Low-density Intergalactic Medium Research Articles

Cosmic evolution of the C iv in high-resolution hydrodynamic simulations

We investigate the properties of triply ionized carbon (C iv) in the intergalactic medium (IGM) using a set of high resolution and large box size cosmological hydrodynamic simulations of a Lambda cold dark matter (ΛCDM) model. We rely on a modification of the publicly available TreeSPH code gadget-2 that self-consistently follows the metal enrichment mechanism by means of a detailed chemical evolution model. We focus on several numerical implementations of galactic feedback: galactic winds in the energy-driven and momentum-driven prescriptions, galactic winds hydrodynamically coupled to the surrounding gas and active galactic nuclei (AGNs) powered by gas accretion on to massive black holes. Furthermore, our results are compared to a run in which galactic feedback is not present and we also explore different initial stellar mass function. After having addressed some global properties of the simulated volume like the impact on the star formation rate and the content in carbon and C iv, we extract mock IGM transmission spectra in neutral hydrogen (H i) and C iv and perform Voigt profile fitting. The results are then compared with high-resolution quasar (QSO) spectra obtained with the Ultraviolet Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) and the High Resolution Echelle Spectrograph (HIRES) at Keck. We find that feedback has little impact on statistics related to the neutral hydrogen, while C iv is more affected by galactic winds and/or AGN feedback. The feedback schemes investigated have a different strength and redshift evolution with a general tendency for AGN feedback to be more effective at lower redshift than galactic winds. When the same analysis is performed over observed and simulated C iv lines, we find a reasonably good agreement between data and simulations over the column density range NC IV= 1012.5-15 cm−2. Also the C iv linewidth distribution appears to be in agreement with the observed values, while the H i Doppler parameters, bH I, are in general too large, showing that the diffuse cosmic web is heated more than what is inferred by observations. The simulation without feedback fails in reproducing the C iv systems at high column densities at all redshift, while the AGN feedback case agrees with observations only at z < 3, when this form of feedback is particularly effective. We also present scatter plots in the b–N and in the NC IV–NH I planes, showing that there is rough agreement between observations and simulations only when feedback is taken into account. Although it seems difficult to constrain the nature and the strength of galactic feedback using the present framework and find a unique model that fits all the observations, these simulations offer the perspective of understanding the galaxy–IGM interplay and how metals produced by stars can reach the low-density IGM.

Pressure support versus thermal broadening in the Lyman α forest - I. Effects of the equation of state on longitudinal structure

In the low density intergalactic medium (IGM) that gives rise to the Lyman-alpha forest, gas temperature and density are tightly correlated. The velocity scale of thermal broadening and the Hubble flow across the gas Jeans scale are of similar magnitude (Hlambda_J ~ sigma_th). To separate the effects of gas pressure support and thermal broadening on the Lya forest, we compare spectra extracted from two smoothed particle hydrodynamics (SPH) simulations evolved with different photoionization heating rates (and thus different Jeans scales), imposing different temperature-density relations on the evolved particle distributions. The turnover scales in the flux power spectrum and flux autocorrelation function are determined mainly by thermal broadening rather than pressure. However, the insensitivity to pressure arises partly from a cancellation effect with a sloped temperature-density relation (T ~ rho^{0.6} in our simulations): the high density peaks in the colder, lower pressure simulation are less smoothed by pressure support than in the hotter simulation, and it is this higher density gas that experiences the strongest thermal broadening. Changes in thermal broadening and pressure support have comparably important effects on the flux probability distribution (PDF), which responds directly to the gas overdensity distribution rather than the scale on which it is smooth. Tests on a lower resolution simulation show that our statistical results are converged even at this lower resolution. While thermal broadening generally dominates the longitudinal structure in the Lya forest, we show in Paper II that pressure support determines the transverse coherence of the forest observed towards close quasar pairs. [ABRIDGED]

