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 Å. Source flux is detected to rest-frame wavelengths as short as ∼237 Å. 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 II)/N(H I) 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 bHe+He+ = 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.

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 logN(O VI) = 13.80 ± 0.03 and logN(O VI) = 13.56 ± 0.06; from Lyman series absorption, we find logN(H I) = 16.37 -0.14+0.22 and 15.23-0.22+0.38. 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.

Les raies d'absorption de quasars sont des outils efficaces pour étudier le milieu interstellaire dans les galaxies. Dans ce travail, nous étudions un échantillon de soixante-six systèmes absorbants à z<1.5 sélectionnés pour la présence de raies d'absorption de CI intenses dans leurs spectres SDSS. Ils sont observés par les spectrographes X-shooter et UVES du VLT de l'ESO. Nous étudions en tout 17 systèmes observés par X-shooter. Nous déduisons la métallicité, la déplétion par la poussière, le taux d'extinction par la poussière et le taux d'absorption des raies de MgII, MgI, CaII et NaI décalées dans l'infrarouge proche. Nous détectons neuf raies d'absorption de CaII avec W(CaII λ3934)>0.23Å. Nous détectons dix raies d'absorption de NaI dans quatorze systèmes susceptibles d'en montrer. La largeur équivalente médiane de W(NaI λ5891)=0.68Å est plus grande que celles observées dans des nuages proches ayant des densités-colonnes de HI similaires ou dans des systèmes CaII à z<0.7 détectés par le SDSS. La présence systématique de raies d'absorption de NaI dans ces systèmes CI suggère fortement que le gaz environnant est neutre et froid, et donc peut faire partie du gaz moléculaire diffus dans le milieu interstellaire de galaxies à fort décalage vers le rouge. Les raies d'absorption de MgII s'étalent sur plus de 400km/s en Δv pour la moitié de l'échantillon; trois systèmes ont un Δv supérieur à 500 km/s. Tout ceci suggère qu'une fraction importante du gaz froid à fort décalage vers le rouge émane d'environnements perturbés. Nous détectons de l'hydrogène moléculaire dans tous les systèmes dans la limite de détection. / Quasar absorption lines are a powerful tool to study the interstellar medium(ISM) in the galaxies. We study a sample of 66 z >1.5 absorbers selected based on the presence of strong CI absorption lines in SDSS spectra and observed with the ESO-VLT spectrograph X-shooter/UVES. I study 17 systems that are re-observed by X-shooter. I derive metallicities, depletion onto dust, extinction by dust and analyse the absorption from MgII, MgI, CaII and NaI that are redshifted into the near infrared wavelength range. I detect 9 CaII absorptions with W(CaII λ3934) > 0.23 Å out of 14 systems. I detect 10 NaI absorptions in the 11 systems where we could observe this absorption. The median equivalent width (W(NaI λ5891) = 0.68 Å) is larger than what is observed in local clouds with similar HI column densities but also in z<0.7 CaII systems detected in the SDSS. The systematic presence of NaI absorption in these CI systems strongly suggests that the gas is neutral and cold, maybe part of the diffuse molecular gas in the ISM of high-redshift galaxies. The MgII absorptions are spread over more than Δv ~ 400 km/s for half of the systems; three absorbers have Δv > 500 km/s. The kinematics is strongly perturbed for most of these systems which probably do not arise in quiet disks and must be close to regions with intense star formation activity. All this suggests that a large fraction of the cold gas at high redshift arises in disturbed environments. We detect molecular hydrogen in all the systems within the detection limit.

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