Lyα Forest Lines Research Articles

Spectral Signatures of Quasar Ages at z ≈ 3

Abstract Insight into quasar ages may be obtained from the proximity effect, but so far only in a limited number of bright quasars. Based on ∼2600 Sloan Digital Sky Survey quasar spectra at 2.5 ≤ z ≤ 3.5, a search for spectral voids between Lyα forest lines finds proximity zones over a wide range of radial distances. The majority of zone sizes are less than 5 Mpc, with their numbers decreasing exponentially toward larger distances. After a normalization by luminosities, the zone sizes are distributed with an e-folding scale of 0.64 as compared with the anticipated values. A group of quasars are selected for their large proximity zones of ≳10 Mpc. Their composite spectrum displays strong narrow cores and large equivalent widths in Lyα and other major UV emission lines. If the proximity zones along lines of sight are indicative of quasar ages, these features may be the signatures of old quasars. Another group of quasars are selected as they show no proximity zone and exhibit intrinsic absorption lines at z ab > z em . They are likely young quasars and exhibit weaker narrow emission-line components. The significant difference of spectral features between the two groups may reflect an evolution pattern over quasars’ lifetimes.

TOWARD DETECTING THE 2175 Å DUST FEATURE ASSOCIATED WITH STRONG HIGH-REDSHIFT Mg II ABSORPTION LINES

We report detections of 39 2175 Å dust extinction bump candidates associated with strong Mg ii absorption lines at z∼ 1–1.8 on quasar spectra in Sloan Digital Sky Survey (SDSS) DR3. These strong Mg ii absorption line systems are detected among 2951 strong Mg ii absorbers with a rest equivalent width Wrλ2796> 1.0 Å at 1.0 < z < 1.86, which is part of a full sample of 7421 strong Mg ii absorbers compiled by Prochter et al. The redshift range of the absorbers is chosen to allow the 2175 Å extinction features to be completely covered within the SDSS spectrograph operation wavelength range. An upper limit of the background quasar emission redshift at z = 2.1 is set to prevent the Lyα forest lines from contaminating the sensitive spectral region for the 2175 Å bump measurements. The FM90 parameterization is applied to model the optical/UV extinction curve in the rest frame of Mg ii absorbers of the 2175 Å bump candidates. The simulation technique developed by Jiang et al. is used to derive the statistical significance of the candidate 2175 Å bumps. A total of 12 absorbers are detected with 2175 Å bumps at a 5σ level of statistical significance, 10 are detected at a 4σ level, and 17 are detected at a 3σ level. Most of the candidate bumps in this work are similar to the relatively weak 2175 Å bumps observed in the Large Magellanic Cloud LMC2 supershell rather than the strong ones observed in the Milky Way. This sample has greatly increased the total number of 2175 Å extinction bumps measured on SDSS quasar spectra. Follow-up observations may rule out some of the possible false detections and reveal the physical and chemical natures of 2175 Å quasar absorbers.

A SURVEY OF METAL LINES AT HIGH-REDSHIFT. I. SDSS ABSORPTION LINE STUDIES— THE METHODOLOGY AND FIRST SEARCH RESULTS FOR O VI

We report the results of a systematic search for signatures of metal lines in quasar spectra of the Sloan Digital Sky Survey (SDSS) data release 3 (DR3), focusing on finding intervening absorbers via detection of their O VI doublet. Here, we present the search algorithm and criteria for distinguishing candidates from spurious Lyα forest lines. In addition, we compare our findings with simulations of the Lyα forest in order to estimate the detectability of O VI doublets over various redshift intervals. We have obtained a sample of 1756 O VI doublet candidates with rest-frame equivalent width (EW) ≥0.05 A in 855 active galactic nuclei spectra (out of 3702 objects with redshifts in the accessible range for O VI detection). This sample is further subdivided into three groups according to the likelihood of being real and the potential for follow-up observation of the candidate. The group with the cleanest and most secure candidates is comprised of 145 candidates. Sixty-nine of these reside at a velocity separation ≥5000 km s–1 from the QSO and can therefore be classified tentatively as intervening absorbers. Most of these absorbers have not been picked up by earlier, automated QSO absorption line detection algorithms. This sample increases the number of known O VI absorbers at redshifts beyond z abs≥ 2.7 substantially.