Intergalactic dust and its photoelectric heating

We have examined dust photoelectric heating in the intergalactic medium (IGM). The heating rate in a typical radiation field of the IGM is represented by Γpe=1.2x10-34 erg s-1 cm-3 (D/10-4)(nH/10-5 cm-3)43(JL/10-21 erg s-1 cm-2 HZ-1 sr-1)2/3(T104 K)-1/6), where D is the dust-to-gas mass ratio, nH is the hydrogen number density, JL is the mean intensity at the hydrogen Lyman limit of the background radiation, and T is the gas temperature, if we assume the new X-ray photoelectric yield model by Weingartner et al. (2006) and the dust size distribution in the Milky Way by Mathis et al. (1977). This heating rate dominates the HI and HeII photoionization heating rates when the hydrogen number density is less than ∼10-6 cm-3 if D = 10-4 which is 1% of that in the Milky Way, although the heating rate is a factor of 2–4 smaller than that with the old yield model by Weingartner and Draine (2001). The grain size distribution is very important. If only large (≥0.1 μm) grains exist in the IGM, the heating rate is reduced by a factor of ≏5. Since dust heating is more efficient in a lower density medium relative to the photoionization heating, it may cause an inverted temperature-density relation in the low-density IGM, as suggested by Bolton et al. (2008). Finally, we have found that dust heating is not very important in the mean IGM before the cosmic reionization.

On the Relationship between a Giant Radio Galaxy MSH 05‐22and the Ambient Large‐Scale Galaxy Structure

We present a comparison of the properties of a giant radio galaxy and the ambient intergalactic medium, whose properties are inferred from the large-scale distribution in galaxies. The double lobes of the radio galaxy MSH 05-22 are giant—1.8 Mpc projected linear size—and interact with the environment outside the interstellar medium and coronal halo associated with the host galaxy. The radio lobes appear to be relicts, and the double structure is asymmetric. We have examined the large-scale structure in the galaxy distribution surrounding the radio source. The host galaxy of MSH 05-22 is associated with a small group that lies close to the boundary of sheetlike and filamentary density enhancements, and adjacent to a void. Assuming that the galaxies trace gas, the asymmetries in the radio morphology in this case study appear related to the anisotropy in the medium. However, the observed overdensities and structure formation models for the heating of the intergalactic medium (IGM) suggest a density-temperature product for the IGM environment that is an order of magnitude below that expected from the properties of the radio source. The discordance suggests that even sources like MSH 05-22, which are observed in the relatively low-density IGM environment associated with the filamentary large-scale structure and have multiple signatures of being relicts, may be overpressured and evolving toward an equilibrium relaxed state with the ambient IGM. Alternately, it is speculated that astrophysical feedback originating in galaxy overdensities observed 1-2 Mpc to the north and northeast of MSH 05-22 might be the mechanism for the heating of the ambient IGM gas.

Possible evidence for an inverted temperature–density relation in the intergalactic medium from the flux distribution of the Lyα forest

We compare the improved measurement of the Lya forest flux probability distribution at 1.7 < z < 3.2 presented by Kim et al. to a large set of hydrodynamical simulations of the Lya forest with different cosmological parameters and thermal histories. The simulations are in good agreement with the observational data if the temperature-density relation for the low-density intergalactic medium (IGM), T = T 0 Δ γ-1 , is either close to isothermal or inverted (y < 1). Our results suggest that the voids in the IGM may be significantly hotter and the thermal state of the low-density IGM may be substantially more complex than is usually assumed at these redshifts. We discuss radiative transfer effects which alter the spectral shape of ionizing radiation during the epoch of He II reionization as a possible physical mechanism for achieving an inverted temperature-density relation at z ≃ 3.

