High-redshift Intergalactic Medium Research Articles

The effect of helium reionization on the Lyα forest hydrogen flux statistics

Abstract We assess the impact of QSOs on the high redshift (z > 4) Intergalactic Medium using Monte Carlo realisations of QSO populations and the He iii regions they generate, applied to the Sherwood-Relics simulations, allowing for uncertainties in the QSO luminosity function, its evolution, and QSO spectra and ages. While QSO luminosity functions based on optical-infrared selection are unable to reproduce the broadening H i Lyα optical depth distributions at z > 5, much broader distributions are found for the higher numbers of QSOs based on x-ray selection, suggesting a large QSO contribution to the ultra-violet background at z > 5 may offer an alternative to late reionization models to account for the broad H i Lyα optical depth distributions. Realisations using QSOs based on the higher QSO counts also much better recover the measured pixel flux auto-correlation function at z > 5. The He iii regions from QSO sources according to both types of luminosity function suppress the pixel flux power spectrum on small scales, k > 0.02 s km−1, while enhancing it on larger, both by amounts of up to tens of percent at z > 4, with the magnitude increasing with the intergalactic He iii filling factor and the boost in temperature within the He iii regions.

Understanding the spatial variation of Mg ii and ionizing photon escape in a local LyC leaker

ABSTRACT Ionizing photons must have escaped from high-redshift galaxies, but the neutral high-redshift intergalactic medium makes it unlikely to directly detect these photons during the Epoch of Reionization. Indirect methods of studying ionizing photon escape fractions present a way to infer how the first galaxies may have reionized the Universe. Here, we use HET/LRS2 observations of J0919 + 4906, a confirmed z≈ 0.4 emitter of ionizing photons to achieve spatially resolved (12.5 kpc in diameter) spectroscopy of Mg iiλ2796, Mg iiλ2803, [O ii]λλ3727, 3729, [Ne iii]λ3869, H γ, [O iiI]λ4363, H β, [O iii]λ4959, [O iii]λ5007, and H α. From these data, we measure Mg ii emission, which is a promising indirect tracer of ionizing photons, along with nebular ionization and dust attenuation in multiple spatially resolved apertures. We find that J0919 + 4906 has significant spatial variation in its Mg ii escape and thus ionizing photon escape fraction. Combining our observations with photoionization models, we find that the regions with the largest relative Mg ii emission and Mg ii escape fractions have the highest ionization and lowest dust attenuation. Some regions have an escape fraction that matches that required by models to reionize the early Universe, while other regions do not. We observe a factor of 36 spatial variation in the inferred LyC escape fraction, which is similar to recently observed statistical samples of indirect tracers of ionizing photon escape fractions. These observations suggest that spatial variations in neutral gas properties lead to large variations in the measured LyC escape fractions. Our results suggest that single sightline observations may not trace the volume-averaged escape fraction of ionizing photons.

The evolution of the Si ivcontent in the Universe from the epoch of reionization to cosmic noon

ABSTRACTWe investigate the abundance and distribution of metals in the high-redshift intergalactic medium and circum-galactic medium through the analysis of a sample of almost 600 Si iv absorption lines detected in high- and intermediate-resolution spectra of 147 quasars. The evolution of the number density of Si iv lines, the column density distribution function, and the cosmic mass density are studied in the redshift interval 1.7 ≲ z ≲ 6.2 and for log N(Si iv) ≥ 12.5. All quantities show a rapid increase between z ∼ 6 and z ≲ 5 and then an almost constant behaviour to z ∼ 2 in very good agreement with what is already observed for C iv absorption lines. The present results are challenging for numerical simulations: When simulations reproduce our Si iv results, they tend to underpredict the properties of C iv, and when the properties of C iv are reproduced, the number of strong Si iv lines [log N(Si iv) > 14] is overpredicted.

