Escape Fraction Of Ionizing Photons Research Articles

Binary stars can provide the ‘missing photons’ needed for reionization

Empirical constraints on reionization require galactic ionizing photon escape fractions fesc>20%, but recent high-resolution radiation-hydrodynamic calculations have consistently found much lower values ~1-5%. While these models include strong stellar feedback and additional processes such as runaway stars, they almost exclusively consider stellar evolution models based on single (isolated) stars, despite the fact that most massive stars are in binaries. We re-visit these calculations, combining radiative transfer and high-resolution cosmological simulations from the Feedback in Realistic Environments (FIRE) project. For the first time, we use a stellar evolution model that includes a physically and observationally motivated treatment of binaries (the BPASS model). Binary mass transfer and mergers enhance the population of massive stars at late times (>3 Myr) after star formation, which in turn strongly enhances the late-time ionizing photon production (especially at low metallicities). These photons are produced after feedback from massive stars has carved escape channels in the ISM, and so efficiently leak out of galaxies. As a result, the time-averaged "effective" escape fraction (ratio of escaped ionizing photons to observed 1500 A photons) increases by factors 4-10, sufficient to explain reionization. While important uncertainties remain, we conclude that binary evolution may be critical for understanding the ionization of the Universe.

HUBBLE FRONTIER FIELDS FIRST COMPLETE CLUSTER DATA: FAINT GALAXIES ATz∼ 5-10 FOR UV LUMINOSITY FUNCTIONS AND COSMIC REIONIZATION

We present the comprehensive analyses of faint dropout galaxies up to $z\sim10$ with the first full-depth data set of Abell 2744 lensing cluster and parallel fields observed by the Hubble Frontier Fields (HFF) program. We identify $54$ dropouts at $z\sim5-10$ in the HFF fields, and enlarge the size of $z\sim9$ galaxy sample obtained to date. Although the number of highly magnified ($\mu\sim10$) galaxies is small due to the tiny survey volume of strong lensing, our study reaches the galaxies' intrinsic luminosities comparable to the deepest-field HUDF studies. We derive UV luminosity functions with these faint dropouts, carefully evaluating the combination of observational incompleteness and lensing effects in the image plane by intensive simulations including magnification, distortion, and multiplication of images, with the evaluations of mass model dependences. Our results confirm that the faint-end slope, $\alpha$, is as steep as $-2$ at $z\sim6-8$, and strengthen the evidence of the rapid decrease of UV luminosity densities, $\rho_\mathrm{UV}$, at $z>8$ from the large $z\sim9$ sample. We examine whether the rapid $\rho_\mathrm{UV}$ decrease trend can reconcile with the large Thomson scattering optical depth, $\tau_\mathrm{e}$, measured by CMB experiments allowing a large space of free parameters such as average ionizing photon escape fraction and stellar-population dependent conversion factor. No parameter set can reproduce both the rapid $\rho_\mathrm{UV}$ decrease and the large $\tau_\mathrm{e}$. It is possible that the $\rho_\mathrm{UV}$ decrease moderates at $z\gtrsim11$, that the free parameters significantly evolve towards high-$z$, or that there exist additional sources of reionization such as X-ray binaries and faint AGNs.

ACCELERATED EVOLUTION OF THE Lyα LUMINOSITY FUNCTION ATz≳ 7 REVEALED BY THE SUBARU ULTRA-DEEP SURVEY FOR Lyα EMITTERS ATz= 7.3

