Damped Lyman-α Absorbers Research Articles

New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-α absorbers

We report the detections of molecular hydrogen (H2), vibrationally-excited H2 (H2∗), and neutral atomic carbon (C I), an efficient tracer of molecular gas, in two new afterglow spectra of GRBs 181020A (z = 2.938) and 190114A (z = 3.376), observed with X-shooter at the Very Large Telescope (VLT). Both host-galaxy absorption systems are characterized by strong damped Lyman-α absorbers (DLAs) and substantial amounts of molecular hydrogen with logN(H I, H2) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and logN(H I, H2) = 22.15 ± 0.05, 19.44 ± 0.04 (GRB 190114A). The DLA metallicites, depletion levels, and dust extinctions are within the typical regimes probed by GRBs with [Zn/H] = −1.57 ± 0.06, [Zn/Fe] = 0.67 ± 0.03, and AV = 0.27 ± 0.02 mag (GRB 181020A) and [Zn/H] = −1.23 ± 0.07, [Zn/Fe] = 1.06 ± 0.08, and AV = 0.36 ± 0.02 mag (GRB 190114A). In addition, we examine the molecular gas content of all known H2-bearing GRB-DLAs and explore the physical conditions and characteristics required to simultaneously probe C I and H2∗. We confirm that H2 is detected in all C I- and H2∗-bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H2 absorbers. We find that a large molecular fraction of fH2 ≳ 10−3 is required for C I to be detected. The defining characteristic for H2∗ to be present is less clear, though a large H2 column density is an essential factor. We also find that the observed line profiles of the molecular-gas tracers are kinematically “cold”, with small velocity offsets of δv < 20 km s−1 from the bulk of the neutral absorbing gas. We then derive the H2 excitation temperatures of the molecular gas and find that they are relatively low with Tex ≈ 100−300 K, however, there could be evidence of warmer components populating the high-J H2 levels in GRBs 181020A and 190114A. Finally, we demonstrate that even though the X-shooter GRB afterglow campaign has been successful in recovering several H2-bearing GRB-host absorbers, this sample is still hampered by a significant dust bias excluding the most dust-obscured H2 absorbers from identification. C I and H2∗ could open a potential route to identify molecular gas even in low-metallicity or highly dust-obscured bursts, though they are only efficient tracers for the most H2-rich GRB-host absorption systems.

Gaia-assisted selection of a quasar reddened by dust in an extremely strong damped Lyman-α absorber at z = 2.226

Damped Lyman-α absorbers (DLAs) as a class of quasi-stellar object (QSO) absorption-line systems are currently our most important source of detailed information on the cosmic chemical evolution of galaxies. However, the degree to which this information is biased by dust remains to be understood. One strategy is to specifically search for QSOs that are reddened by metal-rich and dusty foreground absorbers. In this Letter, we present the discovery of a z = 2.60 QSO that is strongly reddened by dust in an intervening extremely strong DLA at z = 2.226. This QSO was identified through a novel selection that combined the astrometric measurements from ESA’s Gaia satellite with extisting optical and near- to mid-infrared photometry. We infer a total neutral atomic-hydrogen column density of log N(H I) = 21.95 ± 0.15 and a lower limit on the gas-phase metallicity of [Zn/H] > −0.96. This DLA is also remarkable in that it exhibits shielded neutral gas that is visible in C I, and it shows tentative detections of CO molecular bands. The spectral energy distribution of the QSO is well accounted for by a spectral energy distribution that is normal for a QSO and is reddened by dust from a DLA with 10% solar metallicity, a dust extinction of AV = 0.82 ± 0.02 mag, and an extinction curve that resembles that of the Large Magellanic Cloud, including the characteristic 2175 Å extinction feature. Such QSO absorption-line systems have been shown to be very rare in previous surveys, which have mostly revealed sight-lines with low extinction. The present case therefore suggests that previous samples have under-represented the fraction of dusty absorbers. Building a complete sample of such systems is required to assess the significance of this effect.

