Mark L. Giroux

Reionization in a cold dark matter universe: The feedback of galaxy formation on the intergalactic medium

We study the coupled evolution of the intergalactic medium (IGM) and the emerging structure in the universe in the context of the cold dark matter (CDM) model, with a special focus on the consequences of imposing reionization and the Gunn-Peterson constraint as a boundary condition on the model. We have calculated the time-varying density of the IGM by coupling our detailed, numerical calculations of the thermal and ionization balance and radiative transfer in a uniform, spatially averaged IGM of H and He, including the mean opacity of an evolving distribution of gas clumps which correspond to quasar absorption line clouds, to the linearized equations for the growth of density fluctuations in both the gaseous and dark matter components in a CDM universe. We use the linear growth equations to identify the fraction of the gas which must have collapsed out at each epoch, an approach similar in spirit to the so-called Press-Schechter formalism. We identify the IGM density with the uncollapsed baryon fraction. The collapsed fraction is postulated to be a source of energy injection into the IGM, by radiation or bulk hydrodynamical heating (e.g., via shocks) or both, at a rate which is marginally enough to satisfy the Gunn-Peterson constraint at z less than 5. Our results include the following: (1) We find that the IGM in a CDM model must have contained a substantial fraction of the total baryon density of the universe both during and after its reionization epoch. (2) As a result, our previous conclusion that the observed Quasi-Stellar Objects (QSOs) at high redshift are not sufficient to ionize the IGM enough to satisfy the Gunn-Peterson constraint is confirmed. (3) We predict a detectable He II Gunn-Peterson effect at 304(1 + z) A in the spectra of quasars at a range of redshift z greater than or approx. 3, depending on the nature of the sources of IGM reionization. (4) We find, moreover, that a CDM model with high bias parameter b (i.e., b greater than or approx. 2) cannot account for the baryon content of the universe at z approximately 3 observed in quasar absorption line gas unless Omega (sub B) significantly exceeds the maximum value allowed by big bang nucleocynthesis. (5) For a CDM model with bias parameter within the allowed range of (lower) values, the lower limit to Omega(sub B) imposed by big bang nucleosynthesis (Omega(sub B) h(sup 2) greater than or equal to 0.01) combines with our results to yield the minimum IGM density for the CDM fodel. For CDM with b = 1 (Cosmic Background Explorer (COBE) normalization), we find Omega(sub IGM)(sup min) (z approximately 4) approx. equal 0.02-0.03, and Omega(sub IGM)(sup min)(z approximately 0) approx. equal 0.005-0.03, depending upon the nature of the sources of IGM reionization. (6) In general, we find that self-consistent reionization of the IGM by the collapsed baryon fraction has a strong effect on the rate of collapse. (7) As a further example, we show that the feedback effect on the IGM of energy release by the collapsed baryon fraction may explain the slow evolution of the observed comoving QSO number density between z = 5 and z = 2, followed by the sharp decline after z = 2.

Heating and Ionization of the Intergalactic Medium by an Early X-Ray Background

Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts just over 6. A number of models have been developed to describe the process of reionization and the effects of the ionizing photons from the first luminous objects. In this paper we study the impact of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM prior to reionization, before the fully ionized bubbles associated with individual sources have overlapped. X-rays have large mean free paths relative to EUV photons, and their photoelectrons can have significant effects on the thermal and ionization balance. We find that hydrogen ionization is dominated by the X-ray photoionization of neutral helium and the resulting secondary electrons. Thus, the IGM may have been warm and weakly ionized prior to full reionization. We examine several related consequences, including the filtering of the baryonic Jeans mass scale, signatures in the cosmic microwave background, and the H--catalyzed production of molecular hydrogen.

