parameter space supplemented by a low-k cutoff for P(k). The temperature correlations by themselves are better fit by a cutoff spectrum, but including the TE temperature-polarization spectrum reduces this preference to a 1.4σ effect. Inclusion of large scale structure data does not change the conclusion. If taken seriously, the low-k cutoff is correlated with optical depth so that reionization occurs even earlier than indicated by the WMAP analysis.