We present the ultra-deep Subaru narrowband imaging survey for Lya emitters (LAEs) at $z=7.3$ in SXDS and COSMOS fields with a total integration time of 106 hours. Exploiting our new sharp bandwidth filter, NB101, installed on Suprime-Cam, we have reached $L(Lya)=2.4\times10^{42} \ erg \ s^{-1}$ ($5\sigma$) for $z=7.3$ LAEs, about 4 times deeper than previous Subaru $z \gtrsim 7$ studies, which allows us to reliably investigate evolution of Lya luminosity function (LF), for the first time, down to the luminosity limit same as those of Subaru $z=3.1-6.6$ LAE samples. Surprisingly, we only find three and four LAEs in SXDS and COSMOS fields, respectively, while one expects a total of $\sim 65$ LAEs by our survey in the case of no Lya LF evolution from $z=6.6$ to $7.3$.We identify a decrease of Lya LF from $z=6.6$ to $7.3$ at the $>90\%$ confidence level from our $z=7.3$ Lya LF.Moreover, the evolution of Lya LF is clearly accelerated at $z>6.6$ beyond the measurement uncertainties including cosmic variance. Because no such accelerated evolution of UV-continuum LF or cosmic star-formation rate (SFR) is found at $z\sim 7$, but suggested only at $z>8$ (Oesch et al. 2013, Bouwens et al. 2014), this accelerated Lya LF evolution is explained by physical mechanisms different from a pure SFR decrease but related to Lya production and escape in the process of cosmic reionization. Because a simple accelerating increase of IGM neutral hydrogen absorbing Lya would not reconcile with Thomson scattering optical depth measurements from WMAP and Planck, our findings may support new physical pictures suggested by recent theoretical studies, such as the existence of HI clumpy clouds within cosmic ionized bubbles selectively absorbing Lya and the large ionizing photon escape fraction of galaxies making weak Lya emission.

Ionization state of inter-stellar medium in galaxies: evolution, SFR–M*–Z dependence, and ionizing photon escape

We present a systematic study for ionization state of inter-stellar medium in galaxies at z=0-3 with ~140,000 SDSS galaxies and 108 intermediate to high redshift galaxies from the literature, using an ionization-parameter sensitive line ratio of [OIII]5007/[OII]3727 and photoionization models. We confirm that z=2-3 galaxies show an [OIII]/[OII] ratio significantly higher than a typical star-forming galaxy of SDSS by a factor of >~10, and the photoionization models reveal that these high-z galaxies have an ionization parameter of log(qion/cm s^{-1})~7.6-9.0, a factor of ~4-10 higher than local galaxies. For galaxies at any redshift, we identify a correlation between the [OIII]/[OII] ratio and galaxy global properties of star-formation rate (SFR), stellar mass (M_star), and metallicity (Z). We extend the fundamental metallicity relation (FMR) and develop the fundamental ionization relation (FIR), a four-dimensional relation of ionization parameter, SFR, M_star, and Z. The intermediate and high-z galaxies up to z~3 follow the FIR defined with the local galaxies, in contrast with the FMR whose possible evolution from z~2 to 3 is reported. We find that the FMR evolution of z~2-3 appears, if one omits ionization parameter differences, and that the FMR evolution does not exist for an average metallicity solution of z~3 galaxies with a high-ionization parameter. Interestingly, all of two local Lyman-continuum emitting galaxies (LyC leakers) have a high [OIII]/[OII] ratio, indicating a positive correlation between [OIII]/[OII] and ionizing photon escape fraction (fesc), which is successfully explained by our photoionization models. Because [OIII]/[OII] ratios of z~2-3 galaxies, especially Ly-alpha emitters (LAEs), are comparable to, or higher than, those of the local LyC leakers, these high-z galaxies are candidates of Lyman-continuum emitting objects. (abridged)

The visibility of Lyman α emitters: constraining reionization, ionizing photon escape fractions and dust

We build a physical model for high-redshift Lyman Alpha emitters (LAEs) by coupling state of the art cosmological simulations (GADGET-2) with a dust model and a radiative transfer code (pCRASH). We post-process the cosmological simulation with pCRASH using five different values of the escape fraction of hydrogen ionizing photons (f_esc=0.05,0.25,0.5,0.75,0.95) until reionization is complete, i.e. the average neutral hydrogen fraction drops to <X_HI>~10^-4. Then, the only free-parameter left to match model results to the observed Lya and UV luminosity functions of LAEs at z~6.6 is the relative escape of Lyman Alpha (Lya) and continuum photons from the galactic environment (f_alpha/f_c). We find a three-dimensional degeneracy such that the theoretical model can be reconciled with observations for an IGM Lya transmission <T_alpha>_LAE~38-50% (which translates to <X_HI>~0.5-10^-4 for Gaussian emission lines), f_esc~0.05-0.50 and f_alpha/f_c~0.6-1.8.

