Abstract We present the detection of the average H2 absorption signal in the overall population of neutral gas absorption systems at z∼ 3 using composite absorption spectra built from the Sloan ...Digital Sky Survey-III damped Lyman α catalogue. We present a new technique to directly measure the H2 column density distribution function $f_{\rm H_2}(N)$ from the average H2 absorption signal. Assuming a power-law column density distribution, we obtain a slope $\beta = -1.29 \pm 0.06(\rm stat) \pm 0.10 (\rm sys)$ and an incidence rate of strong H2 absorptions with N(H2) ≳ 1018 cm−2 to be $4.0 \pm 0.5(\rm stat) \pm 1.0 (\rm sys)\, \hbox{ per cent}$ in H i absorption systems with N(H i) ≥1020 cm−2. Assuming the same inflexion point where $f_{\rm H_2}(N)$ steepens as at z = 0, we estimate that the cosmological density of H2 in the column density range $\log N(\rm H_2) ({\rm cm}^{-2})= 18{\text{--}}22$ is ${\sim } 15\hbox{ per cent}$ of the total. We find one order of magnitude higher H2 incident rate in a sub-sample of extremely strong damped Lyman α absorption systems (DLAs) $\log N(\rm{H\,\small {I}}) ({\rm cm}^{-2}) \ge 21.7$, which, together with the derived shape of $f_{\rm H_2}(N)$, suggests that the typical H i–H2 transition column density in DLAs is log N(H)(cm−2) ≳ 22.3 in agreement with theoretical expectations for the average (low) metallicity of DLAs at high-z.
We present the results of a search for damped Lyman-α (DLA) systems in the Sloan Digital Sky Survey II (SDSS), Data Release 7. We use a fully automatic procedure to identify DLAs and derive their ...column densities. The procedure is checked against the results of previous searches for DLAs in SDSS. We discuss the agreements and differences and show the robustness of our procedure. For each system, we obtain an accurate measurement of the absorber's redshift, the H I column density and the equivalent width of associated metal absorption lines, without any human intervention. We find 1426 absorbers with $2.15 < z < 5.2$ with log N(H I) ≥ 20, out of which 937 systems have log N(H I) ≥ 20.3. This is the largest DLA sample ever built, made available to the scientific community through the electronic version of this paper. In the course of the survey, we discovered the intervening DLA with highest H I column density known to date with log N(H I) = 22.0±0.1. This single system provides a strong constraint on the high-end of the N(H I) frequency distribution now measured with high accuracy. We show that the presence of a DLA at the blue end of a QSO spectrum can lead to important systematic errors and propose a method to avoid them. This has important consequences for the measurement of the cosmological mass density of neutral gas at z ~ 2.2 and therefore on our understanding of galaxy evolution over the past 10 billion years. We find a significant decrease of the cosmological mass density of neutral gas in DLAs, $\Omega_{\rm g}^{\rm DLA}$, from $z = 4$ to $z = 2.2$, consistent with the result of previous SDSS studies. However, and contrary to other SDSS studies, we find that $\Omega_{\rm g}^{\rm DLA}$(z = 2.2) is about twice the value at $z = 0$. This implies that $\Omega_{\rm g}^{\rm DLA}$ keeps decreasing at $z < 2.2$.
We present the results of a search for cold gas at high redshift along quasar lines of sight carried out without any a priori assumption on the neutral atomic-hydrogen content of the absorption-line ...systems. To do this, we systematically looked for neutral- carbon (C I) AA1560, 1656 transition lines in 41696 low-resolution quasar spectra from the SDSS-II - Data Release 7 - database. C I absorption lines should indeed probe the shielded gas in the neutral interstellar medium of galaxies more efficiently than traditional tracers such as neutral atomic-hydrogen (H I) damped Lyman-a (DLA) and/or Mg II systems. We show that, at equal amount of reddening, the 2175 A feature is weak compared to Galactic lines of sight. This is probably the consequence of current or past star formation in the vicinity of the C I systems. The C I-absorber sample presented here hence provides ideal targets for detailed studies of the dust composition and molecular species at high redshift.