From HARPS to CODEX: exploring the limits of Doppler measurements

Only 3–6 years ago, the Doppler technique was believed to have reached its final limitations in measuring stellar velocities and finding extra-solar planets. The 3–4 m s−1 precision level achieved, at that time, by various teams, was certainly limited by instrumental performances, but also constrained, as believed by a part of the community, by intrinsic stellar limitations. The advent of HARPS drastically changed this view. The instrument demonstrated, through its recent discoveries, that stars more ‘stable’ than 1 m s−1 actually exist, and that their radial velocity (RV) can be measured at that level of precision. Short-term precision of 20 cm s−1 rms and long-term precision of the order of 30–60 cm s−1 rms have been actually achieved on real stars, showing that RVs still harbor a great potential, and not only in the domain of extra-solar planets. Indeed, HARPS inspired the CODEX@ELT experiment for the direct determination of the expansion of the Universe, measuring the Doppler shift of Ly-α forest lines as a function of time. This experiment calls for a Doppler precision as low as 1 cm s−1, which in turn inspires new possibilities in the domain of extra-solar planets. We will investigate the obstacles on the way to cm s−1 precision. The discussion presented here will be based on our experience with HARPS and what we consider to be the current limitations set by the instrument, telescope, atmosphere and star. Finally, we will also provide an outlook on possible improvements and expected performances, which will finally define new scientific opportunities.

Tracing the gas at redshift 1.7-3.5 with the Lyα forest: the FLO approach★

We present FLO (From Lines to Overdensities), a new technique to reconstruct the hydrogen density field for the Lyα forest lines observed in high-resolution QSO spectra. The method is based on the hypothesis that the Lyα lines arise in the low to intermediate density intergalactic gas and that the Jeans length is the typical size of the Lyα absorbers. The reliability of FLO is tested against mock spectra obtained from cosmological simulations. The recovering algorithm gives satisfactory results in the range from the mean density to overdensities of ∼30 and reproduces correctly the correlation function of the density field and the 1D power spectrum on scales between ∼20 and 60 comoving Mpc. A sample of Lyα forests from 22 high-resolution QSO spectra is analysed, covering the redshift range 1.7 � z � 3.5. For each line of sight, we fit Voigt profiles to the lines of the Lyα forest, providing the largest, homogeneous sample of fitted Lyα lines ever studied. The line number density evolution with redshift follows a power-law relation: dn/dz = (166 ± 4) [(1 + z)/3.5] (2.8±0.2) (1σ errors). The two-point correlation function of lines shows a signal up to separations of ∼2 comoving Mpc; weak lines [logN (H i) < 13.8] also show a significant clustering but on smaller scales (r � 1.5 comoving Mpc). We estimate with FLO the hydrogen density field toward the 22 observed lines of sight. The redshift distribution of the average densities computed for each QSO is consistent with the cosmic mean hydrogen density in the analysed redshift range. The two-point correlation function and the 1D power spectrum of the δ field are estimated. They are both consistent with the analogous results computed from hydrosimulated spectra obtained in the framework of the concordance cosmological model. The correlation function shows clustering signal up to ∼4 comoving Mpc.