The contribution of high-redshift galaxies to the near-infrared background

Several independent measurements have confirmed the existence of fluctuations ($\delta F_{\rm obs}\approx 0.1 \rm nW/m^{2}/sr$ at $3.6 \rm \mu m$) up to degree angular scales in the source-subtracted Near InfraRed Background (NIRB) whose origin is unknown. By combining high resolution cosmological N-body/hydrodynamical simulations with an analytical model, and by matching galaxy Luminosity Functions (LFs) and the constraints on reionization simultaneously, we predict the NIRB absolute flux and fluctuation amplitude produced by high-$z$ ($z > 5$) galaxies (some of which harboring Pop III stars, shown to provide a negligible contribution). This strategy also allows us to make an empirical determination of the evolution of ionizing photon escape fraction: we find $f_{\rm esc} = 1$ at $z \ge 11$, decreasing to $\approx 0.05$ at $z = 5$. In the wavelength range $1.0-4.5 \rm \mu m$, the predicted cumulative flux is $F =0.2-0.04 \rm nW/m^2/sr$. However, we find that the radiation from high-$z$ galaxies (including those undetected by current surveys) is insufficient to explain the amplitude of the observed fluctuations: at $l=2000$, the fluctuation level due to $z > 5$ galaxies is $\delta F = 0.01-0.002 \rm nW/m^2/sr$, with a relative wavelength-independent amplitude $\delta F/F = 4$%. The source of the missing power remains unknown. This might indicate that an unknown component/foreground, with a clustering signal very similar to that of high-$z$ galaxies, dominates the source-subtracted NIRB fluctuation signal.

PRESENT-DAY GALACTIC EVOLUTION: LOW-METALLICITY, WARM, IONIZED GAS INFLOW ASSOCIATED WITH HIGH-VELOCITY CLOUD COMPLEX A

The high-velocity cloud (HVC) Complex A is a probe of the physical conditions in the Galactic halo. The kinematics, morphology, distance, and metallicity of Complex A indicate that it represents new material that is accreting onto the Galaxy. We present Wisconsin H-alpha Mapper (WHAM) kinematically resolved observations of Complex A over the velocity range of -250 to -50 km/s in the local standard of rest reference frame. These observations include the first full H-alpha intensity map of Complex A across (l, b) = (124, 18) to (171, 53) and deep targeted observations in H-alpha, [S II]6716, [N II]6584, and [O I]6300 towards regions with high H I column densities, background quasars, and stars. The H-alpha data imply that the masses of neutral and ionized material in the cloud are similar, both being greater than a million solar masses. We find that the Bland-Hawthorn & Maloney (1999, 2001) model for the intensity of the ionizing radiation near the Milky Way is consistent with the known distance of the high-latitude part of Complex A and an assumed cloud geometry that puts the lower-latitude parts of the cloud at a distance of 7 to 8 kpc. This compatibility implies a 5% ionizing photon escape fraction from the Galactic disk. We also provide the nitrogen and sulfur upper abundance solutions for a series of temperatures, metallicities, and cloud configurations for purely photoionized gas; these solutions are consistent with the sub-solar abundances found by previous studies, especially for temperatures above 10,000 K or for gas with a high fraction of singly-ionized nitrogen and sulfur.

