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Item Molecular hydrogen at Zabx=1.973 towards Q0013 - 004: Dust depletion pattern in damped Lyman - alpha systems(2002-03-06) Petitjean, Patrick; Srianand, R.; Ledoux, C.We study the dust depletion pattern in different well separated components of the Zabs = 1.973, log N(H I) = 20.83, damped Lyman-a system toward Q 0013-004. The apparent correlation between [Fe/S] and [Si/S] in the components indicates that the abundance pattern is indeed due to dust-depletion. In particular, we find evidence for depletion similar to what is observed in cold gas of the Galactic disk in one of the weakest components ([Fe/Zn] = -1.62, [Fe/S]= -1.82, [Zn/S] = -0.2, [Si/S]= -0.92) in which molecular hydrogen is detected with log N(H2) '" 16.5. This is the first time that such depletion is seen in a DLA system. Extinction due to this component is negligible owing to small total HI column density, log N(HI ) ::;;19.4.This observation supports the possibility that current samples of DLA systems might be biased against the presence of cold and dusty gas along the line of sight. The global metallicities of this peculiar DLA system in which 0 I and C II are spread over ",1050 km S-1 are [P/H] = -0.64, [Zn/H] = -0.75 and [S/H] = -0.76 relative to solar. The overall molecular fraction is in the range -2.7 < log f < -0.6. which is the highest value found for DLA systems. H2 is detected in four components at -625, -475, 0 and 80 km S-1 relative to the strongest component at Zabs = 1.97296.CO is not detected (log N(CO)/ N(H r) < -8) and HD could be present at Zabs = 1.97380. We show that the presence of H2 is closely related to the physical conditions in the gas: high particle density together with low temperature. Excitation of high J levels and molecular fraction vary largely from one component to the ot.her suggesting that the UV radiation field is highly inhomogeneous through the system. Gas pressure, estimated from C I absorptions, is larger than what is observed in the ISM of our Galaxy. This, together with the complex kinematics, suggests that part of the gas is subject to high compression due to either collapse, merging and/or supernovae explosion. This is probably a consequence of star-formation activity in the vicinity of the absorbing gasItem H2 molecules and the nature of damped Lyman-alpha systems(2000-12-14) Petitjean, Patrick; Srianand, R.; Ledoux, C.We report results from our mini-survey for molecular hydrogen in eight high-redshift damped Lyman- α (DLA) systems using the ESO Ultra-violet and Visible Spectrograph on the VLT. In addition, we investigate two systems using ESO public data. We include in the sam- ple the only system where H2 was previously detected and studied at high-spectral resolution. Altogether our sample consists of eleven absorbers with 1.85 < zabs < 3.4. We confirm the presence of H2 in the zabs = 2.3377, metal-poor ([Si/H] = −1.20), system toward PKS 1232+082. The derived molecular fraction, f = 2N(H2)/(2N(H2)+N(H i)) = 4×10−4, is two orders of magnitude less than what has been claimed previ- ously from low-resolution data. The physical conditions within the cloud can be constrained directly from ob- servation. The kinetic temperature and particle density are in the ranges, respectively, 100 < T < 300 K and 30 < nH < 50 cm−3. In addition, UV pumping is of the same order of magnitude than in our Galaxy. The upper limits on the molecular fraction derived in nine of the systems are in the range 1.2×10−7−1.6×10−5. There is no evidence in this sample for any correlation be- tween H2 abundance and relative heavy element depletion into dust grains. This should be investigated using a larger sample however. The molecular abundance in a few DLA systems (and in particular in the two systems where H2 is detected) is consistent with what is seen in the Magel- lanic clouds. But most of the DLA measurements are well below these values. This is probably partly due to small amounts of dust and/or high UV flux. We argue however that the lack of molecules is a direct consequence of high kinetic temperature (T > 3000 K) implying a low forma- tion rate of H2 onto dust grains. Therefore, most of the DLA systems arise in warm and diffuse neutral gas.Item Cosmic microwave background temperature at a redshift of 2.33771(2000-12-18) Srianand, R.; Petitjean, Patrick; Ledoux, C.The Cosmic Microwave Background radiation is a fundamental prediction of Hot Big Bang cosmology. The temperature of its black-body spectrum has been measured at the present time, TCMBR,0 = 2.726±0.010 K, and is predicted to have been higher in the past. At earlier time, the temperature can be measured, in principle, using the excitation of atomic fine structure levels by the radiation