The line-of-sight proximity effect in individual quasar spectra

We exploit a set of high signal-to-noise (∼70), low-resolution (R ∼ 800) quasar spectra to search for the signature of the so-called proximity effect in the H i Lyα forest. Our sample consists of 17 bright quasars in the redshift range 2.7 < z < 4.1. Analysing the spectra with the flux transmission technique, we detect the proximity effect in the sample at high significance. We use this to estimate the average intensity of the metagalactic UV background, assuming it to be constant over this redshift range. We obtain a value of J = (9 ± 4) × 10 −22 erg cm −2 s −1 Hz −1 sr −1 , in good agreement with previous measurements at similar z. We then apply the same procedure to individual lines of sight, finding a significant deficit in the effective optical depth close to the emission redshift in every single object except one (which by a different line of evidence does nevertheless show a noticeable proximity effect). Thus, we clearly see the proximity effect as a universal phenomenon associated with individual quasars. Using extensive Monte-Carlo simulations to quantify the error budget, we assess the expected statistical scatter in the strength of the proximity effect due to shot noise (cosmic variance). The observed scatter is larger than the predicted one, so that additional sources of scatter are required. We rule out a dispersion of spectral slopes as a significant contributor. Possible effects are long time-scale variability of the quasars and/or gravitational clustering of Lyα forest lines. We speculate on the possibility of using the proximity effect as a tool to constrain individual quasar ages, finding that ages between ∼10 6 and ∼10 8 yrs might produce a characteristic signature in the optical depth profile towards the QSO. We identify one possible candidate for this effect in our sample.

The Low‐Redshift Lyα Forest toward PKS 0405−123

We present results for Lyα forest and metal absorbers from ~7 km s-1 resolution Space Telescope Imaging Spectrograph data for QSO PKS 0405-123 (z = 0.574). We analyze strong and weak samples of Lyα forest lines, containing 60 absorbers with column density log N ≥ 13.3 over 0.002 < z < 0.423 and 44 with log N ≥ 13.1 over 0.020 < z < 0.234, respectively. Seven absorbers contain metals, all with associated O VI and often offset in velocity from Lyα. The strong and weak Doppler parameter distributions have b = 47 ± 22 and 44 ± 21 km s-1, respectively. Simulated spectra suggest that line blending and signal-to-noise ratio effects inflate b. For absorbers with 13.1 < log N < 14.0, we find a redshift overdensity of ~0.2-0.3 dex at 0.127 < z < 0.234, which we attribute to cosmic variance. There is a void in the strong sample at 0.0320 < z < 0.0814 with probability of random occurrence P < 0.0004. We detect Lyα-Lyα clustering in our sample on a scale of Δv ≤ 250 km s-1 for log N ≥ 13.3, consistent with a numerical model of structure evolution. We detect velocity correlations of up to 250 km s-1 between Lyα absorbers and 39 galaxies at z < 0.43 out to a transverse distance of 1.6 h local frame Mpc. The Lyα-galaxy two-point correlation function is significant out to Δv < 250 km s-1 and grows with minimum H I column density. The strongest signal occurs for log N 13.5-14.0 absorbers and is similar to the galaxy-galaxy correlation, implying that such Lyα absorbers have masses log(M/M☉) = 11.3. We find a correlation between local galaxy counts and local summed H I column density, with peak significance on scales of 4000-6000 km s-1 and probability of random occurrence P = 0.0009. Finally, we present column densities for a number of Galactic species.