Lower Metal Enrichment of Virialized Gas in Minihalos

We differentiate between the metal enrichment of the gas in virialized minihalos and that of the intergalactic medium at high redshift, which is pertinent to cosmological reionization, with the initial expectation that gas in the high-density regions within formed dark matter halos may be more robust and thus resistant to mixing with the lower density intergalactic medium. Using detailed hydrodynamic simulations of gas clouds in minihalos subject to destructive processes associated with the encompassing intergalactic shocks carrying metal-enriched gas, we find, as an example, that, for realistic shocks with velocities of 10-100 km s−1, more than (90%, 65%) of the high-density gas with ρ ⩾ 500ρb inside a minihalo virialized at z = 10 with a mass of (107, 106) M☉, respectively, remains at a metallicity lower than 3% of that of the intergalactic medium by redshift z = 6. It may be expected that the high-density gas in minihalos will become fuel for subsequent star formation when they are incorporated into larger halos, where efficient atomic cooling can induce gas condensation and hence star formation. Since minihalos virialize at high redshift, when the universe is not expected to have been significantly reionized, the implication is that gas in virialized minihalos may provide an abundant reservoir of primordial gas that could possibly allow the formation of Population III metal-free stars to extend to much lower redshifts than would have been otherwise expected on the basis of the enrichment of the intergalactic medium.

Continuous statistics of the Ly forest at 0 &lt;z &lt; 1.6: the mean flux, flux distribution and autocorrelation fromHST FOS spectra

We measure the amount of absorption in the Lyα forest at 0 <z < 1.6 inHubble Space Telescope Faint Object Spectrograph spectra of 74 quasi-stellar objects (QSOs). Starting with a 334 QSO sample compiled by Bechtold et al., we selected 74 QSOs that have the highest signal-to-noise ratio and complete coverage of rest-frame wavelengths 1070–1170 Å. We measure the absorption from the flux in each pixel in units of the unabsorbed continuum level. We mask out regions of spectra that contain metal lines, or strong Lyα lines that are accompanied by other Lyman series line or metals at the same redshift, leaving Lyα absorption from the low-density intergalactic medium (IGM). At 0 <z < 1.6 we find that 79 per cent of the absorption is from the low-density IGM, 12 per cent from metals and 9 per cent from the strong H i lines, nearly identical to the percentages (78, 15 and 7) that we measured independently at z= 2 from spectra taken with the Kast spectrograph on the Lick 3-m. At z= 1 the low-density IGM absorbs 0.037 ± 0.004 of the flux. The error includes some but not all of the uncertainty in the continuum level. The remaining part gives relative errors of approximately 0.21 when we report the mean absorption in eight independent redshift intervals, and 0.047 when we average over all redshifts. We find 1.46 times more absorption from the low-density IGM than comes from Lyα lines that Bechtold et al. listed in the same spectra. The amount of absorption increases with z and can be fit by a power law (1 +z)α, with α= 1.01. If the absorption comes entirely from lines with fixed rest equivalent width, this result implies the number density of lines evolves like (1 +z)0.01, or no change in the number of lines per unit redshift, consistent with the Janknecht et al. results on the distribution of lines. When we include similar measurements from higher redshifts, we need more degrees of freedom to fit the amount of absorption at 0 < z < 3.2. A power law with a break in slope, changing from index 1.5 at low z to 3.0 above z∼ 1.1 is a better but only marginally acceptable fit. We also calculate two other continuous statistics, the flux probability distribution function and the flux autocorrelation function that is non-zero out to v∼ 500 km s−1 at 0.5 < z < 1.5.