field. All previous measurements however give only upper limits as they assume that no other significant source of excitation is present. Here we report the detection of absorption from the first and second fine-structure levels of neutral carbon atoms in an isolated remote cloud at a redshift of 2.33771. In addition, the unusual detection of molecular hydrogen in several rotational levels and the presence of ionized carbon in its excited fine structure level make the absorption system unique to constrain, directly from observation, the different excitation processes at play. It is shown for the first time that the cosmic radiation was warmer in the past. We find 6.0 < TCMBR < 14 K at z = 2.33771 when 9.1 K is expected in the Hot Big Bang cosmology.Item Detection of molecular hydrogen in a near solar-metallicity damped Lyman - alpha system at Zabc ~ 2 toward Q 0551 - 366 Authors: Ledoux, C.(2002-05-01) Ledoux, C.; Srianand, R.; Petitjean, PatrickWe report the detection of H2, C I, C 1*, C I ** and Cl I lines in a near Solar-metallicity ([Zn/H] = -0.13) damped Lyman-a (DLA) system at Zabs = 1.962 observed on the line of sight to the quasar Q 0551-366. The iron-peak elements, X = Fe, Cr and Mn are depleted compared to zinc, [X/Zn] "'" -0.8, probably because they are tied up onto dust grains. Among the three detected Hs-bearing clouds, spanning 55 km S-l in velocity space, we derive a total molecular hydrogen column density N(H2) = 2.6 X 1017 cm- 2 and a mean molecular fraction f = 2N(H2)/(2N(H2) + N(H I» = 1.7 x 10- 3. The depletion of heavy elements (S, Si, Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observed in the diffuse neutral gas of the Galactic halo. This depletion is approximately the same in the six C I-detected components independently of the presence or absence of H2. The gas clouds in which H2 is detected always have large densities, nH > 30 cm- 3 , and low temperatures, T01 ~ 100 K. This shows that presence of dust, high particle density and/or low temperature are required for molecules to be present. The photo-dissociation rate derived in the components where H2 is detected suggests the existence of a local UV radiation field similar in strength to the one in the Galaxy. Star formation therefore probably occurs near these H2-bearing clouds.Item Collimated flow driven by radiative pressure from the nucleus of quasar Q 1511 + 091(2011-07-06) Srianand, R.; Petitjean, Patrick; Ledoux, C.; et al.High velocity outflows from quasars are revealed by the absorption signatures they produce in the spectrum of the quasar. Clues on the nature and origin of these flows are important for our understanding of the dynamics of gas in the central regions of the Active Galactic Nucleus (AGNs) but also of the metal enrichment of the intergalactic space. Line radiation pressure has often been suggested to be an important process in driving these outflows, however no convincing evidence has been given so far. Here we report observation of a highly structured flow, toward Q 1511+091, where the velocity separations between distinct components are similar to O vi, N v and C iv doublet splittings with some of the profiles matching perfectly. This strongly favors the idea that the absorbing clumps originate at similar physical location and are driven by radiative acceleration due to resonance lines. The complex absorption can be understood if the flow is highly collimated so that the different optically thick clouds are aligned and cover the same region of the background source. One component shows saturated H i Lyman series lines together with absorptions from excited levels from C ii and Si ii but covers only 40% of the source of continuum. The fact that clouds cover only part of the small continuum source implies that the flow is located very close to it.Item VLT-UVEs survey for molecular hydrogen in high-redshift damped lyman-alfa system(2011-07-05) Ledoux, C.; Petitjean, Patrick; Srianand, R.We have searched for molecular hydrogen in damped Lyman-α (DLA) and sub- DLA systems at high redshift (zabs > 1.8) using UVES at the VLT down to a detection limit of typically N(H2) = 2×1014 cm−2. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, molecular hydrogen is detected in 13 to 20 percent of the newly-surveyed systems.We report new detections of molecular hydrogen at zabs = 2.087 and 2.595 toward, respectively, Q1444+014 and Q0405−443, and also reanalyse the system at zabs = 3.025 toward Q0347−383. In all of the systems, we measure metallicities relative to Solar, [X/H] (with either X=Zn, or S, or Si), and depletion factors of Fe, [X/Fe], supposedly onto dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems. Although H2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2- detected components even has [Zn/Fe] > 1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction, f = 2N(H2)/[2N(H2) + N(Hi)], is generally small in DLA systems (typically log f < −1) and similar to what is observed in the Magellanic Clouds. There is no correlation between the observed amount of H2 and the Hi column density. In fact, two systems where H2 is detected have logN(Hi) < 20.3 and, therefore, are sub-DLA systems. From 58 to 75 percent of the DLA systems have log f < −6. This can be explained if the formation rate of H2 onto dust grains is reduced in those systems, probably because the gas is warm (T > 1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.Item Velocity-metallicity correlation for high-z DLA galaxies: Evidence for a mass-metallicity relation?(2006-06-08) Ledoux, C.; Petitjean, Patrick; Fynbo, J. P. U.; et al.Item Relative abundance pattern along the profile of high redshift damped Lyman-alpha systems(2005-09-01) Rodriguez, E.; Ledoux, C.; Aracil, Bastien; et al.We investigate abundance ratios along the profiles of six high-redshift Damped Lyman-α systems, three of them beeing associated with H2 absorption. We use a new method to derive optical depths in each velocity pixel. The variations of the pixel abundance ratios are found to be remarquably small and usually smaller than a factor of two within a profile. This results holds when considering independent sub-clumps in the same system. Only in the components where H2 is detected is the depletion factor significantly enhanced. There is a strong correlation between [Fe/S] and [Si/S] ratios showing that the abundance ratio patterns are definitely related to the presence of dust. The depletion pattern is usually close to that seen in warm halo gas of our Galaxy.Item Nitrogen and Oxygen abundances in the neutral gas at high redshift(2007-12-17) Petitjean, Patrick; Ledoux, C.; Srianand, R.Aims. We study the Oxygen and Nitrogen abundances in the interstellar medium of high-redshift galaxies. Methods. We use high resolution and high signal-to-noise ratio spectra of Damped Lyman-α (DLA) systems detected along the line-of-sight to quasars to derive robust abundance measurements from unsaturated metal absorption lines. Results. We present results for a sample of 16 high-redshift DLAs and strong sub-DLAs (log N(H i) > 19.5, 2.4 < zabs <3.6) including 13 new measurements. We find that the Oxygen to Iron abundance ratio is pretty much constant with [O/Fe] ∼ +0.32±0.10 for −2.5 < [O/H] < −1.0 with a small scatter around this value. The Oxygen abundance follows quite well the Silicon abundance within ∼0.2 dex although the Silicon abundance could be slightly smaller for [O/H] < −2. The distribution of the [N/O] abundance ratio, measured from components that are detected in both species, is somehow double peaked: five systems have [N/O] > −1 and nine systems have [N/O] < −1.15. In the diagram [N/O] versus [O/H], a loose plateau is possibly present at [N/O] ∼ −0.9 that is below the so-called primary plateau as seen in local metal-poor dwarf galaxies ([N/O] in the range −0.57 to −0.74). No system is seen above this primary plateau whereas the majority of the systems lie well below with a large scatter. All this suggests a picture in which DLAs undergo successive star-bursts. During such an episode, the [N/O] ratio decreases sharply because of the rapid release of Oxygen by massive stars whereas inbetween two bursts, Nitrogen is released by low and intermediate-mass stars with a delay and the [N/O] ratio increases.Item Multiphase plasma in sub-damped Lyman-Alpha systems : A hidden metal reservoir(2007-08-22) Fox, Andrew J.; Petitjean, Patrick; Ledoux, C.; et al.We present a VLT/UVES spectrum of a proximate sub-damped Lyman-α (sub-DLA) system at zabs = 2.65618 toward the quasar Q0331-4505 (zqso = 2.6785±0.0030). Absorption lines of O I, Si II,Si III, Si IV, C II, C III, C IV, Fe II, Al II, and O VI are seen in the sub-DLA, which has a neutral hydrogen column density logNHI = 19.82 ± 0.05. The absorber is at a velocity of 1 820 ± 250 kms−1 from the quasar; however, its low metallicity [O/H]=−1.64 ± 0.07, lack of partial coverage, lack of temporal variations between observations taken in 2003 and 2006, and non-detection of N V imply the absorber is not a genuine intrinsic system. By measuring the O VI column density and assuming equal metallicities in the neutral and ionized gas, we determine the column density of hot ionized hydrogen in this sub-DLA, and in two other sub-DLAs with O VI drawn from the literature. Coupling this with determinations of the typical amount of warm ionized hydrogen in sub-DLAs, we confirm that sub-DLAs are a more important metal reservoir than DLAs, in total comprising at least 6–22% of the metal budget at z ≈ 2.5.