Expansion and Collapse in the Cosmic Web

We study the kinematics of the gaseous cosmic web at high redshift using Lyα forest absorption in multiple QSO sight lines. Observations of the projected velocity shifts between Lyα absorbers common to the lines of sight to a gravitationally lensed QSO and three more widely separated QSO pairs are used to directly measure the expansion of the cosmic web in units of the Hubble velocity, as a function of redshift and spatial scale. The lines of sight used span a redshift range from about 2 to 4.5 and represent transverse scales from the subkiloparsec range to about 300 h physical kpc. Using a simple analytic model and a cosmological hydrodynamic simulation, we constrain the underlying three-dimensional distribution of expansion velocities from the observed line-of-sight distribution of velocity shear across the plane of the sky. The shape of the shear distribution and its width (14.9 km s-1 rms for a physical transverse separation of 61 h kpc at z = 2, 30.0 km s-1 for 261 h kpc at z = 3.6) are found to be in good agreement with the IGM undergoing large-scale motions dominated by the Hubble flow, making this one of the most direct observations possible of the expansion of the universe. However, modeling the Lyα clouds with a simple expanding pancake model, the average expansion velocity of the gaseous structures causing the Lyα forest in the lower redshift (z ~ 2) smaller separation (61 kpc) sample appears about 20% lower than the local Hubble expansion velocity. In order to understand the observed velocity distribution further we investigated the statistical distribution of expansion velocities in cosmological Lyα forest simulations. The mean expansion velocity in the (z ~ 2, separation ~ 60 kpc) simulation is indeed somewhat smaller than the Hubble velocity, as found in the real data. We interpret this finding as tentative evidence for some Lyα forest clouds breaking away from the Hubble flow and undergoing the early stages of gravitational collapse. However, the distribution of velocities is highly skewed, and the majority of Lyα forest clouds at all redshifts from 2 to 3.8 expand with super-Hubble velocities, typically about 5%-20% faster than the Hubble flow. This behavior is explained if most Lyα forest clouds in the column density range typically detectable are expanding filaments that stretch and drain into more massive nodes. The significant difference seen in the velocity distributions between the high- and low-redshift samples may conceivably reflect actual peculiar deceleration, the differences in spatial scale, or our selecting higher densities at lower redshift for a given detection threshold for Lyα forest lines. We also investigate the alternative possibility that the velocity structure of the general Lyα forest could have an entirely different, local origin, as expected if the Lyα forest were produced or at least significantly modified by galactic feedback, e.g., winds from star-forming galaxies at high redshift. However, we find no evidence that the observed distribution of velocity shear is significantly influenced by processes other than Hubble expansion and gravitational instability. To avoid overly disturbing the IGM, galactic winds may be old and/or limp by the time we observe them in the Lyα forest, or they may occupy only an insignificant volume fraction of the IGM. We briefly discuss the observational evidence usually presented in favor of an IGM afflicted by high-redshift extragalactic superwinds and find much of it ambiguous. During the hierarchical buildup of structure, galaxies are expected to spill parts of their interstellar medium and to heat and stir the IGM in ways that make it hard to disentangle this gravitational process from the effects of winds.

Faint Active Galactic Nuclei and the Ionizing Background

We determine the evolution of the faint, high-redshift, optical luminosity function (LF) of active galactic nuclei (AGNs) implied by several observationally motivated models of the ionizing background from 3 < z < 5. Our results depend crucially on whether we use the total ionizing rate measured by the proximity effect technique or the lower determination favored by the flux decrement distribution of Lyα forest lines. Assuming a faint-end LF slope of 1.58 and the SDSS estimates of the bright-end slope and normalization, we find that the LF must break at M = -24.2, -22.3, -20.8 at z = 3, 4, 5 if we adopt the lower ionization rate and assume no stellar contribution to the background. The breaks must occur at M = -20.6, -18.7, -18.7 for the proximity effect estimate. Since stars may also contribute to the background, these values are lower limits on the break luminosity, and they brighten by as much as ~2 mag if the escape fraction of ionizing photons from high-z galaxies is consistent with recent estimates: fesc = 0.16. By comparing our expectations to faint AGN searches in the Hubble Deep Field and high-z galaxy fields, we find that typically quoted proximity effect estimates of the background imply an overabundance of AGNs compared to the faint counts (even with fesc = 1). Even adopting the lower bound on proximity effect measurements, the stellar escape fraction must be high: fesc 0.2. Conversely, the lower flux decrement-derived background requires a smaller number of ionizing sources, and faint AGN counts are consistent with this estimate only if there is a limited stellar contribution, fesc 0.05. Our derived LFs together with the locally estimated black hole density suggest that the efficiency of converting mass to light in optically unobscured AGNs is somewhat lower than expected, 0.05 (all models). Comparison with similar estimates based on X-ray counts suggests that more than half of all AGNs are obscured in the UV/optical. We also derive lower limits on typical AGN lifetimes and obtain 107 yr for favored cases.