Dust-depletion sequences in damped Lyman-α absorbers

Metals in the interstellar medium (ISM) of essentially all types of galaxies are observed to be depleted compared to the expected values. The depletion is most likely due to dust condensation in, for example, cold molecular clouds and various circumstellar and interstellar environments. The relative observed metal abundances should thereby reflect the composition of the ISM dust components. We aim at identifying the most dominant dust species or types, including silicate and iron oxide grains present in the ISM, using recent observations of dust depletion of galaxies at various evolutionary stages. We use the observed elemental abundances in dust of several metals (O, S, Si, Mg, and Fe) in different environments, considering systems with different metallicities and dust content, namely damped Lyman-α absorbers (DLAs) towards quasars and the Galaxy. We derive a possible dust composition by computationally finding the statistically expected elemental abundances in dust assuming a set of key dust species with the iron content as a free parameter. Carbonaceous dust is not considered in the present study. Metallic iron (likely in the form of inclusions in silicate grains) and iron oxides are important components of the mass composition of carbon-free dust. The latter make up a significant mass fraction (~1∕4 in some cases) of the oxygen-bearing dust and there are good reasons to believe that metallic iron constitutes a similar mass fraction of dust. Wüstite (FeO) could be a simple explanation for the depletion of iron and oxygen because it is easily formed. There appears to be no silicate species clearly dominating the silicate mass, but rather a mix of iron-poor as well as iron-rich olivine and pyroxene. To what extent sulphur depletion is due to sulfides remains unclear. In general, there seems to be little evolution of the dust composition (not considering carbonaceous dust) from low-metallicity systems to the Galaxy.

Dissecting cold gas in a high-redshift galaxy using a lensed background quasar

We present a study of cold gas absorption from a damped Lyman-α absorber (DLA) at redshift zabs = 1.946 toward two lensed images of the quasar J144254.78+405535.5 at redshift zQSO = 2.590. The physical separation of the two lines of sight at the absorber redshift is dabs = 0.7 kpc according to our lens model. We observe absorption lines from neutral carbon and H2 along both lines of sight, indicating that cold gas is present on scales larger than dabs. We measure the column densities of H I to be log N(HI) = 20.27 ± 0.02 and 20.34 ± 0.05 and those of H2 to be log N(H2) = 19.7 ± 0.1 and 19.9 ± 0.2. The metallicity inferred from sulphur is consistent with solar metallicity for both sightlines: [S/H]A = 0.0 ± 0.1 and [S/H]B = −0.1 ± 0.1. Based on the excitation of low rotational levels of H2, we constrain the temperature of the cold gas phase to be T = 109 ± 20 and T = 89 ± 25 K for the two lines of sight. From the relative excitation of fine-structure levels of C I, we constrain the hydrogen volumetric densities to lie in the range of 40 − 110 cm−3. Based on the ratio of observed column density and volumetric density, we infer the average individual “cloud” size along the line of sight to be l ≈ 0.1 pc. Using the transverse line-of-sight separation of 0.7 kpc together with the individual cloud size, we are able to place an upper limit to the volume filling factor of cold gas of fvol < 0.1%. Nonetheless, the projected covering fraction of cold gas must be large (close to unity) over scales of a few kpc in order to explain the presence of cold gas in both lines of sight. Compared to the typical extent of DLAs (∼10 − 30 kpc), this is consistent with the relative incidence rate of C I absorbers and DLAs.

Unidentified quasars among stationary objects from Gaia DR2

Here we apply a technique selecting quasar candidates purely as sources with zero proper motions in the Gaia data release 2 (DR2). We demonstrate that this approach is highly efficient toward high Galactic latitudes with ≲25% contamination from stellar sources. Such a selection technique offers very pure sample completeness, since all cosmological point sources are selected regardless of their intrinsic spectral properties within the limiting magnitude of Gaia. We carry out a pilot-study, defining a sample compiled by including all Gaia-DR2 sources within one degree of the north Galactic pole (NGP) selected to have proper motions consistent with zero within 2σ uncertainty. By cross-matching the sample to the optical Sloan Digital Sky Survey (SDSS) and the mid-infrared (MIR) AllWISE photometric catalogues, we investigate the colours of each of our sources. We determine the efficiency of our selection by comparison with previously spectroscopically confirmed quasars. The majority of the zero-proper-motion sources selected here have optical to MIR colours consistent with known quasars. The remaining population may be contaminating stellar sources, but some may also be quasars with colours similar to stars. Spectroscopic follow-up of the zero-proper-motion sources is needed to unveil such a hitherto hidden quasar population. This approach has the potential to allow substantial progress on many important questions concerning quasars, such as determining the fraction of dust-obscured quasars, the fraction of broad absorption line (BAL) quasars, and the metallicity distribution of damped Lyman-α absorbers. The technique could also potentially reveal new types of quasars or even new classes of cosmological point sources.