Thethesanproject: properties of the intergalactic medium and its connection to reionization-era galaxies

ABSTRACTThe high-redshift intergalactic medium (IGM) and the primeval galaxy population are rapidly becoming the new frontier of extragalactic astronomy. We investigate the IGM properties and their connection to galaxies at z ≥ 5.5 under different assumptions for the ionizing photon escape and the nature of dark matter, employing our novel thesan radiation-hydrodynamical simulation suite, designed to provide a comprehensive picture of the emergence of galaxies in a full reionization context. Our simulations have realistic ‘late’ reionization histories, match available constraints on global IGM properties, and reproduce the recently observed rapid evolution of the mean free path of ionizing photons. We additionally examine high-z Lyman-α transmission. The optical depth evolution is consistent with data, and its distribution suggests an even-later reionization than simulated, although with a strong sensitivity to the source model. We show that the effects of these two unknowns can be disentangled by characterizing the spectral shape and separation of Lyman-α transmission regions, opening up the possibility to observationally constrain both. For the first time in simulations, thesan reproduces the modulation of the Lyman-α flux as a function of galaxy distance, demonstrating the power of coupling a realistic galaxy formation model with proper radiation hydrodynamics. We find this feature to be extremely sensitive on the timing of reionization, while being relatively insensitive to the source model. Overall, thesan produces a realistic IGM and galaxy population, providing a robust framework for future analysis of the high-z Universe.

Efficient adiabatic hydrodynamical simulations of the high-redshift intergalactic medium

We present a post-processing tool for GADGET-2 adiabatic simulations to model various observed properties of the Ly$\alpha$ forest at $2.5 \leq z \leq 4$ that enables an efficient parameter estimation. In particular, we model the thermal and ionization histories that are not computed self-consistently by default in GADGET-2. We capture the effect of pressure smoothing by running GADGET-2 at an elevated temperature floor and using an appropriate smoothing kernel. We validate our procedure by comparing different statistics derived from our method with those derived using self-consistent simulations with GADGET-3. These statistics are: line of sight density field power spectrum, flux probability distribution function, flux power spectrum, wavelet statistics, curvature statistics, HI column density (${\rm N_{HI}}$) distribution function, linewidth ($b$) distribution and $b$ versus $\log {\rm N_{HI}}$ scatter. For the temperature floor of $10^4$ K and typical signal-to-noise of 25, the results agree well within 20 percent of the self-consistent GADGET-3 simulation. However, this difference is smaller than the expected $1\sigma$ sample variance for an absorption path length of $\sim 5.35$ at $z=3$. Moreover for a given cosmology, we gain a factor of $\sim N$ in computing time for modelling the intergalactic medium under $N \gg 1$ different thermal histories. In addition, our method allows us to simulate the non-equilibrium evolution of thermal and ionization state of the gas and include heating due to non-standard sources like cosmic rays and high energy $\gamma$-rays from Blazars.

A Space-based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21 cm Global Spectrum

Abstract The redshifted 21 cm monopole is expected to be a powerful probe of the epoch of the first stars and galaxies ( ). The global 21 cm signal is sensitive to the thermal and ionization state of hydrogen gas and thus provides a tracer of sources of energetic photons—primarily hot stars and accreting black holes—which ionize and heat the high redshift intergalactic medium (IGM). This paper presents a strategy for observations of the global spectrum with a realizable instrument placed in a low-altitude lunar orbit, performing night-time 40–120 MHz spectral observations, while on the farside to avoid terrestrial radio frequency interference, ionospheric corruption, and solar radio emissions. The frequency structure, uniformity over large scales, and unpolarized state of the redshifted 21 cm spectrum are distinct from the spectrally featureless, spatially varying, and polarized emission from the bright foregrounds. This allows a clean separation between the primordial signal and foregrounds. For signal extraction, we model the foreground, instrument, and 21 cm spectrum with eigenmodes calculated via Singular Value Decomposition analyses. Using a Markov Chain Monte Carlo algorithm to explore the parameter space defined by the coefficients associated with these modes, we illustrate how the spectrum can be measured and how astrophysical parameters (e.g., IGM properties, first star characteristics) can be constrained in the presence of foregrounds using the Dark Ages Radio Explorer (DARE).