Large‐Scale Correlation of Mass and Galaxies with the Lyα Forest Transmitted Flux

We present predictions of the correlation between the Lyα forest absorption in quasar spectra and the mass within ~5 h-1 Mpc (comoving) of the line of sight, using fully hydrodynamic and hydro-PM numerical simulations of the cold dark matter model supported by present observations. The observed correlation based on galaxies and the Lyα forest can be directly compared to our theoretical results, assuming that galaxies are linearly biased on large scales. Specifically, we predict the average value of the mass fluctuation, δm, conditioned to a fixed value of the Lyα forest transmitted flux, δF, after they have been smoothed over a 10 h-1 Mpc cube and line-of-sight interval, respectively. We find that the relation between δF/σF and δm/σm (where σm and σF are the rms dispersions) has a predicted slope of ~0.6 at δF = 0 that is largely insensitive to the cosmological model and other Lyα forest parameters. Comparison of our predictions to observations should provide a fundamental test of our ideas on the nature of the Lyα forest and the distribution of galaxies and can yield a measurement of the bias factor of any type of galaxy that is observed in the vicinity of Lyα forest lines of sight.

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.

A Uniform Analysis of the Lyα Forest at z = 0–5. I. The Sample and Distribution of Clouds at z &gt; 1.7

We present moderate-resolution data for 39 quasi-stellar objects (QSOs) at z ≈ 2 obtained at the Multiple Mirror Telescope. These data are combined with spectra of comparable resolution of 60 QSOs with redshifts greater than 1.7 found in the literature to investigate the distribution of Lyα forest lines in redshift and equivalent width. We find a value for γ, the parameter describing the number distribution of Lyα forest lines in redshift, of 1.88 ± 0.22 for lines stronger than a rest equivalent width of 0.32 A, in good agreement with some previous studies. The Kolmogorov-Smirnov test was applied to the data, and it is found that this single power law is a good fit over the relevant redshift ranges. Simulations of the Lyα forest were performed to determine the completeness of the line lists and to test how well the analysis recovers the underlying line statistics, given this level of completeness.

Ultraviolet Spectral Variability and the Lyα Forest in the Lensed Quasar Q0957+561

Far-ultraviolet spectra of the gravitational lens components Q0957+561A, B were obtained with the Hubble Space Telescope Faint Object Spectrograph (FOS) at five equally spaced epochs, one every 2 weeks. We confirm the flux variability of the quasar's Lyα and O VI λ1037 emission lines in IUE spectra reported in earlier work of Dolan et al. The fluxes in these lines vary on a timescale of weeks in the observer's rest frame, independently of each other and of the surrounding continuum. The individual spectra of each image were co-added to investigate the properties of the Lyα forest along the two lines of sight to the quasar. Absorption lines having equivalent width Wλ ≥ 0.3 A in the observer's frame not previously identified by Michalitsianos et al. as interstellar lines, metal lines, or higher order Lyman lines were taken to be Lyα forest lines. The existence of each line in this consistently selected set was then verified by its presence in two archival FOS spectra with ~ 1.5 times higher signal to noise than our co-added spectra. Lyα forest lines with Wλ ≥ 0.3 A appear at 41 distinct wavelengths in the spectra of the two images. One absorption line in the spectrum of image A has no counterpart in the spectrum of image B, and one line in image B has no counterpart in image A. Based on the separation of the lines of sight over the redshift range searched for Lyα forest lines, the density of the absorbing clouds in the direction of Q0957+561 must change significantly over a distance R = 160 h kpc in the simplified model where the absorbers are treated as spherical clouds and the characteristic dimension, R, is the radius. (We adopt H0 = 50 h50 km s-1 Mpc-1, q0 = , and Λ = 0 throughout this paper.) The 95% confidence interval on R extends from 50 to 950 h kpc. We show in the Appendix that the fraction of Lyα forest lines that appear in only one spectrum can be expressed as a rapidly converging power series in 1/r, where r the ratio of the radius of the cloud to the separation of the two lines of sight at the redshift of the cloud. This power series can be rewritten to give r in terms of the fraction of Lyα forest wavelengths that appear in the spectrum of only one image. A simple linear approximation to the solution that everywhere agrees with the power series solution to better than 0.8% for r ≥ 2 is derived in the Appendix.

