Celine Peroux

The evolution of ΩHI and the epoch of formation of damped Lyman α absorbers

We present a study of the evolution of the column density distribution, f(N, z), and total neutral hydrogen mass in high column density quasar absorbers using candidates from a recent high-redshift survey for damped Lyman α (DLA) and Lyman-limit system (LLS) absorbers. The observed number of LLS [N(H_i) >1.6 × 10^(17) atom cm^(−2)] is used to constrain f(N, z) below the classical DLA definition of 2 × 10^(20) atom cm^(−2). The evolution of the number density of LLS is consistent with our previous work but steeper than previously published work of other authors. At z= 5, the number density of Lyman-limit systems per unit redshift is ∼5, implying that these systems are a major source of ultraviolet (UV) opacity in the high-redshift Universe. The joint LLS–DLA analysis shows unambiguously that f(N, z) deviates significantly from a single power law and that a Γ-law distribution of the form f(N,z) = (f_*/N_*)(N/N_*)^(−β)exp(−N/N_*) provides a better description of the observations. These results are used to determine the amount of neutral gas contained in DLAs and in systems with lower column density. Whilst in the redshift range 2–3.5, ∼90 per cent of the neutral H i mass is in DLAs, we find that at z > 3.5 this fraction drops to only 55 per cent and that the remaining ‘missing’ mass fraction of the neutral gas lies in sub-DLAs with N(H i) 10^(19)–2 × 10^(20) atom cm^(−2). The characteristic column density, N_*, changes from 1.6 × 10^(21) atom cm^(−2) at z 3.5, supporting a picture where at z > 3.5, we are directly observing the formation of high column density neutral hydrogen DLA systems from lower column density units. Moreover, since current metallicity studies of DLA systems focus on the higher column density systems they may be giving a biased or incomplete view of global galactic chemical evolution at z > 3. After correcting the observed mass in H i for the ‘missing’ neutral gas the comoving mass density now shows no evidence for a decrease above z= 2.

New perspectives on strong z≃ 0.5 Mg ii absorbers: are halo mass and equivalent width anticorrelated?

We measure the mean halo mass of z ≃ 0.5 Mg II absorbers using the cross-correlation (over comoving scales 0.05-13 h -1 Mpc) between 1806 Mg n quasar absorption systems and ∼250 000 luminous red galaxies (LRGs), both selected from the Sloan Digital Sky Survey Data Release 3. The Mg II systems have λ2796 rest-frame equivalent widths W λ2796 r ? 0.3 A. From the ratio of the Mg II-LRG cross-correlation to the LRG-LRG auto-correlation, we find that the bias ratio between Mg II absorbers and LRGs is b Mg II /b LRG = 0.65 ± 0.08, which implies that the absorber host galaxies have a mean halo mass ∼20-40 times smaller than that of the LRGs; the Mg II absorbers have haloes of mean mass = 11.94 ± 0.31 (stat) +0.24 -0.25 (sys). We demonstrate that this statistical technique, which does not require any spectroscopic follow-up, does not suffer from contaminants such as stars or foreground and background galaxies. Finally, we find that the absorber halo mass is anticorrelated with the equivalent width. If Mg II absorbers were virialized in galaxy haloes, a positive M h -W λ2796 r correlation would have been observed since W λ2796 r is a direct measure of the velocity spread of the Mg II subcomponents. Thus, our results demonstrate that the individual clouds of a Mg II system are not virialized in the gaseous haloes of the host galaxies. We review past results in the literature on the statistics of Mg II absorbers and find that they too require a M h -W λ2796 r anticorrelation. When combined with measurements of the equivalent width distribution (d 2 N/d z /dW r ), the M h -W λ2796 r anticorrelation naturally explains why absorbers with W λ2796 r ? 2 A are not seen at large impact parameters. We interpret the M h -W λ2796 r anticorrelation within the starburst scenario where strong Mg II absorbers are produced by supernovae-driven winds.

A homogeneous sample of sub-damped Lyman α systems – II. Statistical, kinematic and chemical properties

Damped Lyman a systems (DLAs), with N(HI) > 2 x 10 2 0 atom cm - 2 , observed in the spectra of quasars have allowed us to quantify the chemical content of the Universe over cosmological scales. Such studies can be extended to lower column densities, in the sub-DLA range [10 1 9 3.5. In this paper, we use a homogeneous sample of sub-DLAs from the European Southern Oberservatory (ESO) Ultraviolet-Visual Echelle Spectrograph (UVES) archives presented in Paper I, to determine observationally for the first time the shape of the column density distribution, f(N), down to N(H I) = 10 1 9 atom cm - 2 . The results are in good agreement with the predictions from Peroux et al. We also present the kinematic and clustering properties of this survey of sub-DLAs, which appear to be marginally different from the DLAs. We compare low- and high-ionization transition widths and find that the properties of the sub-DLAs span roughly the parameter space of DLAs. We also find hints of an increase of metallicity in systems with larger velocity widths in the metal lines, although the statistical significance of this result is low. Then we analyse the chemical content of this sample in conjunction with a compilation of abundances from 72 DLAs taken from the literature. As previously reported, the individual metallicities traced by [Fe/H] of these systems evolve mildly with redshift. Moreover, we analyse the H I column-density-weighted mean abundance, which is believed to be an indicator of the metallicity of the Universe. Although the number statistics is limited in the current sample, the results suggest a slightly stronger evolution of this quantity in the sub-DLA range. The effect is predominant at z < 2 and most of the evolution observed lies in this redshift range. Observational arguments support the hypothesis that the evolution we probe in the sub-DLA range is not due to their lower dust content. Therefore, these systems might be associated with a different class of objects, which better trace the overall chemical evolution of the Universe. Finally, we present abundance ratios of [Si/Fe], [O/Fe], [C/Fe] and [Al/Fe] for sub-DLAs in conjunction with DLA measurements from the literature. The elemental ratios in sub-DLAs are comparable with those from DLAs. It is difficult to decipher whether the observed values are the effect of nucleosynthesis or are due to differential dust depletion. The metallicities are compared with two different sets of models of galaxy evolution in order to provide constraints on the morphology of quasar absorbers.

