Browsing by Author "Ledoux, C´edric"

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Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems
(2007-01-15) Fox, Andrew J.; Petitjean, Patrick; Ledoux, C´edric; et al.
Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z=2.1–3.1. Methods. We search for Ovi absorption and corresponding Si iv, Civ, and Nv in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering Ovi at S/N>10. We then use optical depth proﬁle comparisons and ionization modelling to investigate the properties, phase structure, and origin of the plasma. Results. We report twelve DLAs (nine intervening and three at <5 000 kms −1 from the QSO redshift) with detections of Ovi absorption. There are no clear Ovi non-detections, so the incidence of Ovi in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, Civ and Si iv are seen whenever data is available, and Nv is detected in 3 cases. Analysis of the line widths together with photoionization modelling suggests that two phases of DLA plasma exist: a hot, collisionally ionized phase (seen in broad Ovi components), and a warm, photoionized phase (seen just in narrow Civ and Si iv components). The presence of inﬂows and/or outﬂows is indicated by individual Ovi and Civ components displaced from the neutral gas (either blueshifted or redshifted) by up to 400 kms −1 . We ﬁnd tentative evidence (98% conﬁdence) for correlations between the DLA metallicity (measured in the neutral gas) and high-ion column density, and between the DLA metallicity and high-ion line width, as would be expected if supernova-driven galactic outﬂows rather than accretion produced the high ions. Using conservative ionization corrections, we ﬁnd lower limits to the total hydrogen column densities in the hot (Ovi-bearing) and warm (Civ-bearing) phases in the range logNHot H ii > 19.5 to > 21.1, and logNWarm H ii > 19.4 to > 20.9. On average, the hot and warm phases thus contain > ∼40% and > ∼20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the Ovi phase is ≈ 106 K and so fOvi = Ovi/O ≪ 0.2, the plasma can make a signiﬁcant contribution to the metal budget at high redshift. Additional searches for Ovi in Lyman Limit Systems (QSO absorbers with 17.0 < NHi < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z ≈ 2.
Metal-Enriched Plasma in Protogalactic Halos: A Survey of N V Absorption in High-z Damped & Sub-Damped Lyman-alpha Systems
(2009-01-01) Andrew, Fox; Prochaska, Jason X.; Ledoux, C´edric; et al.
We continue our recent work to characterize the plasma content of high-redshift damped and sub-damped Lyman-α systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey Nv absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67−2.3. The Nv and Civ component b-value distributions in DLAs are statistically similar, but the median b(Nv) of 18 kms −1 is narrower than the median b(Ovi) of ≈25 kms −1 . Some ≈20% of the Nv components have b<10 kms −1 and thus arise in warm, photoionized plasma at log (T/K)<4.92; local sources of ionizing radiation (as opposed to the extragalactic background) are required to keep the cloud sizes physically reasonable. The nature of the remaining ≈80% of (broad) Nv components is unclear; models of radiatively-cooling collisionally-ionized plasma at log (T/K)=5.2–5.4 are fairly successful in reproducing the observed integrated high-ion column density ratios and the component line widths, but we cannot rule out photoionization by local sources. Finally, we identify several unusual DLAs with extremely low metallicity (<0.01 solar) but strong high-ion absorption [logN(Nv)>14 or logN(Ovi)>14.2], which present challenges to either galactic inﬂow or outﬂow models.
VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems: Physical conditions in the neutral gas
(2005-06-01) Srianand, R.; Petitjean, Patrick; Ledoux, C´edric; et al.
We study the physical conditions in damped Lyman-α systems (DLAs), using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H2 by Ledoux et al. (2003). We use the column densities of H2 in diﬀerent rotational levels, together with those of C i, C i ∗, C i ∗∗, C ii ∗ and singly ionized atomic species to discuss the kinetic temperature, the density of hydrogen and the electronic density in the gas together with the ambient UV radiation ﬁeld. Detailed comparisons are made between the observed properties in DLAs, the interstellar medium (ISM) of the Galaxy, the large and small Magellanic clouds (LMC and SMC).The mean kinetic temperature of the gas corresponding to DLA subcomponents in which H2 absorption line is detected, derived from the ortho-to-para ratio (153±78 K), is higher than that measured in the ISM (77±17 K) and the Magellanic clouds (82±21 K). Typical pressure in these components (corresponding to T = 100−300 K and nH = 10−200 cm−3), measured using C i ﬁne-structure excitation, are higher than what is measured along ISM sightlines. This is consistent with the corresponding higher values for N(H2,J=2)/N(H2,J=0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation ﬁeld in the H2 bearing components is of the order of or slightly higher than the mean UV ﬁeld in the Galactic ISM. Determination of electron density in the gas with H2 and C i show the ionization rate is similar to that of a cold neutral medium (CNM) in a moderate radiation ﬁeld. This, together with the fact that we see H2 in 13-20% of the DLAs, can be used to conclude that DLAs at z > 1.9 could contribute as much as 50% star formation rate density seen in Lyman break galaxies (LBGs).C ii ∗ absorption line is detected in all the components where H2 absorption line is seen. The excitation of C ii in these systems is consistent with the physical parameters derived from the excitation of H2 and C i. We detect C ii ∗ in about 50% of the DLAs and therefore in a considerable fraction of DLAs that do not show H2. In part of the later systems, physical conditions could be similar to that in the CNM gas of the Galaxy. However, the absence of C i absorption line and the presence of Al iii absorption lines with a proﬁle similar to the proﬁles of singly ionized species suggest an appreciable contribution from warm (WNM) and/or partially ionized gas. The absence of H2, for the level of metallicity and dust depletion seen in these systems, are consistent with low densities (i.e nH 6 1 cm−3) for a radiation ﬁeld similar to the mean Galactic UV ﬁeld.