The Gunn—Peterson effect and the Lyman alpha forest

We show that spatial correlations in a stochastic large-scale velocity field in an otherwise smooth intergalactic medium (homogeneous comoving density) superposed on the general Hubble flow may cause a ‘line-like’ structure in QSO spectra similar to the population of unsaturated Lyα forest lines which usually are attributed to individual clouds with 1011 ≲NHi 5 × 1013 cm−2. Therefore there is no clear observational distinction between a diffuse intergalactic medium and discrete intergalactic clouds. It follows that the H i density in the diffuse intergalactic medium might be substantially underestimated if it is determined from the observed intensity distribution near the apparent continuum in high-resolution spectra of QSOs. Our tentative estimate implies a diffuse neutral hydrogen opacity τGP ∼ 0.3 at z ∼ 3 and a current baryon density ΩIGM ≃ 0.08, assuming a Hubble constant H0 = 70 km s−1 Mpc−1.

Hubble Space TelescopeObservations of the Gravitationally Lensed Cloverleaf Broad Absorption Line QSO H1413+1143: Spectroscopy of the Lyα Forest and Metal‐Line Systems

The intervening Lyα forest and metal-line absorbers toward the Cloverleaf gravitational lens H1413+1143 (zem = 2.54, mv ≈ 17) are examined along the four individual sight lines using HST-FOS spectra. The observations probe the Lyα forest systems on size scales of up to 4 h-1 kpc (H0 = 100 h km s-1 Mpc-1, q0 = 0.5) and metal-line systems on scales up to 6 h-1 kpc. The equivalent widths of Lyα forest lines in general are well correlated along the four different lines of sight and are consistent with the sight lines piercing the same absorbers. The heavy-element systems clearly show evidence for differences. One striking difference is seen in a system at zabs = 2.0969 in which the Lyα line possesses a column density approaching log N(H I) = 19.7 in component B but not exceeding log N(H I) = 18.4 in any of the other three components. Simulations show that a circular absorber responsible for such a scenario has a radius of ≤10 h-1 kpc with a 90% confidence level, assuming a lens redshift of zlens = 1.55. The zabs = 1.6595 system contains a probable damped Lyα line and shows evidence for rotation. A system at zabs = 1.4377 exhibits very significant Fe II absorption in only two of the components at the level of sensitivity of the observations. Examination of how the absorption-line equivalent widths are correlated between the component spectra can provide information about whether all four lines of sight to the QSO are passing through the same absorbers, placing limits on absorber sizes and shapes. The four sight lines toward H1413+1143 provide a unique geometry for this type of study.

Recovery of the Power Spectrum of Mass Fluctuations from Observations of the Lyα Forest