Abstract
Starting from a summary of detection statistics of our recent X-shooter campaign, we review the major surveys, both space and ground based, for emission counterparts of high-redshift damped ...Ly α absorbers (DLAs) carried out since the first detection 25 yr ago. We show that the detection rates of all surveys are precisely reproduced by a simple model in which the metallicity and luminosity of the galaxy associated to the DLA follow a relation of the form, M
UV = −5 × (M/H + 0.3) − 20.8, and the DLA cross-section follows a relation of the form
$\sigma _{\small {DLA}} \propto L^{0.8}$
. Specifically, our spectroscopic campaign consists of 11 DLAs pre-selected based on their equivalent width of Si ii λ1526 to have a metallicity higher than Si/H > −1. The targets have been observed with the X-shooter spectrograph at the Very Large Telescope to search for emission lines around the quasars. We observe a high detection rate of 64 per cent (7/11), significantly higher than the typical ∼10 per cent for random, H i-selected DLA samples. We use the aforementioned model, to simulate the results of our survey together with a range of previous surveys: spectral stacking, direct imaging (using the ‘double DLA’ technique), long-slit spectroscopy, and integral field spectroscopy. Based on our model results, we are able to reconcile all results. Some tension is observed between model and data when looking at predictions of Ly α emission for individual targets. However, the object-to-object variations are most likely a result of the significant scatter in the underlying scaling relations as well as uncertainties in the amount of dust which affects the emission.
We present the first results from an ongoing survey for damped Lyman-α systems (DLAs) in the spectra of z > 2 quasars observed in the course of the Baryon Oscillation Spectroscopic Survey (BOSS), ...which is part of the Sloan Digital Sky Survey (SDSS) III. Our full (non-statistical) sample, based on Data Release 9, comprises 12 081 systems with log N(H i) ≥ 20, out of which 6839 have log N(H i) ≥ 20.3. This is the largest DLA sample ever compiled, superseding that from SDSS-II by a factor of seven. Using a statistical sub-sample and estimating systematics from realistic mock data, we probe the N(H i) distribution at ⟨z⟩ = 2.5. Contrary to what is generally believed, the distribution extends beyond 1022 cm-2 with a moderate slope of index ≈−3.5. This result matches the opacity-corrected distribution observed at z = 0 surprisingly well. The cosmological mass density of neutral gas in DLAs is found to be \hbox{$\omegagdla \approx 10^{-3}$}ΩgDLA≈10-3, evolving only mildly over the past 12 billion years.
Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 < z_{\rm abs} \le 4.2) Damped Lyman- \alpha systems (DLAs) capitalising on observations performed ...with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log N (H I) \ge 20 and z_{\rm abs} >1.8, which have data that include redshifted H sub(2) Lyman and/or Werner-band absorption lines. This sample of H I, H sub(2) and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H sub(2) could be completed so far, including all non-detections. Results. H sub(2) is detected in thirteen of the systems, which have molecular fractions of values between f \simeq 5\times10 super(- 7) and f \simeq 0.1, where f = 2 N (H sub(2)) /(2 N (H sub(2)) +N(H I)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N (H sub(2)) similar to 14.3, corresponding to log f <-5. We find that about 35% of the DLAs with metallicities relative to solar X/H \ge-1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log f >-4.5, while H sub(2) is detected-regardless of the molecular fraction-in \sim 50% of them. In contrast, only about 4% of the X/H <-1.3 DLAs have log f >-4.5. We show that the presence of H sub(2) does not strongly depend on the total neutral hydrogen column density, although the probability of finding log f >-4.5 is higher for log N (H I) \ge 20.8 than below this limit (19% and 7% respectively). The overall H sub(2) detection rate in log N (H I) \ge 20 DLAs is found to be about 16% (10% considering only log f >-4.5 detections) after correction for a slight bias towards large N (H I). There is a strong preference for H sub(2)-bearing DLAs to have significant depletion factors, X/Fe > 0.4. In addition, all H sub(2)-bearing DLAs have column densities of iron into dust grains larger than log N({\rm Fe}) _{\rm dust} similar to 14.7, and about 40% of the DLAs above this limit have detected H sub(2) lines with log f >-4.5. This demonstrates the importance of dust in governing the detectability of H sub(2) in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H sub(2)-bearing DLAs nor that of the molecular fraction in systems with H sub(2) detections over the redshift range 1.8 < z_{\rm abs} \le 3.