Cosmological simulations of intergalactic medium enrichment from galactic outflows

We investigate models of self-consistent chemical enrichment of the intergalactic medium (IGM) from z = 6.0 → 1.5, based on hydrodynamic simulations of structure formation that explicitly incorporate outflows from star-forming galaxies. Our main result is that outflow parametrizations derived from observations of local starburst galaxies, in particular momentum-driven wind scenarios, provide the best agreement with observations of C iv absorption at z ∼ 2-5. Such models sufficiently enrich the high-z IGM to produce a global mass density of Civ absorbers that is relatively invariant from z = 5.5 → 1.5, in agreement with observations. This occurs despite continual IGM enrichment causing an increase in volume-averaged metallicity by ∼ x 5-10 over this redshift range, because energy input accompanying the enriching outflows causes a drop in the global ionization fraction of C iv. Comparisons to observed C IV column density and linewidth distributions and C IV-based pixel optical depth ratios provide significant constraints on wind models. Our best-fitting outflow models show mean IGM temperatures only slightly above our no-outflow case, metal filling factors of just a few per cent with volume-weighted metallicities around 10 -3 at z ∼ 3, significant amounts of collisionally ionized C iv absorption and a metallicity-density relationship that rises rapidly at low overdensities and flattens at higher ones. In general, we find that outflow speeds must be high enough to enrich the low-density IGM at early times but low enough not to overheat it, and concurrently must significantly suppress early star formation while still producing enough early metals. It is therefore non-trivial that locally calibrated momentum-driven wind scenarios naturally yield the desired strength and evolution of outflows, and suggest that such models represent a significant step towards understanding the impact of galactic outflows on galaxies and the IGM across cosmic time.

X-ray pre-ionization powered by accretion on the first black holes - II. Cosmological simulations and observational signatures

We use numerical simulations of a cosmological volume to study the X-ray ionization and heating of the intergalactic medium (IGM) by an early population of accreting black holes (BHs). By considering theoretical limits on the accretion rate and observational constraints from the X-ray background and faint X-ray source counts, we find that the maximum value of the optical depth to Thompson scattering which can be produced using these models is τe≃ 0.17, in agreement with previous semi-analytic results. The redshifted soft X-ray background produced by these early sources is important in producing a fully ionized atomic hydrogen in the low-density intergalactic medium before stellar reionization at redshift z∼ 6–7. As a result, stellar re-ionization is characterized by an almost instantaneous ‘overlap phase’ of H ii regions. The background also produces a second He ii re-ionization at about redshift 3 and maintains the temperature of the IGM at about 10 000 K even at low redshifts. If the spectral energy distribution of these sources has a non-negligible high-energy power-law component, the luminosity in the soft X-ray band of the ‘typical’ galaxies hosting intermediate-mass accreting BHs is maximum at z∼ 15 and is about one or two orders of magnitude below the sensitivity limit of the Chandra Deep Field. We find that about a thousand of these sources may be present per square arcmin of the sky, producing potentially detectable fluctuations. We also estimate that a few rare objects, not present in our small simulated volume, could be luminous enough to be visible in the Chandra Deep Field. The XEUS and Constellation-X satellites will be able to detect more of these sources that, if radio loud, could be used to study the 21-cm forest in absorption. A signature of an early X-ray pre-ionization is the production of secondary cosmic microwave background (CMB) anisotropies on small angular scales (<1 arcmin). We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (ΔT∼ 16 μK at ∼1-arcsec scales), while for stellar re-ionization scenarios the power decreases on smaller scales. We also show that the redshifted 21-cm radiation from neutral hydrogen can be marginally detected in emission at redshifts 7 < z < 12. At a redshift of about z∼ 30 a stronger and narrower (in redshift space) signal in absorption against the CMB, that is peculiar to these models, could be detectable.

Cosmological Parameters σ8, the Baryon Density Ωb, the Vacuum Energy Density ΩΛ, the Hubble Constant and the UV Background Intensity from a Calibrated Measurement of HiLyα Absorption atz = 1.9