CANDELS: THE CONTRIBUTION OF THE OBSERVED GALAXY POPULATION TO COSMIC REIONIZATION

We present measurements of the specific ultraviolet luminosity density from a sample of 483 galaxies at 6<z<8. These galaxies were selected from new deep near-infrared HST imaging from the CANDELS, HUDF09 and ERS programs. In contrast to the majority of previous analyses, which assume that the distribution of galaxy ultraviolet (UV) luminosities follows a Schechter distribution, and that the distribution continues to luminosities far below our observable limit, we investigate the contribution to reionization from galaxies which we can observe, free from these assumptions. We find that the observable population of galaxies can sustain a fully reionized IGM at z=6, if the average ionizing photon escape fraction (f_esc) is ~30%. A number of previous studies have measured UV luminosity densities at these redshifts that vary by 5X, with many concluding that galaxies could not complete reionization by z=6 unless a large population of galaxies fainter than the detection limit were invoked, or extremely high values of f_esc were present. The observed UV luminosity density from our observed galaxy samples at z=7-8 is not sufficient to maintain a fully reionized IGM unless f_esc>50%. Combining our observations with constraints on the emission rate of ionizing photons from Ly-alpha forest observations at z=6, we can constrain f_esc<34% (2-sigma) if the observed galaxies are the only contributors to reionization, or <13% (2-sigma) if the luminosity function extends to M_UV = -13. These escape fractions are sufficient to complete reionization by z=6. These constraints imply that the volume ionized fraction of the IGM becomes less than unity at z>7, consistent with a number of complementary reionization probes. If faint galaxies dominate reionization, future JWST observations will probe deep enough to see them, providing an indirect constraint on the ionizing photon escape fraction [abridged].

A critical analysis of the ultraviolet continuum slopes (β) of high-redshift galaxies: no evidence (yet) for extreme stellar populations at z &gt; 6

Following the discovery of the first significant samples of galaxies at z > 6.5 with Wide Field Camera 3/Infra-Red (WFC3/IR) on board Hubble Space Telescope (HST), it has been claimed that the faintest high-redshift galaxies display extremely blue ultraviolet (UV) continuum slopes, with a UV power-law index β≃−3 (where fλ∝λβ). Such slopes are bluer than previously reported for any other galaxy population, and are most readily explained theoretically by extinction-free, young and very low metallicity stellar populations with a high ionizing photon escape fraction. Here we undertake a critical study of the evidence for such extreme values of β, combining three new WFC3/IR-selected samples of galaxies spanning nearly two decades in UV luminosity over the redshift range z≃ 4.5–8. We explore the impact of inclusion/exclusion of less robust high-redshift candidates and use the varying depths of the samples to explore the effects of noise and selection bias at a given UV luminosity. Simple data-consistency arguments suggest that artificially blue average values of β can result when the analysis is extended into the deepest ≃0.5 mag bin of these WFC3/IR-selected galaxy samples, regardless of the actual luminosity or redshift range probed. By confining attention to robust high-redshift galaxy candidates, with at least one 8σ detection in the WFC3/IR imaging, we find that the average value of β is consistent with 〈β〉=−2.05 ± 0.10 over the redshift range z= 5–7 and the UV absolute magnitude range −22 < MUV,AB < − 18, and that 〈β〉 shows no significant trend with either redshift or MUV. We create and analyse a set of simple end-to-end simulations based on the WFC3/IR+ACS Hubble Ultra Deep Field (HUDF) and Early Release Science data sets which demonstrate that a bias towards artificially low/blue average values of β is indeed ‘expected’ when the UV slope analysis is extended towards the source detection threshold, and conclude that there is as yet no clear evidence for UV slopes significantly bluer than β≃−2, the typical value displayed by the bluest star-forming galaxies at more modest redshifts. A robust measurement of 〈β〉 for the faintest galaxies at z≃ 7 (and indeed z≃ 8) remains a key observational goal, as it provides a fundamental test for high escape fractions from a potentially abundant source of re-ionizing photons. This goal is achievable with HST, but requires still deeper WFC3/IR imaging in the HUDF.

