George D. Becker

The Evolution of Optical Depth in the Lyα Forest: Evidence Against Reionization at z~6*

We examine the evolution of the IGM Lyα optical depth distribution using the transmitted flux probability distribution function (PDF) in a high-resolution sample of 55 QSOs spanning absorption redshifts 1.7 5.7, while simultaneously reproducing the mean transmitted flux down to z = 1.6. In this empirical sense, the evolution of the Lyα forest at z ~ 6 is consistent with observed trends at lower redshift. If the evolution of the forest at z ≾ 5 reflects a slowly evolving density field, temperature, and UV background, then no sudden change in the IGM, such as one due to late reionization, appears necessary to explain the disappearance of transmitted flux at z ~ 6. If the MHR00 density distribution is correct, then a nonuniform UV background and/or IGM temperature may be required to produce the correct distribution of optical depths. We find that an inverse temperature-density relation for the MHR00 model significantly improves the PDF fits, but with a large scatter in the equation-of-state index.

How cold is cold dark matter? Small-scales constraints from the flux power spectrum of the high-redshift Lyman-α forest

We present constraints on the mass of warm dark matter (WDM) particles derived from the Lyman-alpha flux power spectrum of 55 high-resolution HIRES spectra at 2.0 or approximately 1.2 keV (2sigma) if the WDM consists of early decoupled thermal relics and m(WDM) > or approximately 5.6 keV (2sigma) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-alpha flux power spectrum, we get m(WDM) > or approximately 4 keV and m(WDM) > or approximately 28 keV (2sigma) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor of 2.

Observations of Chemically Enriched QSO Absorbers near z~2.3 Galaxies: Galaxy Formation Feedback Signatures in the Intergalactic Medium*

We present a comparative study of galaxies and intergalactic gas toward the z = 2.73 quasar HS 1700+6416, to explore the effects of galaxy formation feedback on the IGM. Our observations and ionization simulations indicate that the volume within 100-200 h physical kpc of high-redshift galaxies is populated by very small (ΔL 1 kpc), dense (ρ/ ~ 1000), and metal-rich (Z - Z☉) absorption-line regions. These systems often contain shock-heated gas seen in O VI and may exhibit [Si/C] abundance enhancements suggestive of preferential enrichment by Type II supernovae. We argue that the absorber geometries resemble thin sheets or bubbles and that their unusual physical properties can be explained using a simple model of radiatively efficient shocks plowing through moderately overdense intergalactic filaments. The high metallicities suggest that these shocks are being expelled from, rather than falling into, star-forming galaxies. There is a drop-off in the intergalactic gas density at galaxy impact parameters of 300 physical kpc (1 comoving Mpc) that may represent boundaries of the gas structures where galaxies reside. The heavy-element enhancement near galaxies covers smaller distances: at galactocentric radii between 100 and 200 h kpc the observed abundances blend into the general metallicity field of the IGM. Our results suggest that either supernova-driven winds or dynamical stripping of interstellar gas alters the IGM near massive galaxies, even at R 100 kpc. However, only a few percent of the total mass in the Lyα forest is encompassed by this active feedback at z ~ 2.5. The effects could be more widespread if the more numerous metal-poor C IV systems at impact parameters 200 h kpc are the tepid remnants of very powerful late-time winds. However, based on present observations it is not clear that this scenario is to be favored over one involving preenrichment by smaller galaxies at z 6.

Stellar population effects on the inferred photon density at reionization

The relationship between stellar populations and the ionizing flux with which they irradiate their surroundings has profound implications for the evolution of the intergalactic medium (IGM). We quantify the ionizing flux arising from synthetic stellar populations which incorporate the evolution of interacting binary stars. We determine that these show ionizing flux boosted by 60 per cent at 0.05 ≤ Z ≤ 0.3 Z⊙ and a more modest 10–20 per cent at near-solar metallicities relative to star-forming populations in which stars evolve in isolation. The relation of ionizing flux to observables such as 1500 A continuum and ultraviolet spectral slope is sensitive to attributes of the stellar population including age, star formation history and initial mass function (IMF). For a galaxy forming 1 M⊙ yr−1, observed at >100 Myr after the onset of star formation, we predict a production rate of photons capable of ionizing hydrogen, Nion = 1.4 × 1053 s−1 at Z = Z⊙ and 3.5 × 1053 s−1 at 0.1 Z⊙, assuming a Salpeter-like IMF. We evaluate the impact of these issues on the ionization of the IGM, finding that the known galaxy populations can maintain the ionization state of the Universe back to z ∼ 9, assuming that their luminosity functions continue to MUV = −10, and that constraints on the IGM at z ∼ 2–5 can be satisfied with modest Lyman-continuum photon escape fractions of 4–24 per cent depending on assumed metallicity.