We present a method to recover the shape and amplitude of the power spectrum of mass fluctuations, P(k), from observations of the high-redshift Lyα forest. The method is motivated by the physical picture that has emerged from hydrodynamic cosmological simulations and related semianalytic models, in which typical Lyα forest lines arise in a diffuse, continuous, fluctuating intergalactic medium. The thermal state of this low-density gas (δρ/ρ 10) is governed by simple physical processes, which lead to a tight correlation between the Lyα optical depth and the underlying matter density. To recover the mass power spectrum, we (1) apply a monotonic Gaussian mapping to convert the QSO spectrum to an approximate line-of-sight density field with arbitrary normalization, (2) measure the power spectrum of this continuous density field and convert it to the equivalent three-dimensional P(k), and (3) evolve cosmological simulations with this P(k) shape and a range of normalizations and choose the normalization for which the simulations reproduce the observed power spectrum of the transmitted QSO flux. Imposing the observed mean Lyα opacity as a constraint in step (3) makes the derived P(k) normalization insensitive to the choice of cosmological parameters, ionizing background spectrum, or reionization history. Thus, in contrast to estimates of P(k) from galaxy clustering, there are no uncertain bias parameters in the recovery of the mass power spectrum from the Lyα forest. We test the full recovery procedure on smoothed particle hydrodynamics (SPH) simulations of three different cosmological models and show that it recovers the true mass power spectrum of the models on comoving scales ~1-10 h-1 Mpc, the upper scale being set by the size of the simulation boxes. The procedure works well even when it is applied to noisy (S/N ~ 10), moderate-resolution (~40 km s-1 pixels) spectra. We present an illustrative application to Songaila & Cowie's Keck HIRES spectrum of Q1422+231; the recovered P(k) is consistent with that of an Ω = 1, h = 0.5, σ8(z = 0) ≈ 0.5 cold dark matter model. The statistical uncertainty in this result is large because it is based on a single QSO, but the method can be applied to large samples of existing QSO spectra and should thereby yield the power spectrum of mass fluctuations on small and intermediate scales at redshifts z ~ 2-4.

The H[CLC]e[/CLC] [CSC]ii[/CSC] Opacity of the L[CLC]y[/CLC]α Forest and the Intergalactic Medium

Primordial baryonic matter in the vast intergalactic space may be traced with Lyα resonance absorption by neutral hydrogen and singly ionized helium. The wavelength-averaged He II opacity shortward of 304(1 + z) A, as measured by low-resolution UV spectroscopy, is at least 4.5 times greater than the H I opacity shortward of 1216(1 + z) A. While a part of the He II opacity arises from the intergalactic regions that produce the known Lyα forest, it has been argued whether the He II opacity may be entirely attributable to these observed lines. Based on the empirical formulas governing the distribution of Lyα forest absorption, we use a Monte Carlo technique to calculate the average He II optical depth produced by these forest lines. The He II counterparts of the Lyα forest lines are highly saturated, and hence their contribution to the observed opacity is limited. Assuming an He+ to H0 population ratio of 100 and that the power-law distribution dn/dN ∝ N-1.5 can be extended to a neutral hydrogen column density of NH I = 2 × 1012cm-2, the contribution from these forest lines may account for an He II opacity that is 3 times the H I opacity. Our simulated He II spectrum of the quasar Q0302-003, based on the fitted lines in a high-resolution Keck spectrum, yields a forest optical depth of ~0.9, less than half the observed He II opacity. Therefore, a substantial contribution to He II absorption arises from extremely tenuous regions of intergalactic gas that are beyond the observational limits for H I absorption. He II spectra at higher resolution are a sensitive tool to explore the properties of these small-scale fluctuations that fill ~80% of the intergalactic space and contain a significant part of the baryonic matter in the early universe.

Neutral Hydrogen Gunn‐Peterson Absorption from an Overdensity in the Diffuse Intergalactic Medium at Redshift 0.8?

The results of a search for diffuse neutral hydrogen absorption, including possible absorption from any overdensity in the diffuse intergalactic medium, at low to moderate redshifts (z < 1.2) are discussed. The relatively low incidence of Lyα forest lines at these redshifts makes this work more straightforward than would be the case at higher redshifts. Criteria for the selection of QSO spectra leading to conservative estimates of Gunn-Peterson absorption are discussed, including the requirement that spectra shortward of the O VI λ1032 emission line be available. Thus, extrapolation of the continuum from longward to shortward of the Lyα emission line can be replaced by interpolation of the continuum in the spectral interval between Lyα and O VI emission. Analyses of Hubble Space Telescope Faint Object Spectrograph data on two QSOs with zem < 1 are presented here for illustrative purposes. PKS 1354+195 (zem ≈ 0.72) shows no evidence for an absorption trough to an optical depth limit of τGP-H I(z ≈ 0.5) < 0.02, which is the normal case. However, PKS 2145+067 (zem ≈ 0.99) shows evidence for a rare 12%-14% depression. If this is indeed Gunn-Peterson absorption, there must be an overdensity in the intergalactic medium near redshift z ≈ 0.8 toward PKS 2145+067 which is ≈ 3-7 times the density required for a typical nondetection. The region extends more than 500 h−1100 Mpc along the sight line and, for J912 ≈ 10-22 ergs s-1 Hz-1 cm-2 sr-1, has typical gas density ng ≈ 10-6 cm-3 and temperature T ≈ (1-2) × 105 K. If the proposed overdense region is spherical and not filamentary in shape, this same effect may be visible in objects near the sight line toward PKS 2145+067. Alternative possibilities are discussed.