Constraining high-redshift X-ray sources with next generation 21-cm power spectrum measurements

We use the Fisher matrix formalism and semi-numerical simulations to derive quantitative predictions of the constraints that power spectrum measurements on next-generation interferometers, such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA), will place on the characteristics of the X-ray sources that heated the high redshift intergalactic medium. Incorporating observations between $z=5$ and $z=25$, we find that the proposed 331 element HERA and SKA phase 1 will be capable of placing $\lesssim 10\%$ constraints on the spectral properties of these first X-ray sources, even if one is unable to perform measurements within the foreground contaminated "wedge" or the FM band. When accounting for the enhancement in power spectrum amplitude from spin temperature fluctuations, we find that the observable signatures of reionization extend well beyond the peak in the power spectrum usually associated with it. We also find that lower redshift degeneracies between the signatures of heating and reionization physics lead to errors on reionization parameters that are significantly greater than previously predicted. Observations over the heating epoch are able to break these degeneracies and improve our constraints considerably. For these two reasons, 21\,cm observations during the heating epoch significantly enhance our understanding of reionization as well.

Empirical covariance modeling for 21 cm power spectrum estimation: A method demonstration and new limits from early Murchison Widefield Array 128-tile data

The separation of the faint cosmological background signal from bright astrophysical foregrounds remains one of the most daunting challenges of mapping the high-redshift intergalactic medium with the redshifted 21 cm line of neutral hydrogen. Advances in mapping and modeling of diffuse and point source foregrounds have improved subtraction accuracy, but no subtraction scheme is perfect. Precisely quantifying the errors and error correlations due to missubtracted foregrounds allows for both the rigorous analysis of the 21 cm power spectrum and for the maximal isolation of the "EoR window" from foreground contamination. We present a method to infer the covariance of foreground residuals from the data itself in contrast to previous attempts at a priori modeling. We demonstrate our method by setting limits on the power spectrum using a 3 h integration from the 128-tile Murchison Widefield Array. Observing between 167 and 198 MHz, we find at 95% confidence a best limit of Delta^2(k) < 3.7 x 10^4 mK^2 at comoving scale k = 0.18 hMpc^-1 and at z = 6.8, consistent with existing limits.

THE IDENTIFICATION OF z -DROPOUTS IN PAN-STARRS1: THREE QUASARS AT 6.5&lt; z &lt; 6.7

Luminous distant quasars are unique probes of the high redshift intergalactic medium (IGM) and of the growth of massive galaxies and black holes in the early universe. Absorption due to neutral Hydrogen in the IGM makes quasars beyond a redshift of z~6.5 very faint in the optical $z$-band, thus locating quasars at higher redshifts require large surveys that are sensitive above 1 micron. We report the discovery of three new z>6.5 quasars, corresponding to an age of the universe of <850 Myr, selected as z-band dropouts in the Pan-STARRS1 survey. This increases the number of known z>6.5 quasars from 4 to 7. The quasars have redshifts of z=6.50, 6.52, and 6.66, and include the brightest z-dropout quasar reported to date, PSO J036.5078+03.0498 with M_1450=-27.4. We obtained near-infrared spectroscopy for the quasars and from the MgII line we estimate that the central black holes have masses between 5x10^8 and 4x10^9 M_sun, and are accreting close to the Eddington limit (L_Bol/L_Edd=0.13-1.2). We investigate the ionized regions around the quasars and find near zone radii of R_NZ=1.5-5.2 proper Mpc, confirming the trend of decreasing near zone sizes with increasing redshift found for quasars at 5.7<z<6.4. By combining R_NZ of the PS1 quasars with those of 5.7<z<7.1 quasars in the literature, we derive a luminosity corrected redshift evolution of R_NZ,corrected=(7.2+/-0.2)-(6.1+/-0.7)x(z-6) Mpc. However, the large spread in R_NZ in the new quasars implies a wide range in quasar ages and/or a large variation in the neutral Hydrogen fraction along different lines of sight.