Metals and dust in the neutral ISM: the Galaxy, Magellanic Clouds, and damped Lyman-α absorbers

Context. The presence of dust in the neutral interstellar medium (ISM) dramatically changes the metal abundances that we measure. Understanding the metal content in the neutral ISM, and a direct comparison between different environments, has been hampered to date because of the degeneracy to the observed ISM abundances caused by the effects of metallicity, the presence of dust, and nucleosynthesis. Aims. We study the metal and dust content in the neutral ISM consistently in different environments, and assess the universality of recently discovered sequences of relative abundances. We also intend to assess the validity of [Zn/Fe] as a tracer of dust in the ISM. This has recently been cast into doubt based on observations of stellar abundances, and needs to be addressed before we can safely use it to study the ISM. Methods. In this letter we present a simple comparison of relative abundances observed in the neutral ISM in the Galaxy, the Magellanic Clouds, and damped Lyman-α absorbers (DLAs). The main novelty in this comparison is the inclusion of the Magellanic Clouds. Results. The same sequences of relative abundances are valid for the Galaxy, Magellanic Clouds, and DLAs. These sequences are driven by the presence of dust in the ISM and seem “universal”. Conclusions. The metal and dust properties in the neutral ISM appear to follow a similar behaviour in different environments. This suggests that a dominant fraction of the dust budget is built up from grain growth in the ISM depending of the physical conditions and regardless of the star formation history of the system. In addition, the DLA gas behaves like the neutral ISM, at least from a chemical point of view. Finally, despite the deviations in [Zn/Fe] observed in stellar abundances, [Zn/Fe] is a robust dust tracer in the ISM of different environments, from the Galaxy to DLAs.

Multiple origins for the DLA at zabs = 0.313 toward PKS 1127–145 indicated by a complex dust depletion pattern of Ca, Ti, and Mn

Aims. We aim to investigate the dust depletion properties of optically thick gas in and around galaxies and its origin we study in detail the dust depletion patterns of Ti, Mn, and Ca in the multi-component damped Lymanα (DLA) absorber at zabs = 0.313 toward the quasar PKS 1127−145. Methods. We performed a detailed spectral analysis of the absorption profiles of Ca ii, Mn ii, Ti ii, and Na i associated with the DLA toward PKS 1127−145, based on optical high-resolution data obtained with the UVES instrument at the Very Large Telescope. We obtained column densities and Doppler-parameters for the ions listed above and determine their gas-phase abundances, from which we conclude on their dust depletion properties. We compared the Ca and Ti depletion properties of this DLA with that of other DLAs. Results. One of the six analyzed absorption components (component 3) shows a striking underabundance of Ti and Mn in the gas-phase, indicating the effect of dust depletion for these elements and a locally enhanced dust-to-gas ratio. In this DLA and in other similar absorbers, the Mn ii abundance follows that of Ti ii very closely, implying that both ions are equally sensitive to the dust depletion effects. Conclusions. Our analysis indicates that the DLA toward PKS 1127−145 has multiple origins. With its narrow line width and its strong dust depletion, component 3 points toward the presence of a neutral gas disk from a faint LSB galaxy in front of PKS 1127−145, while the other, more diffuse and dust-poor, absorption components possibly are related to tidal gas features from the interaction between the various, optically confirmed galaxy-group members. In general, the Mn/Ca ii ratio in sub-DLAs and DLAs possibly serves as an important indicator to discriminate between dust-rich and dust-poor in neutral gas in and around galaxies.