We identify a concordant model for the intergalactic medium (IGM) at redshift z = 1.9 that uses popular values for cosmological and astrophysical parameters and accounts for all baryons with an uncertainty of 5%. The amount of absorption by H I in the IGM provides the best evidence on the physical conditions in the IGM, especially the combination of the mean gas density, the density fluctuations, the intensity of the ionizing flux, and the level of ionization. We have measured the amount of absorption, known as the flux decrement (DA), in the Lyα forest at redshift 1.9. We used spectra of 77 QSOs that we obtained with 250 km s-1 resolution from the Kast spectrograph on the Lick observatory 3 m telescope. We fitted unabsorbed continua to these spectra using b-splines. We also fitted equivalent continua to 77 artificial spectra that we made to match the real spectra in most obvious ways: redshift, resolution, signal-to-noise ratio (S/N), emission lines and absorption lines. The typical relative error in our continuum fits to the artificial spectra is 3.5%. Averaged over all 77 QSOs, the mean level is within 1%-2% of the correct value, except at S/N 17.2 cm-2 are responsible for a DA = 1.0% ± 0.4% at z = 1.9. These lines arise in higher density regions than the bulk of the IGM Lyα absorption, and hence they are harder to simulate in the huge boxes required to represent the large-scale variations in the IGM. If we subtract these lines, for comparison with simulations of the lower density bulk of the IGM, we are left with DA = 11.8% ± 1.0%. The mean DA in segments of individual spectra with Δz = 0.1, or 153 Mpc comoving at z = 1.9, has a large dispersion, σ = 6.1% ± 0.3% including Lyman limit systems (LLSs) and metal lines, and σ(Δz = 0.1) = 3.9% for the Lyα from the lower density IGM alone, excluding LLSs and metal lines. This is consistent with the usual description of large-scale structure and accounts for the large variations from QSO to QSO. Although the absorption at z = 1.9 is mostly from the lower density IGM, the Lyα of LLSs and the metal lines are both major contributors to the variation in the mean flux on 153 Mpc scales at z = 1.9, and they make the flux field significantly different from a random Gaussian field with an enhanced probability of a large amount of absorption. We find that a hydrodynamic simulation on a 10243 grid in a 75.7 Mpc box reproduces the observed DA from the low-density IGM alone when we use popular parameters values H0 = 71 km s-1 Mpc-1, Ωb = 0.044, Ωm = 0.27, ΩΛ = 0.73, σ8 = 0.9, and an ultraviolet background (UVB) that has an ionization rate per H I atom of Γ912 = (1.44 ± 0.11) × 10-12 s-1. This is 1.08 ± 0.08 times the prediction by Madau et al. with 61% from QSOs and 39% from stars. Our measurement of the DA gives a new joint constraint on these parameters, and the DA is very sensitive to each parameter. Given fixed values for all other parameters, and assuming the simulation has insignificant errors, the error of our DA measurement gives an error on H0 of 10%, ΩΛ of 6%, Ωb of 5%, and σ8 of 4%, comparable to the best measurements by other methods.

Metals in the intergalactic medium

We use high spectral resolution () and high signal-to-noise ratio ( per pixel) spectra of 19 high-redshift () quasars to investigate the metal content of the low-density intergalactic medium using pixel-by-pixel procedures. This high quality homogeneous survey gives the possibility to statistically search for metals at H i optical depths smaller than unity. We find that the gas is enriched in carbon and oxygen for neutral hydrogen optical depths > 1. Our observations strongly suggest that the C iv/H i ratio decreases with decreasing with . We do not detect C iv absorption statistically associated with gas of < 1. However, we observe that a small fraction of the low density gas is associated with strong metal lines as a probable consequence of the IGM enrichment being highly inhomogeneous. We detect the presence of O vi down to ~ 0.2 with log / ~ -2.0. We show that O vi absorption in the lowest density gas is located within ~300 km s-1 of strong H i lines. This suggests that this O vi phase may be part of winds flowing away from overdense regions. This effect is more important at the largest redshifts (z > 2.4). Therefore, at the limit of present surveys, the presence of metals in the underdense regions of the IGM is still to be demonstrated.