PROBING REIONIZATION WITH THE CROSS-POWER SPECTRUM OF 21 cm AND NEAR-INFRARED RADIATION BACKGROUNDS

The cross-correlation between the 21 cm emission from the high-redshift intergalactic medium and the near-infrared (NIR) background light from the high-redshift galaxies promises to be a powerful probe of cosmic reionization. In this paper, we investigate the cross power spectrum during the epoch of reionization. We employ an improved halo approach to derive the distribution of the density field and consider two stellar populations in the star formation model: metal-free stars and metal-poor stars. The reionization history is further generated to be consistent with the electron-scattering optical depth from cosmic microwave background measurements. Then the intensity of NIR background is estimated by collecting emission from stars in the first-light galaxies. On large scales, we find the 21 cm and NIR radiation backgrounds are positively correlated during the very early stages of reionization. However, these two radiation backgrounds quickly become anti-correlated as reionization proceeds. The maximum absolute value of the cross power spectrum is $|\Delta^2_{\rm{21,NIR}}|\sim10^{-4}$ mK nW $\rm{m}^{-2}$ $\rm{sr}^{-1}$ reached at $\ell\sim1000$, when the mean fraction of ionized hydrogen is $\bar{x}_{i}\sim0.9$. We find that SKA can measure the 21 cm-NIR cross power spectrum in conjunction with mild extensions to the existing CIBER survey, provided that the integration time independently adds up to 1000 and 1 hours for 21 cm and NIR observations, and that the sky coverage fraction of CIBER survey is extended from $4\times10^{-4}$ to 0.1. Measuring the cross-correlation signal as a function of redshift provides valuable information on reionization and helps confirm the origin of the "missing" NIR background.

Star-forming galactic contrails as a source of metal enrichment and ionizing radiation at high redshift★

A spectroscopically detected Lyman alpha emitting halo at redshift 3.216 in the GOODS-N field is found to reside at the convergence of several Lyman alpha filaments. HST images show that some of the filaments are inhabited by galaxies. Several of the galaxies in the field have pronounced head-tail structures, which are partly aligned with each other. The blue colors of most tails suggest the presence of young stars, with the emission from at least one of the galaxies apparently dominated by high equivalent width Lyman alpha. Faint, more diffuse, and similarly elongated, apparently stellar features, can be seen over an area with a linear extent of at least 90 kpc. The region within several arcseconds of the brightest galaxy exhibits spatially extended emission by HeII, NV and various lower ionization metal lines. The gas-dynamical features present are strongly reminiscent of ram-pressure stripped galaxies, including evidence for recent star formation in the stripped contrails. Spatial gradients in the appearance of several galaxies may represent a stream of galaxies passing from a colder to a hotter intergalactic medium. The stripping of gas from the in-falling galaxies, in conjunction with the occurrence of star formation and stellar feedback in the galactic contrails suggests a mechanism for the metal enrichment of the high redshift intergalactic medium that does not depend on long-range galactic winds, at the same time opening a path for the escape of ionizing radiation from galaxies.

Lyman-αforest constraints on decaying dark matter

We present an analysis of high-resolution $N$-body simulations of decaying dark matter cosmologies focusing on the statistical properties of the transmitted Lyman-$\ensuremath{\alpha}$ ($\mathrm{Ly}\ensuremath{\alpha}$) forest flux in the high-redshift intergalactic medium (IGM). In this type of model a dark matter particle decays into a slightly less massive stable dark matter daughter particle and a comparably light particle. The small mass splitting provides a nonrelativistic kick velocity ${V}_{k}=c\ensuremath{\Delta}M/M$ to the daughter particle resulting in free-streaming and subsequent damping of small-scale density fluctuations. Current $\mathrm{Ly}\ensuremath{\alpha}$ forest power spectrum measurements probe comoving scales up to $\ensuremath{\sim}2--3{h}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{Mpc}$ at redshifts $z\ensuremath{\sim}2--4$, providing one of the most robust ways to probe cosmological density fluctuations on relatively small scales. The suppression of structure growth due to the free-streaming of dark matter daughter particles also has a significant impact on the neutral hydrogen cloud distribution, which traces the underlying dark matter distribution well at high redshift. We exploit $\mathrm{Ly}\ensuremath{\alpha}$ forest power spectrum measurements to constrain the amount of free-streaming of dark matter in such models and thereby place limits on decaying dark matter based only on the dynamics of cosmological perturbations without any assumptions about the interactions of the decay products. We use a suite of dark-matter-only simulations together with the fluctuating Gunn-Peterson approximation to derive the $\mathrm{Ly}\ensuremath{\alpha}$ flux distribution. We argue that this approach should be sufficient for our main purpose, which is to demonstrate the power of the $\mathrm{Ly}\ensuremath{\alpha}$ forest to constrain decaying dark matter models. We find that Sloan Digital Sky Survey 1D $\mathrm{Ly}\ensuremath{\alpha}$ forest power spectrum data place a lifetime-dependent upper limit ${V}_{k}\ensuremath{\lesssim}30--70\text{ }\text{ }\mathrm{km}/\mathrm{s}$ for decay lifetimes $\ensuremath{\lesssim}10\text{ }\text{ }\mathrm{Gyr}$. This is the most stringent model-independent bound on invisible dark matter decays with small mass splittings. For larger mass splittings (large ${V}_{k}$), $\mathrm{Ly}\ensuremath{\alpha}$ forest data restrict the dark matter lifetime to ${\ensuremath{\Gamma}}^{\ensuremath{-}1}\ensuremath{\gtrsim}40\text{ }\text{ }\mathrm{Gyr}$. We leave the calibration of IGM properties using high-resolution hydrodynamic simulations for future work, which might become necessary if we consider data with higher precision such as the Baryon Oscillation and Spectroscopic Survey (BOSS) $\mathrm{Ly}\ensuremath{\alpha}$ data. Forthcoming BOSS data should be able to provide more stringent constraints on exotic dark matter, mainly because the larger BOSS quasar spectrum sample will significantly reduce statistical errors.