Evolution of the cold gas fraction and the star formation history: Prospects with current and future radio facilities

AbstractIt has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the long-standing issue that there is a clear disparity between the total abundance of neutral gas and star-forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-α absorption, which traces all of the gas, provides a measure of the cold gas fraction, or the spin temperature, Tspin. The recent study has shown that the spin temperature (degenerate with the ratio of the absorber/emitter extent) appears to be anti-correlated with the star formation density, ψ*, with 1/Tspin undergoing a similar steep evolution as ψ* over redshifts of 0 ≲ z ≲ 3, whereas the total neutral hydrogen exhibits little evolution. Above z ∼ 3, where ψ* shows a steep decline with redshift, there are insufficient 21-cm data to determine whether 1/Tspin continues to follow ψ*. Knowing this is paramount in ascertaining whether the cold neutral gas does trace the star formation over the Universe’s history. We explore the feasibility of resolving this with 21-cm observations of the largest contemporary sample of reliable damped Lyman-α absorption systems and conclude that, while today’s largest radio interferometers can reach the required sensitivity at z ≲ 3.5, the Square Kilometre Array is required to probe higher redshifts.

The gas and stellar mass of low-redshift damped Lyman-α absorbers

Abstract We report Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet and Arecibo Telescope H i 21 cm spectroscopy of six damped and sub-damped Lyman-α absorbers (DLAs and sub-DLAs, respectively) at z ≲ 0.1, which have yielded estimates of their H i column density, metallicity and atomic gas mass. This significantly increases the number of DLAs with gas mass estimates, allowing the first comparison between the gas masses of DLAs and local galaxies. Including three absorbers from the literature, we obtain H i masses ≈(0.24–5.2) × 109 M⊙, lower than the knee of the local H i mass function. This implies that massive galaxies do not dominate the absorption cross-section for low-z DLAs. We use Sloan Digital Sky Survey photometry and spectroscopy to identify the likely hosts of four absorbers, obtaining low stellar masses, ≈107–108.4 M⊙, in all cases, consistent with the hosts being dwarf galaxies. We obtain high H i 21 cm or CO emission line widths, ΔV20 ≈ 100–290 km s−1, and high gas fractions, fH i ≈ 5–100, suggesting that the absorber hosts are gas-rich galaxies with low star formation efficiencies. However, the H i 21 cm velocity spreads (≳100 km s−1) appear systematically larger than the velocity spreads in typical dwarf galaxies.

Statistical properties of damped Lyman-alpha systems from Sloan Digital Sky Survey DR12

We present new estimates for the statistical properties of damped Lyman-α absorbers (DLAs). We compute the column density distribution function at z > 2, the line density, dN/dX, and the neutral hydrogen density, ΩDLA. Our estimates are derived from the DLA catalogue of Garnett et al. (2016), which uses the Sloan Digital Sky Survey III Data Release 12 (SDSS-III DR12) quasar spectroscopic survey. This catalogue provides a probability that a given spectrum contains a DLA. It allows us to use even the noisiest data without biasing our results and thus substantially increases our sample size. We measure a non-zero column density distribution function at 95 per cent confidence for all column densities |$N_\mathrm{H\,\small {I}}< 5\times 10^{22}$| cm−2. We make the first measurements from SDSS of dN/dX and ΩDLA at z > 4. We show that our results are insensitive to the signal-to-noise ratio of the spectra, but that there is a residual dependence on quasar redshift for z < 2.5, which may be due to remaining systematics in our analysis.

Modeling the neutral hydrogen distribution in the post-reionization Universe: intensity mapping

We model the distribution of neutral hydrogen (HI) in thepost-reionization era and investigate its detectability in 21 cmintensity mapping with future radio telescopes like the SquareKilometer array (SKA). We rely on high resolution hydrodynamicalN-body simulations that have a state-of-the-art treatment of the lowdensity photoionized gas in the inter-galactic medium (IGM). The HI is assigned a-posteriori to the gas particlesfollowing two different approaches: a halo-based method in which HI isassigned only to gas particles residing within dark matter halos;a particle-based method that assigns HI to all gas particlesusing a prescription based on the physical properties of theparticles.The HI statistical properties are then compared to the observationalproperties of Damped Lyman-α Absorbers (DLAs) and of lowercolumn density systems and reasonable good agreement is found for allthe cases. Among the halo-based method, we further considertwo different schemes that aim at reproducing the observed propertiesof DLAs by distributing HI inside halos: one of this results in a muchhigher bias for DLAs, in agreement with recent observations, whichboosts the 21 cm power spectrum by a factor ∼ 4 with respect tothe other recipe. Furthermore, we quantify the contribution of HI inthe diffuse IGM to both ΩHI and the HI power spectrumfinding to be subdominant in both cases.We compute the 21 cm power spectrum from the simulated HI distributionand calculate the expected signal for both SKA1-mid and SKA1-lowconfigurations at 2.4 ⩽ z ⩽ 4. We find that SKA will be ableto detect the 21 cm power spectrum, in the non-linear regime, up tok ∼ 1 h/Mpc for SKA1-mid and k ∼ 5 h/Mpc for SKA1-low with100 hours of observations.We also investigate the perspective of imaging the HI distribution.Our findings indicate that SKA1-low could detect the most massive HIpeaks with a signal to noise ratio (SNR) higher than 5 for anobservation time of about 1000 hours at z = 4, for a synthesized beamwidth of 2′. Detection at redshifts z⩾2.4 with SKA1-midwould instead require a much longer observation time to achieve acomparable SNR level.