THE CONTRIBUTION OF SUPERNOVAE TO COSMIC REIONIZATION

While stars are widely discussed as the source of the high energy photons which reionized the universe, an additional source of ionizing photons that must also contribute to reionization in this scenario is the supernovae (SNe) which mark the end of the life of massive stars. Here we estimate the relative contributions of SNe and stars to reionization. While the rate at which ionizing photons are produced in SN shocks is well below that at which they are produced by stars, the harder spectra of radiation emitted from SNe leads to an enhanced escape fraction of SN generated photons relative to that of stellar photons. In particular, along a given line of sight out of a galaxy, we find that for neutral hydrogen column densities N_H >~10^18 cm^-2 the contribution to reionization from SNe is greater than that from stars. Drawing on the results of simulations presented in the literature, we find that the overall (line of sight-averaged) SNe shock-generated ionizing photon escape fraction is larger than the stellar photon escape fraction by a factor of \simeq 4 to \simeq 7, depending on the metallicity of the stellar population. Overall, our results suggest that the effect of SNe is an enhancement of up to ~ 10 percent in the fraction of hydrogen reionized by stellar sources. We briefly discuss the implications of our results for the population of galaxies responsible for reionization.

IONIZING PHOTON ESCAPE FRACTIONS FROM HIGH-REDSHIFT DWARF GALAXIES

It has been argued that low-luminosity dwarf galaxies are the dominant source of ionizing radiation during cosmological reionization. The fraction of ionizing radiation that escapes into the intergalactic medium from dwarf galaxies with masses less than ~10^9.5 solar masses plays a critical role during this epoch. Using an extensive suite of very high resolution (0.1 pc), adaptive mesh refinement, radiation hydrodynamical simulations of idealized and cosmological dwarf galaxies, we characterize the behavior of the escape fraction in galaxies between 3 x 10^6 and 3 x 10^9 solar masses with different spin parameters, amounts of turbulence, and baryon mass fractions. For a given halo mass, escape fractions can vary up to a factor of two, depending on the initial setup of the idealized halo. In a cosmological setting, we find that the time-averaged photon escape fraction always exceeds 25% and reaches up to 80% in halos with masses above 10^8 solar masses with a top-heavy IMF. The instantaneous escape fraction can vary up to an order of magnitude in a few million years and tend to be positively correlated with star formation rate. We find that the mean of the star formation efficiency times ionizing photon escape fraction, averaged over all atomic cooling (T_vir > 8000 K) galaxies, ranges from 0.02 for a normal IMF to 0.03 for a top-heavy IMF, whereas smaller, molecular cooling galaxies in minihalos do not make a significant contribution to reionizing the universe due to a much lower star formation efficiency. These results provide the physical basis for cosmological reionization by stellar sources, predominately atomic cooling dwarf galaxies.

How Very Massive Metal‐Free Stars Start Cosmological Reionization

The initial conditions and relevant physics for the formation of the earliest galaxies are well specified in the concordance cosmology. Using ab initio cosmological Eulerian adaptive mesh refinement radiation hydrodynamical calculations, we discuss how very massive stars start the process of cosmological reionization. The models include nonequilibrium primordial gas chemistry and cooling processes and accurate radiation transport in the case B approximation using adaptively ray-traced photon packages, retaining the time derivative in the transport equation. Supernova feedback is modeled by thermal explosions triggered at parsec scales. All calculations resolve the local Jeans length by at least 16 grid cells at all times and as such cover a spatial dynamic range of approx.10(exp 6). These first sources of reionization are highly intermittent and anisotropic and first photoionize the small-scale voids surrounding the halos they form in, rather than the dense filaments they are embedded in. As the merging objects form larger, dwarf-sized galaxies, the escape fraction of UV radiation decreases and the H II regions only break out on some sides of the galaxies, making them even more anisotropic. In three cases, SN blast waves induce star formation in overdense regions that were formed earlier from ionization front instabilities. These stars form tens of parsecs away from the center of their parent DM halo. Approximately five ionizing photons are needed per sustained ionization when star formation in 10(exp 6) stellar Mass halos is dominant in the calculation. As the halos become larger than approx.10(exp 7) Stellar Mass, the ionizing photon escape fraction decreases, which in turn increases the number of photons per ionization to 15-50, in calculations with stellar feedback only. Radiative feedback decreases clumping factors by 25% when compared to simulations without star formation and increases the average temperature of ionized gas to values between 3000 and 10,000 K.