New measurements of the ionizing ultraviolet background over 2 &lt; z &lt; 5 and implications for hydrogen reionization

We present new measurements of the intensity of the ionizing ultraviolet background and the global emissivity of ionizing photons over 2 < z < 5. Our results are based on a suite of updated measurements of physical properties of the high-redshift intergalactic medium (IGM), including gas temperatures and the opacity of the IGM to Ly-alpha and ionizing photons. Consistent with previous works, we find a relatively flat hydrogen photoionization rate over 2 < z < 5, although our measurements are roughly a factor of two higher than the 2008 values of Faucher-Giguere et al., due primarily to our lower gas temperatures. The ionizing emissivity we derive is also generally higher than other recent estimates due to a combination of lower gas temperatures, higher ionizing opacity, and an accounting of cosmological radiative transfer effects. We find evidence that the emissivity increases from z~3 to 5, reaching ~5 ionizing photons per atom per gigayear at z=4.75 for realistic galaxy spectra. We further find that galaxies must dominate the emissivity near 1 Ryd at z > 4, and possibly at all redshifts z > 2.4. Our results suggest that the globally-averaged ionizing "efficiency" of star-forming galaxies increases substantially with redshift over 3.2 < z < 4.75. This trend is consistent with the conclusion often drawn from reionization models that the ionizing efficiency of galaxies must be higher during reionization in order for galaxies to reionize the IGM by z=6. Our emissivity values at z~5 suggest that ionizing photons may have been a factor of two more abundant during the final stages of reionization than previously indicated. The evolution of the ionizing emissivity over 2 < z < 5 suggests, moreover, that the steep decline in the photoionization rate from z~5 to 6 may indicate a rapid evolution in the mean free path at z > 5.

Energy deposition by weakly interacting massive particles: a comprehensive study

We present results obtained with the updated version of our code MEDEA2, which includes all physical processes necessary to study the energy deposition in the surrounding environment from primary photons and fast leptons produced by dark matter (DM) particle decay/annihilation. Such interactions now include also Compton scattering of primary photons off electrons and pair creation of photons on atoms. Our ultimate aim is a thorough study of the impact of DM annihilations on the thermal and ionization history of the high-redshift intergalactic medium (IGM) during the dark ages. In addition, a precise determination of the effects of DM decays/annihilations can help constrain its nature. We present the results for some selected DM candidates: (i) a 10-GeV bino-like neutralino; (ii) a heavy DM candidate of rest mass 1 TeV that pair annihilates into muons; and (iii) a 200-GeV wino-like neutralino with a pair annihilation into W + W − pairs. An interface to DARKSUSY allows us to use the computed annihilation spectra in input for our code and follow the complete secondary cascade. The fractional energy depositions into the IGM depend strongly on the DM particle rest mass: whereas for the 10-GeV particle the absorbed energy fraction, in the redshift range 10 < z < 1000, is 50 per cent, higher mass candidates deposit their energy less efficiently (∼1–10 per cent), making their impact on the high-z IGM considerably weaker. Noticeably, our approach allows us to consistently follow the low-energy deposition of the cascade products, which can be of interest for a broad range of applications. Finally, we provide both tabulated results and analytical fits that can be readily implemented in theoretical studies of the effects and detectability of the most popular DM candidates.

