Browsing by Author "Ledoux, C."

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C iv absorption in damped and sub-damped Lyman-alpha systems: Correlations with metallicity and implications for galactic winds at z~2-3
(2007-07-27) Fox, Andrew J.; Ledoux, C.; Petitjean, Patrick; et al.
We present a study of Civ absorption in a sample of 63 damped Lyman-α (DLA) systems and 11 sub-DLAs in the redshift range 1.75
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.
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.
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, Patrick
We 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.
Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 A* dust feature at z=1.64
(2009-06-01) Noterdaeme, P.; Ledoux, C.; Srianand, R.
We present the detection of carbon monoxide molecules (CO) at z = 1.6408 towards the quasar SDSS J160457.50+220300.5 using the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. CO absorption is detected in at least two components in the ﬁrst six A-X bands and one d-X(5-0) inter-band system. This is the second detection of this kind along a quasar line of sight. The CO absorption proﬁles are well modelled assuming a rotational excitation of CO in the range 6 < Tex < 16 K, which is consistent with or higher than the temperature of the Cosmic Microwave Background Radiation at this redshift. We derive a total CO column density of N(CO) = 4 × 1014 cm−2 . The measured column densities of S i, Mg i, Zn ii, Fe ii and Si ii indicate a dust depletion pattern typical of cold gas in the Galactic disc. The background quasar spectrum is signiﬁcantly reddened (u−K ∼ 4.5 mag) and presents a pronounced 2175 Å dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars we ﬁnd the chance probability for this feature to be spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well ﬁtted with a QSO composite spectrum reddened with a Large Magellanic Cloud supershell extinction law at the redshift of the absorber. It is noticeable that this quasar is absent from the colour-selected SDSS quasar sample. This demonstrates our current view of the Universe may be biased against dusty sightlines. These direct observations of carbonaceous molecules and dust open up the possibility of studying physical conditions and chemistry of diﬀuse molecular gas in high redshift galaxies.
Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 A* dust feature at z=1.64
(2009-06-01) Noterdaeme, P.; Ledoux, C.; Srianand, R.; et al.
We present the detection of carbon monoxide molecules (CO) at z = 1.6408 towards the quasar SDSS J160457.50+220300.5 using the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. CO absorption is detected in at least two components in the ﬁrst six A-X bands and one d-X(5-0) inter-band system. This is the second detection of this kind along a quasar line of sight. The CO absorption proﬁles are well modelled assuming a rotational excitation of CO in the range 6 < Tex < 16 K, which is consistent with or higher than the temperature of the Cosmic Microwave Background Radiation at this redshift. We derive a total CO column density of N(CO) = 4 × 1014 cm−2 . The measured column densities of S i, Mg i, Zn ii, Fe ii and Si ii indicate a dust depletion pattern typical of cold gas in the Galactic disc. The background quasar spectrum is signiﬁcantly reddened (u−K ∼ 4.5 mag) and presents a pronounced 2175 Å dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars we ﬁnd the chance probability for this feature to be spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well ﬁtted with a QSO composite spectrum reddened with a Large Magellanic Cloud supershell extinction law at the redshift of the absorber. It is noticeable that this quasar is absent from the colour-selected SDSS quasar sample. This demonstrates our current view of the Universe may be biased against dusty sightlines. These direct observations of carbonaceous molecules and dust open up the possibility of studying physical co
Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions
(2007-07-30) Noterdaeme, P.; Ledoux, C.; Petitjean, Patrick; et al.
Aims.We probe the physical conditions in high-redshift damped Lyman-α systems (DLAs) using the observed molecular fraction and the rotational excitation of molecular hydrogen. Methods. We search for Lyman- and Werner-band absorption lines of molecular hydrogen in the VLT/UVES spectra of background QSOs at the redshift of known DLAs. Results. We report two new detections of molecular hydrogen in the systems at zabs = 2.402 and 1.989 toward, respectively, HE 0027−1836 and HE 2318−1107, discovered in the course of the Hamburg-ESO DLA survey. We also present a detailed analysis of our recent H2 detection toward Q2343+125. All three systems have low molecular fractions, log f ≤ −4, with f = 2N(H2)/(2N(H2) + N(H)). Only one such H2 system was known previously. Two of them (toward Q2343+125 and HE 2318−1107) have high-metallicities, [X/H] > −1, whereas the DLA toward HE 0027−1836 is the system with the lowest metallicity ([Zn/H] = −1.63) among known H2-bearing DLAs. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Ni in the three systems are found to be very similar to what is observed in diﬀuse gas of the Galactic halo. Molecular hydrogen absorption from rotational levels up to J = 5 is observed in a single well-deﬁned component toward HE 0027−1836. We show that the width (Doppler parameter) of the H2 lines increases with increasing J and that the kinetic energy derived from the Doppler parameter is linearly dependent on the relative energy of the rotational levels. There is however no velocity shift between lines from diﬀerent rotational levels. The excitation temperature is found to be 90 K for J = 0 to J = 2 and ∼500 K for higher J levels. Single isothermal PDR models fail to reproduce the observed rotational excitations. A two-component model is needed: one component of low density (∼50 cm−3 ) with weak illumination (χ = 1) to explain the J ≤ 2 rotational levels and another of high density (∼500 cm−3 ) with strong illumination (χ = 30) for J ≥ 3 levels. However, the juxtaposition of these two PDR components may be ad-hoc and the multicomponent structure could result either from turbulent dissipation or C-shocks.