The Distribution of Metallicity in the Intergalactic Medium atz ∼ 2.5: Oviand CivAbsorption in the Spectra of Seven QSOs

We present a direct measurement of the metallicity distribution function for the high-redshift intergalactic medium (IGM). We determine the shape of this function using survival statistics, which incorporate both detections and nondetections of O VI and C IV absorption, associated with H I lines in high-resolution quasar spectra. The O VI sample (taken from seven QSOs at zabs ~ 2.5) contains lines with N ≥ 1013.6, encompassing ~50% of all baryons at z ~ 2.5. Our survey accounts for ≈98.8% of the C IV mass and ≈90% of the O VI mass in the universe at this epoch. We find a median intergalactic abundance of [O,C/H] = -2.82; the differential abundance distribution is approximately lognormal with mean ≈ -2.85 and σ = 0.75 dex. We discuss the sensitivity of these results to the assumed form of the ionizing UV radiation field. Some ~60%-70% of lines in the Lyα forest are enriched to observable levels of [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 scenarios. 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 strong 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,C/H] -3.5. Using a simple closed box model for the metallicity evolution of the IGM, we estimate the chemical yield of galaxies formed prior to z ~ 2.5, finding 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.

Early Reionization by Miniquasars

Motivated by the recent detection by the Wilkinson Microwave Anisotropy Probe of a large optical depth to Thomson scattering, implying a very early reionization epoch, we assess a scenario where the universe was reionized by 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 > 20, get incorporated through mergers into larger and larger systems, sink to the center as a result of 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 ΛCDM cosmology. Our model is based on the assumptions that quasar activity is driven by major mergers and nuclear IMBHs accrete at the Eddington rate a fraction of the gas in the merger remnant. The long dynamical frictional timescales leave many IMBHs wandering in galaxy halos after a minor merger. While seed IMBHs that are as rare as the 3.5 σ peaks of the primordial density field evolve largely in isolation, a significant number of BH binary systems will form if IMBHs populate the more numerous 3 σ peaks instead. In the case of rapid binary coalescence a fraction of IMBHs will be displaced from galaxy centers and ejected into the intergalactic medium (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, and not their stellar progenitors, 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 farther than EUV radiation. Energetic photons will permeate the universe more uniformly, make the low-density 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 cosmic microwave background.

COSMIC RAY ACCELERATION AT COSMOLOGICAL SHOCKS: NUMERICAL SIMULATIONS OF CR MODIFIED PLANE-PARALLEL SHOCKS

In order to explore the cosmic ray acceleration at the cosmological shocks, we have performed numerical simulations of one-dimensional, plane-parallel, cosmic ray (CR) modified shocks with the newly developed CRASH (Cosmic Ray Amr SHock) numerical code. Based on the hypothesis that strong Alfven waves are self-generated by streaming CRs, the Bohm diffusion model for CRs is adopted. The code includes a plasma-physics-based 'injection' model that transfers a small proportion of the thermal proton flux through the shock into low energy CRs for acceleration there. We found that, for strong accretion shocks with Mach numbers greater than 10, CRs can absorb most of shock kinetic energy and the accretion shock speed is reduced up to <TEX>$20\%$</TEX>, compared to pure gas dynamic shocks. Although the amount of kinetic energy passed through accretion shocks is small, since they propagate into the low density intergalactic medium, they might possibly provide acceleration sites for ultra-high energy cosmic rays of <TEX>$E\ll10^{18}eV$</TEX>. For internal/merger shocks with Mach numbers less than 3, however, the energy transfer to CRs is only about <TEX>$10-20\%$</TEX> and so nonlinear feedback due to the CR pressure is insignificant. Considering that intracluster medium (ICM) can be shocked repeatedly, however, the CRs generated by these weak shocks could be sufficient to explain the observed non-thermal signatures from clusters of galaxies.