ABSTRACT
We present the detection of excited fine-structure energy levels of singly ionized silicon and neutral carbon associated with the proximate damped Lyman-α system at zabs = 2.811 towards Q ...0528−250. This absorber has an apparent relative velocity that is inconsistent with the Hubble flow indicating motion along the line-of-sight towards the quasar, i.e. zabs > zem. We measure the metallicity of the system to be Zn/H = −0.68 ± 0.02. Using the relative populations of the fine-structure levels of Si ii and C i, as well as the populations of H2 rotational levels, we constrain the physical conditions of the gas. We derive hydrogen number densities of $n_{\rm H}=190^{+70}_{-50}$ cm−3 and $260^{+30}_{-20}$ cm−3 in two velocity components where both C i and H2 are detected. Taking into account the kinetic temperature in each component, ∼150 K, we infer high values of thermal pressure in the cold neutral medium probed by the observations. The strengths of the UV field in Draine’s unit are $I_{\rm UV} = 10^{+5}_{-3}$ and $14^{+3}_{-3}$ in each of these two components, respectively. Such enhanced UV fluxes and thermal pressure compared to intervening DLAs are likely due to the proximity of the quasar. The typical size of the absorber is ∼104 au. Assuming the UV flux is dominated by the quasar, we constrain the distance between the quasar and the absorber to be ∼150−200 kpc. This favours a scenario where the absorption occurs in a companion galaxy located in the group where the quasar-host galaxy resides. This is in line with studies in emission that revealed the presence of several galaxies around the quasar.
A milestone of modern cosmology was the prediction and serendipitous discovery of the cosmic microwave background (CMB), the radiation leftover after decoupling from matter in the early evolutionary ...stages of the Universe. A prediction of the standard hot Big-Bang model is the linear increase with redshift of the black-body temperature of the CMB (TCMB). This radiation excites the rotational levels of some interstellar molecules, including carbon monoxide (CO), which can serve as cosmic thermometers. Using three new and two previously reported CO absorption-line systems detected in quasar spectra during a systematic survey carried out using VLT/UVES, we constrain the evolution of TCMB to z ~ 3. Combining our precise measurements with previous constraints, we obtain TCMB(z) = (2.725 ± 0.002) × (1 + z)1-β K with β = -0.007 ± 0.027, a more than two-fold improvement in precision. The measurements are consistent with the standard (i.e. adiabatic, β = 0) Big-Bang model and provide a strong constraint on the effective equation of state of decaying dark energy (i.e. weff = -0.996 ± 0.025). Based on observations carried out at the European Southern Observatory (ESO) using the Ultraviolet and Visual Echelle Spectrograph (UVES) at the Very Large Telescope (VLT, UT2-Kueyen) under Prgm. IDs 278.A-5062(A), 081.A-0334(B), 082.A-0544(A), and 083.A-0454(A).
Absorption lines from molecular hydrogen (H2) in the spectra of background sources are a powerful probe of the physical conditions in intervening cold neutral medium. At high redshift, z>2, H2 lines ...are conveniently shifted in the optical domain, allowing the use of ground-based telescopes to perform high-resolution spectroscopy, which is essential for a proper analysis of the cold gas. We describe recent observational progress, based on the development of efficient pre-selection techniques in low-resolution spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS). The next generation of spectrographs with high blue-throughput, such as CUBES, will certainly significantly boost the efficiency and outcome of follow-up observations. In this paper, we discuss high priority science cases for CUBES, building on recent H2 observations at high-z: probing the physical conditions in the cold phase of regular galaxies and outflowing gas from active galactic nucleus.
ABSTRACT
We discuss observations of C ii*/C ii ratios and cooling rates due to C ii 158μm emission in high-redshift intervening damped Lyman-α (DLA) systems towards quasars. We show that the observed ...bimodality in the C ii cooling rates actually reflects a bimodality in the C ii*/C ii−metallicity plane that can be naturally explained by phase segregation of the neutral medium, without invoking differences in star-formation scenarios. Assuming realistic distributions of the physical parameters to calculate the phase diagrams, we also reproduce qualitatively the metallicity dependence of this bimodality. We emphasize that high-z DLAs mostly probe low-metallicity gas ($Z\lesssim 0.1 \, \mathrm{Z}_{\odot }$), where heating is dominated by cosmic rays (and/or turbulence), and not by photoelectric heating. Therefore, even if the gas of DLA is predominantly cold (where the cooling is dominated by C ii), the excitation of C ii can be used to derive the cosmic ray ionization rate (and/or turbulent heating), but not the UV field, as was previously considered. Alternatively, if the gas in DLA is predominantly warm, C ii*/C ii can be used to constrain its number density. Finally, we also discuss the importance of the ionized medium, which, if also present along the line of sight, can significantly increase the average C ii*/C ii ratio.
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