The Role of Sub-Damped Lyα Absorbers in the Cosmic Evolution of Metals

Observations of low mean metallicity of damped Lyα (DLA) quasar absorbers at all redshifts studied appear to contradict the predictions for the global mean interstellar metallicity in galaxies from cosmic chemical evolution models. On the other hand, a number of metal-rich sub-DLA systems have been identified recently, and the fraction of metal-rich sub-DLAs appears to be considerably larger than that of metal-rich DLAs, especially at z < 1.5. In view of this, here we investigate the evolution of metallicity in sub-DLAs. We find that the mean Zn metallicity of the observed sub-DLAs may be higher than that of the observed DLAs, especially at low redshifts, reaching a near-solar level at z 1. This trend does not appear to be an artifact of sample selection, the use of Zn, the use of NH i weighting, or observational sensitivity. While a bias against very low metallicity could be present in the sub-DLA sample in some situations, this cannot explain the difference between the DLA and sub-DLA metallicities at low z. The primary reason for the difference between the DLAs and sub-DLAs appears to be the dearth of metal-rich DLAs. We estimate the sub-DLA contribution to the total metal budget using measures of their metallicity and comoving gas density. These calculations suggest that at z 1, the contribution of sub-DLAs to the total metal budget may be several times that of DLAs. At higher redshifts also, there are indications that the sub-DLAs may contribute significantly to the cosmic metal budget.

Absorption Systems in the Spectra of 66 z ≳ 4 Quasars

We present high signal-to-noise, ~5 A resolution (FWHM) spectra of 66 z ≳ 4 bright quasars obtained with the 4 m Cerro Tololo Inter-American Observatory and 4.2 m William Hershel telescopes. The primary goal of these observations was to undertake a new survey for intervening absorption systems detected in the spectra of background quasars. We look for both Lyman-limit systems (column densities N_(H I) ≥ 1.6 × 10^(17) atoms cm^(-2)) and damped Lyα systems (column densities N_(H I) ≥ 2 × 10^(20) atoms cm^(-2)). This work resulted in the discovery of 49 Lyman-limit systems, 15 of which are within 3000 km s^(-1) of the quasar emission and thus might be associated with the quasar itself, 26 new damped Lyα absorption candidates, 15 of which have z > 3.5, and numerous metal absorption systems. In addition, 10 of the quasars presented here exhibit intrinsic broad absorption lines.

The dust obscuration bias in damped Lyman

We present a new study of the effects of quasar obscuration on the statistics of Damped Ly α (DLA) systems. We show that the extinction of any Galactic or extragalactic H i region, Aλ, increases linearly with the column density of zinc, with a turning point ∂Aλ/∂(logNZn) = 1, above which background sources are suddenly obscured. We derive a relation Aλ = Aλ(NH,Z,z) between the extinction of a DLA system and its H i column density, NH, metallicity, Z, fraction of iron in dust, fFe(Z), and redshift, z. From this relation we estimate the fraction of DLA systems missed as a consequence of their own extinction in magnitude-limited surveys. We derive a method for recovering the true frequency distributions of NH and Z in DLAs, fNH and fZ, using the biased distributions measured in the redshift range where the observations have sufficient statistics (1.8 ≤ z ≤ 3). By applying our method we find that the well-known empirical thresholds of DLA column densities, NZn < 10 13.1 atoms cm −2 and NHi < 10 22 atoms cm −2 , can be successfully explained in terms of the obscuration effect without tuning of the local dust parameters. The obscuration has a modest effect on the distribution of quasar apparent magnitudes, but plays an important role in shaping the statistical distributions of DLAs. The exact estimate of the bias is still limited by the paucity of the data (� 40 zinc measurements at 1.8 ≤ z ≤ 3). We find that the fraction of DLAs missed as a consequence of obscuration is ∼30% to 50%, consistent with the results of surveys of radio-selected quasars. By modelling the metallicity distribution with a Schechter function we find that the mean metallicity can be ∼ 5t o 6 times higher than the value commonly reported for DLAs at z ∼ 2.3.