Simulating the Sunyaev‐Zeldovich Effect(s): Including Radiative Cooling and Energy Injection by Galactic Winds

We present results on the thermal and kinetic Sunyaev-Zeldovich (SZ) effects from a sequence of high-resolution hydrodynamic simulations of structure formation, including cooling, feedback, and metal injection. These simulations represent a self-consistent thermal model that incorporates ideas from the preheating scenario while preserving good agreement with the low-density intergalactic medium at z ~ 3 probed by the Lyα forest. Four simulations were performed, at two different resolutions with and without radiative effects and star formation. The long-wavelength modes in each simulation were the same, so we can compare the results on an object-by-object basis. We demonstrate that our simulations are converged to the subarcminute level. The effect of the additional physics is to suppress the mean Comptonization parameter by 20% and to suppress the angular power spectrum of fluctuations by just under a factor of 2 in this model, while leaving the source counts and properties relatively unchanged. We quantify how non-Gaussianity in the SZ maps increases the sample variance over the standard result for Gaussian fluctuations. We identify a large scatter in the Y-M relation that will be important in searches for clusters using the SZ effect(s).

Temperature fluctuations in the intergalactic medium

ABSTRA C T The temperature of the low-density intergalactic medium (IGM) is set by the balance between adiabatic cooling resulting from the expansion of the Universe, and photoheating by the ultraviolet (UV) background. We have analysed the Lya forest of 11 high-resolution quasar spectra using wavelets, and find strong evidence of a marked jump in the temperature at the mean density, T0 ,o f 60^ 14 per cent over the redshift interval zaâ3:5; 3:1a, which we attribute to reionization of He II. The jump can be seen in all three of our spectra that straddle redshift 3.3, at a significance of > 99 per cent. Below z , 3:1, our results are consistent with a smooth cooling down of the universe, as expected when adiabatic expansion dominates over photoheating by a UV background from quasi-stellar objects (QSOs) and galaxies. We find no evidence of thermal fluctuations on scales > 5000 km s 21 larger than 50 per cent, which could be detected by our method, suggesting that the IGM follows a reasonably well-defined temperature ‐ density relation. We demonstrate that the mean wavelet amplitude kAl/ 1/T 0, and calibrate the relation with hydrodynamical simulations. We find T 0 > 1:2 £ 10 4 Ka t z > 3:6. Such high temperatures suggest that H I reionization occurred relatively recently.

The Fate of the First Galaxies. II. Effects of Radiative Feedback

We use three-dimensional cosmological simulations with radiative transfer to study the formation and evolution of the first galaxies in a ΛCDM cosmology. The simulations include continuum radiative transfer using the optically thin variable Eddington tensor (OTVET) approximation and line radiative transfer in the H2 Lyman-Werner bands of the UV background radiation. Chemical and thermal processes are treated in detail, particularly the ones relevant for H2 formation and destruction. We find that the first luminous objects ("small-halo objects") are characterized by bursting star formation (SF) that is self-regulated by a feedback process acting on cosmological instead of galactic scales. The global SF history is regulated by the mean number of ionizing photons that escape from each source, UV⟨fesc⟩. It is almost independent of the assumed SF efficiency parameter, *, and the intensity of the dissociating background. The main feedback process that regulates the SF is the reformation of H2 in front of H II regions and inside relic H II regions. The H II regions remain confined inside filaments, maximizing the production of H2 in overdense regions through cyclic destruction/reformation of H2. If UV⟨fesc⟩ > 10-7/*, the SF is self-regulated, photoevaporation of small-halo objects dominates the metal pollution of the low-density intergalactic medium, and the mass of produced metals depends only on ⟨fesc⟩. If UV⟨fesc⟩ ≲ 10-7/*, positive feedback dominates, and small-halo objects constitute the bulk of the mass in stars and metals until at least redshift z ~ 10. Small-halo objects cannot reionize the universe because the feedback mechanism confines the H II regions inside the large-scale structure filaments. In contrast to massive objects ("large halos"), which can reionize voids, small-halo objects partially ionize only the dense filaments while leaving the voids mostly neutral.