Dust-to-metal ratios in damped Lyman-αabsorbers

Dust-to-metal ratios in damped Lyman-<i>α</i>absorbers

The H i column density distribution function in faint dwarf galaxies

We present the HI column density distribution function,f(NHI), as measured from dwarf galaxies observed as part of the Faint Irregular Galaxy GMRT (FIGGS) survey. We find that the shape of the dwarf galaxy f(NHI) is significantly different from the f(NHI) for high redshift Damped Lyman − α absorbers (DLAs) or the f(NHI) for a representative sample of z = 0 gas rich galaxies. The dwarf f(NHI) falls much more steeply at high HI column densities as compared to the other determinations. While ∼ 10% of the cross section above NHI = 10 20.3 atoms cm at z = 0 is provided by dwarf galaxies, the fraction falls to . 1% by NHI ∼ 10 atoms cm. In the local universe, the contribution to the high NHI end of the f(NHI) distribution comes predominantly from the inclined disks of large galaxies. Dwarf galaxies, both because of their smaller scale lengths, and their larger intrinsic axial ratios do not produce large HI column densities even when viewed edge-on. If high column density DLAs/GRB hosts correspond to galaxies like the local dwarfs, this would require either that (i) the absorption arises from merging and not isolated systems or (ii) the observed lines of sight are strongly biased towards high column density regions.

Observed High-ion Absorption-line Signatures of Galactic Winds

In this paper observations are presented of interstellar high-ion absorption-line profiles in three diverse environments: the extended halo of the Milky Way, the host galaxies of gamma-ray bursts at z > 2, and intervening damped Lyman-α absorbers at z ≈ 3. In the first two cases, outflow is directly detected, and in the remaining case outflow is suggested (but not strictly required) by the absorber kinematics. Together these observations demonstrate the ability of the high-ions to trace large-scale galactic outflows.

HI-selected Galaxies As a Probe of Quasar Absorption Systems

AbstractWe investigate the properties of local HI-selected galaxies detected in blind radio surveys by using semi-analytic galaxy formation models in the hierarchical structure formation scenario. By drawing a detailed comparison between the properties of the HI-selected galaxies and the quasar absorption systems, we investigate a link between the local galaxy population and the quasar absorption systems, particularly for Damped Lymanα absorption (DLA) systems (NHI ≥ 1020.3 cm−2) and sub-DLA systems (1019 &lt; NHI &lt; 1020.3 cm−2).We find that DLA galaxies, in which the HI column densities are as high as those of DLA systems, contribute primarily to the population of local HI-selected galaxies at MHI ≥ 108M⊙. By contrast, in the low-mass range MHI ≤ 107M⊙, sub-DLA galaxies replace DLA galaxies as the dominant population. The HI-selected galaxies would be a strong probe of DLA/sub-DLA systems.

Zeeman splitting in the diffuse interstellar medium–The Milky Way and beyond

AbstractWe begin with a brief review of Zeeman-splitting fundamentals and the importance of circular polarization, i.e. Stokes V. We then turn to modern results in several areas, emphasizing the diffuse interstellar medium in the Galaxy. The median field in the Cold Neutral Medium is determined from HI absorption lines and is about 6 μG; the magnetic and turbulent pressures are comparable. Using HI emission lines the field has been mapped in several areas: the field reverses across the Orion Molecular Cloud; the 3-d field structure has been determined in the ρ Oph region; and in regions having shock-like morphology the field is generally stronger, strong enough to limit further compression. We briefly present new field measurements for: photo-dissociation regions at the edges of HII regions, determined from carbon recombination lines; Ultra Luminous Infrared Galaxies, from OH megamasers; and the 3C 286 damped Lyman-α absorption system, determined from the 21-cm line in absorption. We show the sidelobe response of the Green Bank Telescope, which is surprisingly severe and makes the telescope less than optimum for Zeeman-splitting measurements of HI emission lines. Finally, we compare two techniques for determining field strengths, i.e. Zeeman splitting and the Chandrasekhar-Fermi method, and show why the latter usually gives higher field strengths – and sometimes unrealistically high fields.