A Study of the Reionization History of Intergalactic Helium with FUSE and the Very Large Telescope

We obtained high-resolution Far Ultraviolet Spectroscopic Explorer (FUSE; R ~ 20,000) and Very Large Telescope (VLT; R ~ 45,000) spectra of the quasar HE 2347-4342 in order to study the properties of the intergalactic medium between redshifts z = 2.0 and 2.9. The high-quality optical spectrum allows us to identify approximately 850 H I absorption lines with column densities between N ~ 5 × 1011 and 1018 cm-2. The reprocessed FUSE spectrum extends the wavelength coverage of the He II absorption down to an observed wavelength of 920 A. Source flux is detected to rest-frame wavelengths as short as ~237 A. Approximately 1400 He II absorption lines are identified, including 917 He II Lyα systems and some of their He II Lyβ, Lyγ, and Lyδ counterparts. The ionization structure of He II is complex, with approximately 90 absorption lines that are not detected in the hydrogen spectrum. These features may represent the effect of soft ionizing sources. The ratio η = N(He )/N(H ) varies approximately from unity to more than a thousand, with a median value of 62 and a distribution consistent with the intrinsic spectral indexes of quasars. This provides evidence that the dominant ionizing field is from the accumulated quasar radiation, with contributions from other soft sources such as star-forming regions and obscured active galactic nuclei, which do not ionize helium. We find an evolution in η toward smaller values at lower redshift, with the gradual disappearance of soft components. At redshifts z > 2.7, the large but finite increase in the He II opacity, τ = 5 ± 1, suggests that we are viewing the end stages of a reionization process that began at an earlier epoch. Fits of the absorption profiles of unblended lines indicate comparable velocities between hydrogen and He+ ions. For line widths b = ξbH, we find ξ = 0.95 ± 0.12, indicating a velocity field in the intergalactic medium dominated by turbulence. At hydrogen column densities N < 3 × 1012 cm-2, the number of forest lines shows a significant deficit relative to a power law and becomes negligible below N = 1011 cm-2.

The Ionization and Metallicity of the Intervening O VI Absorber at z = 0.1212 in the Spectrum of H1821+643

We use high-resolution UV spectra of the radio-quiet quasar (QSO) H1821+643 (zem = 0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE), to study the ionization and metallicity of an intervening O VI absorption line system at zabs = 0.1212. This absorber has the following notable properties: (1) several galaxies are close to the sight line at the absorber redshift, including an actively star-forming galaxy at a projected distance of 144 h kpc, (2) there is a complex cluster of H I Lyα absorption lines near the O VI redshift, including at least five components spread over a velocity range of ~700 km s-1, (3) the strongest Lyα line in the cluster appears to be composed of a mildly saturated component with a typical b-value blended with a remarkably broad component with b ≈ 85 km s-1, (4) the O VI absorption is not aligned with the strongest (saturated) H I absorption, but instead is well aligned with the very broad component, and (5) the only detected species (at the 4 σ level) are O VI and H I, despite coverage of strong transitions of abundant elements (e.g., C II, C III, and C IV). Based on these constraints, we find that the absorption line properties can be produced in collisionally ionized gas with 105.3 ≤ T ≤ 105.6 K and -1.8 ≤ [O/H] ≤ -0.6. However, we find that photoionization is also viable if the path length l through the absorbing gas is long enough; simple photoionization models require 85 ≤ l ≤ 1900 kpc and -1.1 ≤ [O/H] ≤ -0.3. We briefly discuss how observations of X-ray absorption lines due to O VII and O VIII could be used, in principle, to break the ionization mechanism degeneracy, and we conclude with some comments regarding the nature of O VI absorbers.

Highly Ionized High‐Velocity Clouds toward PKS 2155−304 and Markarian 509

To gain insight into four highly ionized high-velocity clouds (HVCs) discovered by Sembach et al., we have analyzed data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) for the PKS 2155-304 and Mrk 509 sight lines. We measure strong absorption in O VI and column densities of multiple ionization stages of silicon (Si II, III, and IV) and carbon (C II, III, and IV). We interpret this ionization pattern as a multiphase medium that contains both collisionally ionized and photoionized gas. Toward PKS 2155-304, for HVCs at -140 and -270 km s-1, respectively, we measure log N(O ) = 13.80 ± 0.03 and log N(O ) = 13.56 ± 0.06; from Lyman series absorption, we find log N(H ) = 16.37 and 15.23. The presence of high-velocity O VI spread over a broad (100 km s-1) profile, together with large amounts of low-ionization species, is difficult to reconcile with the low densities, ne ≈ 5 × 10-6 cm-3, in the collisional/photoionization models of Nicastro et al., although the HVCs show a similar relation in N(Si IV)/N(C IV) versus N(C II)/N(C IV) to that of high-z intergalactic clouds. Our results suggest that the high-velocity O VI in these absorbers does not necessarily trace the warm-hot intergalactic medium but instead may trace HVCs with low total hydrogen column density. We propose that the broad high-velocity O VI absorption arises from shock ionization, at bow shock interfaces produced from infalling clumps of gas with velocity shear. The similar ratios of high ions for HVC Complex C and these highly ionized HVCs suggest a common production mechanism in the Galactic halo.