Radio views of cosmic reionization

Abstract We use numerical simulations of cosmic reionization and radiative processes related to the H i 21-cm emission line to produce synthetic radio maps as seen by next-generation telescopes that will operate at low radio frequencies (e.g. the Low Frequency Array, LOFAR). Two different scenarios, in which the end of reionization occurs early (z ≈ 13) or late (z ≈ 8) depending on the initial mass function (IMF) of the first stars and ionizing photon escape fraction, have been explored. For each of these models we produce synthetic H i 21-cm emission maps by convolving the simulation outputs with the provisional LOFAR sampling function in the frequency range 76–140 MHz. If reionization occurs late, LOFAR will be able to detect individual H i structures on arcmin scales, emitting at a brightness temperature of ≈35 mK as a 3σ signal in about 1000 h of observing time. In the case of early reionization, the detection would be unlikely, due to decreased sensitivity and increased sky temperatures. These results assume that ionospheric, interference and foreground issues are fully under control.

Formation of First Stars Triggered by Collisions and Shock Waves: Prospect for High Star Formation Efficiency and High Ionizing Photon Escape Fraction

We show that large, high-redshift (z > 10) galaxies with virial temperature in excess of 104 K may be composed mostly of cold atomic clouds that were formerly minihalos. These clouds move at a speed of ~15-30 km s-1 and may collide with one another on an average time interval of ~107 yr. The supersonic collisions may result in spatially distributed and efficient star formation. Most of the subsequent star formation in cold atomic clouds may be triggered by shock waves launched from the first stars formed in colliding clouds. Those shock-wave-compressed clouds may be more widespread spatially, because of the large imparted velocities, and some may escape into the intergalactic medium. The resultant widespread star formation would allow the possibility of a much higher ionizing photon escape fraction. These favorable conditions may form a physical basis to enable the standard cosmological model to produce a reasonably high Thomson optical depth τe = 0.11-0.14. Furthermore, a chain reaction of star formation in minihalos in the intergalactic space may be triggered by explosions in intergalactic medium, if minihalos are strongly clustered. In this case, a still higher τe would be achievable.