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The metallicity in portions of high-redshift galaxies has been successfully measured thanks to the gas observed in absorption in the spectra of quasars, in the Damped Lyman-α systems (DLAs). Surprisingly, the global mean metallicity derived from DLAs is about 1/10th solar at 0 < z < 4 leading to the so-called “missing-metals problem”. In this paper, we present high-resolution observations of a sub-DLA system at zabs = 0.716 with super-solar metallicity toward SDSS J1323−0021. This is the highest metallicity intervening high-H i quasar absorber currently known, and is only the second super-solar such absorber known to date. We provide a detailed study of this unique object from VLT/UVES spectroscopy. We derive [Zn/H] =+ 0.61, [Fe/H] = −0.51, [Cr/H] = < −0.53, [Mn/H] = −0.37, and [Ti/H] = −0.61. Observations and photoionisation models using the CLOUDY software confirm that the gas in this sub-DLA is predominantly neutral and that the abundance pattern is probably significantly different from a Solar pattern. Fe/Zn and Ti/Zn vary among the main velocity components by factors of ∼ 3a nd ∼35, respectively, indicating non-uniform dust depletion. Mn/Fe is super-solar in almost all components, and varies by a factor of ∼3 among the dominant components. It would be interesting to observe more sub-DLA systems and determine whether they might contribute significantly toward the cosmic budget of metals.

systems

Damped Lyman a absorbers (DLAs), seen in absorption against a background quasar, provide the most detailed probes available of element abundances in the Universe over >90 per cent of its age. DLAs can be used to observationally measure the global mean metallicity in the Universe and its evolution with time. Paradoxically, these observations are more difficult at lower redshifts, where the absorber rest-frame ultraviolet (UV) spectra are cut-off due to the atmospheric absorption. We present here high-resolution Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph observations of several elements contained in three DLAs and one sub-DLA with 0.6 < z abs < 0.9. We detect Mg I Mg II, Fen, Zn II, Cr II, Mn II, Ti II and Ca II. Our observations more than double the high-resolution sample of [Zn/H] at z < 1. We also report the discovery of three metal-rich systems, whereas most previous measurements show low N HI -weighted mean metallicity projecting to about 1/6th solar level at z = 0. We derive [Zn/H] = -0.11 ± 0.04 at z abs = 0.725, [Zn/H] = -0.54 ± 0.20 at z abs = 0.740 and [Zn/H] = -0.49 ± 0.22 at z abs = 0.652, plus one additional upper limit ([Zn/H] < -0.36 at z abs = 0.842). These measurements confirm the existence of quasar absorbers with relatively high metallicities based on abundance estimates free from the effect of dust depletion. Possible implications of these results for the metallicity of neutral gas phase in the past ≈8 Gyr are presented and compared with models.

The most metal-rich intervening quasar absorber known

We present chemical abundance measurements from high resolution observations of 7 subdamped Lyman-� absorbers and 1 damped Lyman-� system at z < 1.5. Three of these objects have high metallicity, with near or super-solar Zn abundance. Grids of Cloudy models for each system were constructed to look for possible ionization effects in these systems. For the systems in which we could constrain the ionization parameter, we find that the ionization corrections as predicted by the Cloudy models are generally small and within the typical error bars (� 0.15 dex), in general agreement with previous studies. The Al III to Al II ratio for these and other absorbers from the literature are compared, and we find that while the subDLAs have a larger scatter in the Al III to Al II ratios than the DLAs, there appears to be little correlation between the ratio and NH I. The relationship between the metallicity and the velocity width of the profile for these systems is investigat ed. We show that the sub-DLAs that have been observed to date follow a similar trend as DLA absorbers, with the more metal rich systems exhibiting large velocity widths. We also find that t he systems at the upper edge of this relationship with high metallicities and large velocity wi dths are more likely to be sub-DLAs than DLA absorbers, perhaps implying that the sub-DLA absorbers are more representative of massive galaxies.

Metal-rich damped/subdamped Lyman α quasar absorbers at z < 1★

We present arguments based on the measured abundances in individual damped Lyman alpha systems (DLAs) and sub-damped Lyman alpha systems (sub-DLAs), and also the average abundances inferred in large samples of QSO absorption line systems, to suggest that the amount of dust in intervening QSO absorbers is small and is not responsible for missing many QSOs in magnitude limited QSO surveys. While we can not totally rule out a bimodal dust distribution with a population of very dusty, metal rich, absorbers which push the background QSOs below the observational threshold of current optical spectroscopic studies, based upon the current samples it appears that the metallicity in QSO absorbers decreases with increase in H I column densities beyond 10^{19} cm^{-2}. Thus the sub-DLA population is more metal rich than the DLAs, a trend which may possibly extend to the non-damped Lyman limit systems (NDLLS). Based on the recently discovered mass-metallicity relation for galaxies, we suggest that most sub-DLAs and possibly NDLLS, are associated with massive spiral/elliptical galaxies while most DLAs are associated with low mass galaxies. The sub-DLA galaxies will then contribute a larger fraction of total mass (stellar and ISM) and therefore metals, to the cosmic budget, specially at low redshifts, as compared to the DLAs.