The Influence of the Photoionizing Radiation Spectrum on Metal-Line Ratios in Ly(alpha) Forest Clouds

Recent measurements of Si IV/C IV ratios in the high-redshift Ly(alpha) forest (Songaila & Cowie, AJ, 112, 335 (1996a); Savaglio et at., AA values Si/C approx. 2(Si/C)(solar mass) fit the measurements better than solar abundances. Ultimately, an adequate interpretation of the ratios of C IV, Si IV, and C II may require hot, collisionally ionized gas in a multiphase medium.

Metallicity and Ionization in High-Velocity Cloud Complex C

We analyze HST and FUSE ultraviolet spectroscopic data for 11 sight lines passing through the infalling high-velocity cloud (HVC) Complex C. These sight lines pass through regions with H I column densities in the range N = 1018.1-1020.1 cm-2. From [O I/H I] abundances, we find that Complex C metallicities range from 0.09 to 0.29 Z☉, with a column density weighted mean of 0.13 Z☉. Nitrogen (N I) is underabundant by factors of (0.01-0.07)(N/H)☉, significantly less than oxygen relative to solar abundances. This pattern suggests nucleosynthetic enrichment by Type II SNe, consistent with an origin in the Galactic fountain or infalling gas produced in winds from Local Group galaxies. The range of metallicity and its possible (2 σ) dependence on N could indicate some mixing of primordial material with enriched gas from the Milky Way, but the mixing mechanism is unclear. We also investigate the significant highly ionized component of Complex C, detected in C IV, Si IV, and O VI, but not in N V. High-ion column density ratios show little variance and are consistent with shock ionization or ionization at interfaces between Complex C and a hotter surrounding medium. Evidence for the former mechanism is seen in the Mrk 876 line profiles, where the offset in line centroids between low and high ions suggests a decelerating bow shock.

A Survey of Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope Sight Lines through High-Velocity Cloud Complex C

Using archival Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope (HST) data, we have assembled a survey of eight sightlines through high-velocity cloud Complex C. Abundances of the observed ion species vary significantly for these sightlines, indicating that Complex C is not well characterized by a single metallicity. Reliable metallicities based on [O I/H I] range from 0.1-0.25 solar. Metallicities based on [S II/H I] range from 0.1-0.6 solar, but the trend of decreasing abundance with H I column density indicates that photoionization corrections may affect the conversion to [S/H]. We present models of the dependence of the ionization correction on H I column density; these ionization corrections are significant when converting ion abundances to elemental abundances for S, Si, and Fe. The measured abundances in this survey indicate that parts of the cloud have a higher metallicity than previously thought and that Complex C may represent a mixture of “Galactic fountain” gas with infalling low-metallicity gas. We find that [S/O] and [Si/O] have a solar ratio, suggesting little dust depletion. Further, the measured abundances suggest an over-abundance of O, S, and Si relative to N and Fe. The enhancement of these α-elements suggests that the bulk of the metals in Complex C were produced by Type II supernovae and then removed from the star-forming region, possibly via supernovae-driven winds or tidal stripping, before the ISM could be enriched by N and Fe. Also at JILA, University of Colorado and National Institute of Standards and Technology.Using archival Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope (HST) data, we have assembled a survey of eight sight lines through high-velocity cloud Complex C. Abundances of the observed ion species vary significantly for these sight lines, indicating that Complex C is not well characterized by a single metallicity. Reliable metallicities based on [O I/H I] range from 0.1 to 0.25 Z☉. Metallicities based on [S II/H I] range from 0.1 to 0.6 Z☉, but the trend of decreasing abundance with H I column density indicates that photoionization corrections may affect the conversion to [S/H]. We present models of the dependence of the ionization correction on H I column density; these ionization corrections are significant when converting ion abundances to elemental abundances for S, Si, and Fe. The measured abundances in this survey indicate that parts of the cloud have a higher metallicity than previously thought and that Complex C may represent a mixture of Galactic fountain gas with infalling low-metallicity gas. We find that [S/O] and [Si/O] have a solar ratio, suggesting little dust depletion. Further, the measured abundances suggest an overabundance of O, S, and Si relative to N and Fe. The enhancement of these α-elements suggests that the bulk of the metals in Complex C were produced by Type II supernovae and then removed from the star-forming region, possibly via supernova-driven winds or tidal stripping, before the ISM could be enriched by N and Fe.