Early preheating and galaxy formation

Winds from pregalactic starbursts and 'miniquasars' may pollute the intergalactic medium (IGM) with metals and raise its temperature to a much higher adiabat than expected from photoionization, and so inhibit the formation of early galaxies by increasing the cosmological Jeans mass. We compute the thermal history of the IGM when it experiences a period of rapid, homogeneous 'preheating' at high redshifts, and the impact of such a global feedback mechanism on the IGM ionization state and the subsequent galaxy formation and evolution. Measurements of the temperature of the Lyα forest at redshift z ∼ 3 constrain the redshift and energy of preheating, and rule out models that preheat too late or to too high a temperature, i.e. to T I G M ? 10 6 K at z? 10. The IGM thermal history is used to estimate the effects of preheating on the formation of galaxies at later epochs, allowing us to predict galaxy luminosity functions in preheated universes. The results depend crucially on whether the baryonic smoothing scale in the IGM is computed globally, or in a local, density-dependent fashion (since the IGM temperature can become highly inhomogeneous in the post-preheating epoch). Using a globally averaged smoothing scale, we find that models with excessive preheating produce too few L * and fainter galaxies, and are therefore inconsistent with observational data. More moderate preheating scenarios, with T I G M ? 10 5 K at z ∼ 10, are able to flatten the faint-end slope of the luminosity function, producing excellent agreement with observations, without the need for any local feedback mechanism within galaxies. A density-dependent smoothing scale requires more energetic preheating to achieve the same degree of suppression in the faint-end slope. All models, however, appear unable to explain the sharp cut-off in the luminosity function at bright magnitudes - a problem that is also common to more conventional local feedback prescriptions. Supernova-driven preheating scenarios tend to raise the mean metallicity of the universe well above the minimum levels observed in the Lya clouds. The high energies associated with preheating cause a sharp drop in the abundance of neutral hydrogen in the IGM and are often sufficient to double ionize helium at high redshift, well before the 'quasar epoch'. We find that ionizing photon escape fractions must be significantly higher than 10 per cent in order to explain the low inferred H I fraction at z 6, particularly when using a globally averaged smoothing scale. While early preheating causes strong suppression of dwarf galaxy formation, we show that it is not able to reproduce the observed abundance of satellite galaxies in the Local Group in detail. The detailed thermal history of the universe during the formative early stages around z = 10-15 remains one of the crucial missing links in galaxy-formation and evolution studies.

The Implications of Wilkinson Microwave Anisotropy Probe Observations for Population III Star Formation Processes

In an earlier paper, we pointed out the strong likelihood for universal reionization to occur twice, giving rise to a much larger Thomson optical depth due to the intergalactic medium than that in the case of a single rapid reionization at z ~ 6. The latest Wilkinson Microwave Anisotropy Probe (WMAP) observations of Kogut et al. indicate that the universe indeed appears to have entered a significantly ionized state at a very high redshift. In light of this new development, we perform a more focused analysis of the Thomson optical depth in the context of the spatially flat, cosmological constant-dominated cold dark matter model (?CDM) constrained by WMAP observations. While the current uncertainties of the observed Thomson optical depth are still relatively large, with ?e = 0.17 ? 0.04 (68%), important implications for Population III (Pop III) star formation processes at high redshift can already be inferred. We are able to draw four conclusions: (1) In the absence of a top-heavy initial stellar mass function (IMF) for Pop III metal-free stars and without a dramatic upturn in the star formation efficiency and in the ionizing photon escape fraction at high redshift (z > 6), we find ?e ? 0.09. (2) With a top-heavy IMF for the Pop III metal-free stars, a plausible star formation efficiency, and an ionizing photon escape fraction ?20%, it is expected that ?e ? 0.12, which could be raised to ?e = 0.14, if the metal-enrichment efficiency of the intergalactic medium by Pop III stars is reduced by a factor of 5. (3) To reach ?e = 0.15-0.17 requires at least one of the following three conditions: the cosmological model power index n is positively tilted to n ? 1.03, Pop III star formation in minihalos with molecular hydrogen cooling has an efficiency c*(H2, III) > 0.01, or the ionizing photon escape fraction from Pop III galaxies is close to unity. The highest ?e that we have obtained is 0.24 for a WMAP-consistent ?CDM model with n = 1.03, a Pop III star formation efficiency of 1% for minihalos, and an ionizing photon escape fraction of 100% from Pop III galaxies. (4) For the WMAP-normalized running index ?CDM model, we obtain ?e = 0.12 with a Pop III star formation efficiency of 1% for minihalos and an ionizing photon escape fraction of 100% from Pop III galaxies. If the current observed value of the Thomson optical depth withstands future data, we will have strong observational evidence that Pop III stars are massive and that their formation efficiency may be much higher than current theoretical works suggest. Alternatively, there may be unknown, nonstellar ionizing sources at very high redshift.

The Universe Was Reionized Twice

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.