Lyman‐Alpha Absorption Lines from Minipancakes

Recent numerical simulations show that many Lyα absorption lines of column densities NH I 1015 cm-2 are produced in transient minipancakes. Such pancakes are modeled here, approximating the initial perturbation leading to the formation of the pancake as a single sinusoidal wave. The density and temperature profiles of the gas in the pancake are determined in the case where cooling and heating rate of the gas is larger than the Hubble time, which is shown to hold for zc ~ 3, where zc is the collapse redshift. The Lyα absorption-line profiles for a line of sight through the pancake are then calculated. The absorption lines in general have wings signifying bulk motions in the gas. It is shown that the deviation from a single Voigt profile is large for small H I column density lines, in which the effect of bulk motions is large. For lines with NH I > 1013 cm-2, high temperatures tend to wash out the signatures of bulk motion. The analytical modeling of minipancakes associated with Lyα forest lines—with 1013 NH I 1015 cm-2—gives the corresponding mass scales. In structure formation models with cold dark matter and with reheated intergalactic matter (IGM), the gas in the IGM is inhibited from falling into dark matter potential wells with very small velocity dispersions. Structures with masses larger than this limit (the Jeans mass) can have an infall of gas and form minipancakes. It is shown here that, for typical values of cosmological parameters, absorption lines with NH I ~ 1014 cm-2 correspond to structures with baryonic mass Mb ~ 1010 M☉ with an overdensity of ~10 at z ~ 3. The value of NH I can change by a factor of ~3 in the course of evolution of the pancake in time. It is also shown that there is an upper limit to NH I from a pancake due to the slow recombination rate and the importance of collisional ionization at high temperatures. Minipancakes do not give rise to Lyα lines with NH I 1014.5 cm-2, for a temperature of the IGM of =104 K, J-21 = 1, and ΩIGM ~ 0.03.

Large-Scale Structure at Z approximately 2.5

We have made a statistically complete, unbiased survey of C IV systems toward a region of high QSO density near the south Galactic pole using 25 lines of sight spanning 1.5 &lt; z &lt; 2.8. Such a survey makes an excellent probe of large-scale structure at early epochs. We find evidence for structure on the 15-35 h^-1^ proper Mpc scale (H_0_ = 100 km s^-1^ Mpc^- 1^) as determined by the two-point C IV -C IV absorber correlation function, and we reject the null hypothesis that C IV systems are distributed randomly on such scales at the ~3.5 σ level. The structure likely reflects the distance between two groups of absorbers subtending ~13 x 5 x 21 h^-3^ and ~7 x 1 x 15 h^-3^ Mpc^3^ at z ~ 2.3 and z ~ 2.5, respectively. There is also a marginal trend for the association of high rest equivalent width C IV absorbers and QSOs at similar redshifts but along different lines of sight. The total number of C IV systems detected is consistent with that which would be expected based on a survey using many widely separated lines of sight. Using the same data, we also find 11 Mg II absorbers in a complete survey toward 24 lines of sight; there is no evidence for Mg II-Mg II or Mg II-QSO clustering, though the sample size is likely still too small to detect such structure if it exists. We present also the Lyα forest lines found during the course of the survey.