RADIATIVE TRANSFER IN A CLUMPY UNIVERSE. IV. NEW SYNTHESIS MODELS OF THE COSMIC UV/X-RAY BACKGROUND

We present improved synthesis models of the evolving spectrum of the UV/X-ray diffuse background, updating and extending our previous results. Five new main components are added to our radiative transfer code CUBA: (1) the sawtooth modulation of the background intensity from resonant line absorption in the Lyman series of cosmic hydrogen and helium; (2) the X-ray emission from obscured and unobscured quasars; (3) a piecewise parameterization of the distribution in redshift and column density of intergalactic absorbers that fits recent measurements of the mean free path of 1 ryd photons; (4) an accurate treatment of the photoionization structure of absorbers; and (5) the UV emission from star-forming galaxies at all redshifts. We provide tables of the predicted HI and HeII photoionization and photoheating rates for use, e.g., in cosmological hydrodynamics simulations of the Lya forest, and a new metallicity-dependent calibration to the UV luminosity density-star formation rate density relation. A "minimal cosmic reionization model" is also presented in which the galaxy UV emissivity traces recent determinations of the cosmic history of star formation, the luminosity-weighted escape fraction of hydrogen-ionizing radiation increases rapidly with lookback time, the clumping factor of the high-redshift intergalactic medium evolves following the results of recent hydrodynamic simulations, and Population III stars and miniquasars make a negligible contribution to the metagalactic flux. The model provides a good fit to the hydrogen-ionization rates inferred from flux decrement and proximity effect measurements, predicts that cosmological HII (HeIII) regions overlap at redshift 6.7 (2.8), and yields an optical depth to Thomson scattering that is in agreement with WMAP results. (Abridged)

THE EFFECT OF GALACTIC PROPERTIES ON THE ESCAPE FRACTION OF IONIZING PHOTONS

The escape fraction, fesc, of ionizing photons from early galaxies is a crucial parameter for determining whether the observed galaxies at z ⩾ 6 are able to reionize the high-redshift intergalactic medium. Previous attempts to measure fesc have found a wide range of values, varying from less than 0.01 to nearly 1. Rather than finding a single value of fesc, we clarify through modeling how internal properties of galaxies affect fesc through the density and distribution of neutral hydrogen within the galaxy, along with the rate of ionizing photons' production. We find that the escape fraction depends sensitively on the covering factor of clumps, along with the density of the clumped and interclump medium. One must therefore be cautious when dealing with an inhomogeneous medium. Fewer high-density clumps lead to a greater escape fraction than more numerous low-density clumps. When more ionizing photons are produced in a starburst, fesc increases, as photons escape more readily from the gas layers. Large variations in the predicted escape fraction, caused by differences in the hydrogen distribution, may explain the large observed differences in fesc among galaxies. Values of fesc must also be consistent with the reionization history. High-mass galaxies alone are unable to reionize the universe, because fesc >1 would be required. Small galaxies are needed to achieve reionization, with greater mean escape fraction in the past.

The impact of coupled dark energy cosmologies on the high-redshift intergalactic medium

Abstract We present an analysis of high-resolution hydrodynamical N-body simulations of coupled dark energy cosmologies which focuses on the statistical properties of the transmitted Lyman α flux in the high-redshift intergalactic medium (IGM). In these models the growth of the diffuse cosmic web differs from the standard ΛCDM case: the density distribution is skewed towards underdense regions and the matter power spectra are typically larger (in a scale-dependent way). These differences are also appreciable in the Lyman α flux and are larger than 5 per cent (10 per cent) at z= 2–4 in the flux probability distribution function (pdf) for high-transmissivity regions and for values of the coupling parameter β= 0.08 (β= 0.2). The flux power spectrum is also affected at the ∼2 per cent (∼5–10 per cent) level for β= 0.08 (β= 0.2) in a redshift-dependent way. We infer the behaviour of flux pdf and flux power for a reasonable range of couplings and present constraints using present high- and low-resolution data sets. We find an upper limit β≲ 0.15 (at 2σ confidence level), which is obtained using only IGM data and is competitive with those inferred from other large-scale structure probes.