The Hot Intergalactic Medium-Galaxy Connection: Two Strong O VI Absorbers in the Sight Line toward PG 1211+143

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) and Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of the QSO PG 1211+143 (zem = 0.081) and a galaxy survey of the surrounding field. This sight line shows two strong intergalactic absorption systems at cz 15,300 and 19,300 km s-1. This sight line addresses the nature and origin of the O VI absorbers and their connection to galaxies. We explore the relationship of these absorbers to the nearby galaxies and compare them to other O VI-bearing absorbers in diverse environments. At 15,300 km s-1, we find four distinct H I components and associated C II, C III, C IV, Si II, Si III, Si IV, N V, and O VI, lying near a spiral-dominated galaxy group with a bright member galaxy 137 h kpc from the sight line. The observed ions of C, Si, and N are likely to be photoionized, but the O VI is more consistent with collisional ionization. The ion ratios in this absorber resemble the highly ionized Galactic high-velocity clouds (HVCs); it may also trace the hot intragroup medium gas or the unbound wind of an undiscovered dwarf galaxy. At 19,300 km s-1, we find five H I components and associated C III, Si III, and collisionally ionized O VI lying 146 h kpc from an isolated galaxy. The properties of the O VI-bearing gas are consistent with an origin in strong shocks between low-metallicity gas (?2%-6% solar) and one or more of the warm photoionized components. It is likely that these absorbers are related to the nearby galaxies, perhaps by outflows or gas stripped from unseen satellite galaxies by interactions. However, we cannot reject completely the hypothesis that they reside in the same large-scale structure in which the galaxies are embedded but are otherwise not directly related.

The UV spectrum of HS 1700+6416 II. Fuse observations of the He II Lyman alpha forest

Aims. We present the far-UV spectrum of the quasar HS 1700+6416 taken with FUSE. This QSO provides the second line of sight with the He ii absorption resolved into a Lyα forest structure. Since HS 1700+6416 is slightly less redshifted (zem = 2.72) than HE 2347-4342, we only probe the post-reionization phase of He ii, seen in the evolution of the He ii opacity, which is consistent with as imple power law. Methods. The He ii/H i ratio η is estimated using a line profile-fitting procedure and an apparent optical depth approach, respectively. The expected metal line absorption in the far-UV is taken into account as well as molecular absorption of galactic H2. About 27% of the η values are affected by metal line absorption. In order to investigate the applicability of the analysis methods, we create simple artificial spectra based on the statistical properties of the H i Lyα forest. Results. The analysis of the artificial data demonstrates that the apparent optical depth method as well as the line profile-fitting procedure lead to confident results for restricted data samples only (0.01 ≤ τHi ≤ 0.1 and 12.0 ≤ logNHi ≤ 13.0, respectively). The reasons are saturation in the case of the apparent optical depth and thermal line widths in the case of the profile fits. Furthermore, applying the methods to the unrestricted data set may mimic a correlation between the Heii/Hi ratio and the strength of the H i absorption. For the restricted data samples a scatter of 10−15% in η would be expected even if the underlying value is constant. The observed scatter is significantly larger than expected, indicating that the intergalactic radiation background is indeed fluctuating. In the redshift range 2.58 < z < 2.72, where the data quality is best, we find η ∼ 100, suggesting a contribution of soft sources like galaxies to the UV background.