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.
Metallicity as a criterion to select H2 bearing damped lyman-alpha systems
(2006-07-17) Petitjean, Patrick; Ledoux, C.; Noterdaeme, P.; et al.
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 gas
Molecular hydrogen in a damped lyman-alpha system at Zabs = 4.224
(2006-02-09) Ledoux, C.; Petitjean, Patrick; Srianand, R.
We present the direct detection of molecular hydrogen at the highest redshift known today (zabs = 4.224) in a Damped Lyman-α (DLA) system toward the quasar PSS J 1443+2724. This absorber is remarkable for having one of the highest metallicities amongst DLA systems at zabs > 3, with a measured iron abundance relative to Solar of −1.12 ± 0.10. We provide for the ﬁrst time in this system accurate measurements of Ni, Mg ii, S ii and Ar i column densities. The sulfur and nitrogen abundances relative to Solar, −0.63 ± 0.10 and −1.38 ± 0.10 respectively, correspond exactly to the primary nitrogen production plateau. H2 absorption lines are detected in four diﬀerent rotational levels (J = 0, 1, 2 and 3) of the vibrational ground-state in three velocity components with total column densities of logN(H2) = 17.67, 17.97, 17.48 and 17.26 respectively. The J = 4 level is tentatively detected in the strongest component with logN(H2) ∼ 14. The mean molecular fraction is log f = −2.38 ± 0.13, with f = 2N(H2)/(2N(H2) + N(Hi)). We also measure logN(HD)/N(H2) < −4.2. The excitation temperatures T01 for the two main components of the system are 96 and 136 K respectively. We argue that the absorbing galaxy, whose star formation activity must have started at least 2− 5× 108 yrs before z = 4.224, is in a quiescent state at the time of observation. The density of the gas is small, nH ≤ 50 cm−3 , and the temperature is of the order of T ∼ 90 − 180 K. The high excitation of neutral carbon in one of the components can be explained if the temperature of the Cosmic Microwave Background Radiation has the valueexpected at the absorber redshift, T = 14.2 K. These observations demonstrate the feasibility to study H2 at the highest redshifts provided high enough spectral resolution and good S/N ratio are achieved.
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.
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.
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.
Uncovering strong MgII absorbing galaxies: Imaging below the Lyman limit
(2009-08-01) Christensen, L.; Petitjean, Patrick; Ledoux, C.
Context. The nature of the galaxies that give rise to absorption lines, such as damped Lyman-α systems (DLAs) or strong Mg ii lines, in quasar spectra is diﬃcult to investigate in emission. These galaxies can be very faint and located close to the lines of sight of the much brighter background quasars. Aims. Taking advantage of the total absorption of the QSO light bluewards of the Lyman limit of two DLAs at z > 3.4, we look for the continuum emission from intervening galaxies at z ≈ 2 that are identiﬁed via strong metal absorption lines. The Mg ii absorbers have equivalent width large enough to be potential DLA systems. Methods. Deep images are obtained with the FOcal Reducer and Spectrograph (FORS1) on the Very Large Telescope for the ﬁelds towards SDSS J110855+120953 and SDSS J140850+020522. These quasars have Mg ii absorption lines at z = 1.87 (Wr(Mg ii) = 2.46 Å) and z = 1.98 (Wr(Mg ii) = 1.89 Å), respectively, and each QSO has two intervening higher redshift DLAs at z > 3. The U and R bands of FORS1 lie blue and redwards of the Lyman limit of the background DLAs, allowing us to search for emission from the foreground galaxies directly along the lines of sight to the QSOs. Results. No galaxies are found close to the sight line of the QSO to a point source limit of UAB ∼ 28.0. In both ﬁelds, the closest objects lie at an impact parameter of ∼5′′ corresponding to ∼40 kpc in projection at z = 2, and have typical colours of star forming galaxies at that redshift. However, the currently available data do not allow us to conﬁrm if the galaxies lie at the same redshifts as the absorption systems. A more extended structure is visible in the SDSS J14085+020522 ﬁeld at an impact parameter of 0. ′′ 8 or 7 kpc. If these objects are at z ≈ 2 their luminosities are 0.03–0.04 L∗ in both ﬁelds. The star formation rates estimated from the UV ﬂux are 0.5–0.6 M⊙ yr −1 , while the SFRs are half these values if the U band ﬂux is due to Lyα emission alone. Conclusions. The non-detection of galaxies near to the line of sight is most likely explained by low metallicities and luminosities of the Mg ii galaxies. Alternatively, the Mg ii clouds are part of extended halos or in outﬂows from low-metallicity galaxies.
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.
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.