The neutral hydrogen cosmological mass density at z = 5

We present the largest homogeneous survey of z > 4.4 damped Lyα systems (DLAs) using the spectra of 163 QSOs that comprise the Giant Gemini GMOS (GGG) survey. With this survey we make the most precise high-redshift measurement of the cosmological mass density of neutral hydrogen, Ω_(Hi). At such high redshift, important systematic uncertainties in the identification of DLAs are produced by strong intergalactic medium absorption and QSO continuum placement. These can cause spurious DLA detections, result in real DLAs being missed or bias the inferred DLA column density distribution. We correct for these effects using a combination of mock and higher resolution spectra, and show that for the GGG DLA sample the uncertainties introduced are smaller than the statistical errors on Ω_(Hi). We find Ω_(HI)=0.98^(+0.20)_(−0.18) × 10^(−3) at 〈z〉 = 4.9, assuming a 20 per cent contribution from lower column density systems below the DLA threshold. By comparing to literature measurements at lower redshifts, we show that Ω_(Hi) can be described by the functional form Ω_(HI)(z)∝(1+z)^(0.)4. This gradual decrease from z = 5 to 0 is consistent with the bulk of H I gas being a transitory phase fuelling star formation, which is continually replenished by more highly ionized gas from the intergalactic medium and from recycled galactic winds.

Discovery of Excess O?I Absorption toward the z=6.42 QSO SDSS?J1148+5251

We present a search for O?I in the spectra of nine 4.9 ? zQSO ? 6.4 QSOs taken with Keck HIRES. We detect six systems with N > 1013.7 cm-2 in the redshift intervals where O?I??1302 falls redward of the Ly? forest. Four of these lie toward SDSS?J1148+5251 (zQSO = 6.42). This imbalance is unlikely to arise from variations in sensitivity among our data or from a statistical fluctuation. The excess O?I occurs over a redshift interval that also contains transmission in Ly? and Ly?. Therefore, if these O?I systems represent pockets of neutral gas, then they must occur within or near regions of the IGM that are highly ionized. In contrast, no O?I is detected toward SDSS?J1030+0524 (zQSO = 6.30), whose spectrum shows complete absorption in Ly? and Ly? over ?z ~ 0.2. Assuming no ionization corrections, we measure mean abundance ratios [O/Si] = -0.04 ? 0.06, [C/O] = -0.31 ? 0.09, and [C/Si] = -0.34 ? 0.07 (2 ?), which are consistent with enrichment dominated by Type II supernovae. The O/Si ratio limits the fraction of silicon in these systems contributed by metal-free very massive stars to 30%, a result that is insensitive to ionization corrections. The ionic comoving mass densities along the zQSO > 6.2 sight lines, including only the detected systems, are ? = (7.0 ? 0.6) ? 10-8, ? = (9.6 ? 0.9) ? 10-9, and ? = (1.5 ? 0.2) ? 10-8.

Reionisation and High-Redshift Galaxies: The View from Quasar Absorption Lines

Determining when and how the first galaxies reionised the intergalactic medium promises to shed light on both the nature of the first objects and the cosmic history of baryons. Towards this goal, quasar absorption lines play a unique role by probing the properties of diffuse gas on galactic and intergalactic scales. In this review, we examine the multiple ways in which absorption lines trace the connection between galaxies and the intergalactic medium near the reionisation epoch. We first describe how the Ly α forest is used to determine the intensity of the ionising ultraviolet background and the global ionising emissivity budget. Critically, these measurements reflect the escaping ionising radiation from all galaxies, including those too faint to detect directly. We then discuss insights from metal absorption lines into reionisation-era galaxies and their surroundings. Current observations suggest a buildup of metals in the circumgalactic environments of galaxies over z ~ 6 to 5, although changes in ionisation will also affect the evolution of metal line properties. A substantial fraction of metal absorbers at these redshifts may trace relatively low-mass galaxies. Finally, we review constraints from the Ly α forest and quasar near zones on the timing of reionisation. Along with other probes of the high-redshift Universe, absorption line data are consistent with a relatively late end to reionisation (5.5 ≲ z ≲ 7); however, the constraints are still fairly week. Significant progress is expected to come through improved analysis techniques, increases in the number of known high-redshift quasars from optical and infrared sky surveys, large gains in sensitivity from next-generation observing facilities, and synergies with other probes of the reionisation era.

The evolution of neutral gas in damped Lyman α systems from the XQ-100 survey

We present a sample of 38 intervening Damped Lyman

Expansion and Collapse in the Cosmic Web

\alpha

Metals in the z ∼ 3 intergalactic medium: results from an ultra-high signal-to-noise ratio UVES quasar spectrum

(DLA) systems identified towards 100