Small‐Scale Structure at High Redshift. III. The Clumpiness of the Intergalactic Medium on Subkiloparsec Scales

Spectra obtained with the Keck HIRES instrument of the Lyα forests in the lines of sight to the A and C components of the gravitationally lensed quasi-stellar object Q1422+231 were used to investigate the structure of the intergalactic medium at mean redshift z ~ 3.3 on subkiloparsec scales. We measured the cross-correlation amplitude between the two Lyα forests for a mean transverse separation of 120 h pc and computed the rms column density and velocity differences between individual absorption systems seen in both lines of sight. The rms differences between the velocity centroids of the Lyα forest lines were found to be less than about 400 ms-1 for unsaturated H I absorption lines with column densities in the range 12 < log N(H I) < 14.13. The rate of energy transfer into the low-density intergalactic medium on a typical scale of 100 pc seems to be lower by 3-4 orders of magnitude than the rate measured earlier for strong C IV metal absorption systems. The tight correlation between H I column density and baryonic density in the intergalactic medium was used to obtain a conservative upper limit on the rms fluctuations of the baryonic density field at z = 3.26, namely, [(Δ log ρ)2]1/2 ≤ 3.1 × 10-2 on a scale of 110 h pc. The fraction of the absorption lines that are different across the lines of sight was used to determine the filling factor of the universe for gas that has suffered recent hydrodynamic disturbances. We thereby derived upper limits on the filling factor of galactic outflows at high redshift. Short-lived, short-range ancient winds are essentially unconstrained by this method, but strong winds blowing for a substantial fraction of a Hubble time (at z = 3.3) appear to fill less than 20% of the volume of the universe.

Metal Enrichment of the Intergalactic Medium atz = 3 by Galactic Winds

Studies of quasar absorption lines reveal that the low-density intergalactic medium (IGM) at z ~ 3 is enriched to between 10-3 and 10-2 solar metallicity. This enrichment may have occurred in an early generation of Population III stars at redshift z 10, by protogalaxies at 6 z 10, or by larger galaxies at 3 z 6. This paper addresses the third possibility by calculating the enrichment of the IGM at z 3 by galaxies of baryonic mass 108.5 M☉. We use already completed cosmological simulations, to which we add a prescription for chemical evolution and metal ejection by winds, assuming that the winds have properties similar to those observed in local starbursts and Lyman break galaxies. Results are given for a number of representative models, and we also examine the properties of the galaxies responsible for the enrichment as well as the physical effects responsible for wind escape and propagation. We find that winds of velocity 200-300 km s-1 are capable of enriching the IGM to the mean level observed, although many low-density regions would remain metal free. Calibrated by observations of Lyman break galaxies, our calculations suggest that most galaxies at z 3 should drive winds that can escape and propagate to large radii. The primary effect limiting the enrichment of low-density intergalactic gas in our scenario is then the travel time from high- to low-density regions, implying that the metallicity of low-density gas is a strong function of redshift.

The Detection of Oxygen in the Low-Density Intergalactic Medium

The abundances of metals in the intergalactic medium (IGM) can be used to constrain the amount of star formation at high redshift and the spectral shape of the ionizing background radiation. For both purposes, it is essential to measure the abundances in regions of low density, away from local sources of metals and ionizing photons. Here we report the first detection of O VI in the low-density IGM at high redshift. We perform a pixel-by-pixel search for O VI absorption in eight high-quality quasar spectra spanning the redshift range z = 2.0-4.5. At 2 z 3, we clearly detect O VI in the form of a positive correlation between the H I Lyα optical depth and the optical depth in the corresponding O VI pixel, down to τ ~ 10-1. This is an order of magnitude lower in τ than the best C IV measurements can probe, and it constitutes the first clear detection of metals in underdense gas. The nondetection of O VI at z > 3 is consistent with the enhanced photoionization from a hardening of the UV background below z ~ 3 but could also be caused by the high level of contamination from Lyman series lines.