RESONANT H – PHOTODETACHMENT: ENHANCED PHOTODESTRUCTION AND CONSEQUENCES FOR RADIATIVE FEEDBACK

The hydrogen negative ion plays a crucial role in the formation of hydrogen molecules in the early universe. Cooling through excitation of H2 drives the formation of the first cosmological objects. The H2 molecules are produced primarily by a reaction sequence initiated by H − . We explore the influence of enhanced photodestruction rates that arise due to absorption by resonance states of H − lying near 11 eV. We examine the feedback effects that occur in radiation fields characteristic of Population III stars, blackbody sources, power-law spectra, and the hydrogen Lyman modulated sawtooth spectra of the high-redshift intergalactic medium.

Probing re-ionization with quasar spectra: the impact of the intrinsic Lymanαemission line shape uncertainty

Arguably the best hope of understanding the tail end of the reionization of the intergalactic medium (IGM) at redshift z > 6 is through the detection and characterization of the Gunn-Peterson (GP) damping wing absorption of the IGM in bright quasar spectra. However, the use of quasar spectra to measure the IGM damping wing requires a model of the quasar's intrinsic Lyman-alpha emission line. Here we quantify the uncertainties in the intrinsic line shapes, and how those uncertainties affect the determination of the IGM neutral fraction. We have assembled a catalog of high-resolution HST spectra of the emission lines of unobscured low-redshift quasars, and have characterized the variance in the shapes of their lines. We then add simulated absorption from the high-redshift IGM to these quasar spectra in order to determine the corresponding uncertainties in reionization constraints using current and future samples of z > 6 quasar spectra. We find that, if the redshift of the Lyman-alpha emission line is presumed to coincide with the systemic redshift determined from metal lines, the inferred IGM neutral fraction is systematically biased to low values due to a systematic blueshift of the Lyman-alpha line relative to the metal lines. If a similar blueshift persists in quasars at z > 6, this bias strengthens previous claims of a significant neutral hydrogen fraction at z ~ 6. This technique is capable of making a robust distinction between a highly ionized (x_IGM ~ 10^-3) and a neutral (x_IGM = 1) IGM with even a few bright quasars.

Keeping the Universe ionized: photoheating and the clumping factor of the high-redshift intergalactic medium

The critical star formation rate (SFR) density required to keep the intergalactic hydro- gen ionized depends crucially on the average rate of recombinations in the intergalac- tic medium (IGM). This rate is proportional to the clumping factor C ≡� ρ 2� IGM/�ρ b� 2 , where ρb and �ρ bare the local and cosmic mean baryon density, respectively, and the brackets �� IGM indicate spatial averaging over the recombining gas in the IGM. We per- form a suite of cosmological smoothed particle hydrodynamic simulations that include ra- diative cooling to calculate the volume-weighted clumping factor of the IGM at redshifts z ≥ 6. We focus on the effect of photoionization heating by a uniform ultraviolet background and find that photoheating strongly reduces the clumping factor because the increased pressure support smoothes out small-scale density fluctuations. Photoionization heating is often said to provide a negative feedback on the re-ionization of the IGM because it suppresses the cosmic SFR by boiling the gas out of low-mass haloes. However, because of the reduction of the clumping factor it also makes it easier to keep the IGM ionized. Photoheating therefore also provides a positive feedback which, while known to exist, has received much less attention. We demonstrate that this positive feedback is in fact very strong. Using conservative assumptions, we find that if the IGM was reheated at z 9, the observed population of star-forming galaxies at z ≈ 6 may be sufficient to keep the IGM ionized, provided that the fraction of ionizing photons that escape the star-forming regions to ionize the IGM is larger than ∼0.2.