An integral field spectroscopic survey for high redshift damped Lyman-α galaxies

Aims.We search for galaxy counterparts to damped Lyman-α absorbers (DLAs) at towards nine quasars, which have 14 DLAs and 8 sub-DLAs in their spectra.

H i 21-cm absorption at z∼ 3.39 towards PKS 0201+113

We report the Giant Metrewave Radio Telescope detection of H i 21-cm absorption from the z∼ 3.39 damped Lyman α absorber (DLA) towards PKS 0201+113, the highest redshift at which 21-cm absorption has been detected in a DLA. The absorption is spread over ∼115 km s−1 and has two components, at z= 3.387 144(17) and z= 3.386 141 (45). The stronger component has a redshift and velocity width in agreement with the tentative detection of Briggs, Brinks & Wolfe, but a significantly lower optical depth. The core size and DLA covering factor are estimated to be ≲100 pc and f∼ 0.69, respectively, from a Very Long Baseline Array 328-MHz image. If one makes the conventional assumption that the H i column densities towards the optical and radio cores are the same, this optical depth corresponds to a spin temperature of Ts∼[(955 ± 160) × (f/0.69)] K. However, this assumption may not be correct, given that no metal-line absorption is seen at the redshift of the stronger 21-cm component, indicating that this component does not arise along the line of sight to the optical quasi-stellar object (QSO), and that there is structure in the 21-cm absorbing gas on scales smaller than the size of the radio core. We model the 21-cm absorbing gas as having a two-phase structure with cold dense gas randomly distributed within a diffuse envelope of warm gas. For such a model, our radio data indicate that, even if the optical QSO lies along a line of sight with a fortuitously high (∼50 per cent) cold gas fraction, the average cold gas fraction is low, ≲17 per cent, when averaged over the spatial extent of the radio core. Finally, the large mismatch between peak 21-cm and optical redshifts and the complexity of both profiles makes it unlikely that the z∼ 3.39 DLA will be useful in tests of fundamental constant evolution.

Velocity-metallicity correlation for high-zDLA galaxies: evidence of a mass-metallicity relation?

We used our database of VLT-UVES quasar spectra to build up a sample of 70 Damped Lyman-α (DLA) or strong sub-DLA systems with total neutral hydrogen column densities of H i and redshifts in the range . For each of the systems, we measured the metallicities relative to solar in an homogeneous manner, [X/H] (with , or S or Si), and the velocity widths of low-ionization line profiles, . For the first time, we provide evidence for a correlation between DLA metallicity and line profile velocity width, which is detected at the significance level. This confirms the trend previously observed in a much smaller sample by Wolfe & Prochaska (1998). The best-fit linear relation is , with expressed in km s-1. The slope of the DLA velocity-metallicity relation is the same within uncertainties between the higher () and the lower () redshift halves of our sample. However, the two populations of systems are statistically different. There is a strong redshift evolution in the sense that the median metallicity and median velocity width increase with decreasing redshift. We argue that the existence of a DLA velocity-metallicity correlation, over more than a factor of 100 spread in metallicity, is probably the consequence of an underlying mass-metallicity relation for the galaxies responsible for DLA absorption lines. Assuming a simple linear scaling of the galaxy luminosity with the mass of the dark-matter halo, we find that the slope of the DLA velocity-metallicity relation is consistent with that of the luminosity-metallicity relation derived for local galaxies. If the galaxy dynamical mass is indeed the dominant factor setting up the observed DLA velocity-metallicity correlation, then the DLA systems exhibiting the lowest metallicities among the DLA population should, on average, be associated with galaxies of lower masses (e.g., gas-rich dwarf galaxies). In turn, these galaxies should have the lowest luminosities among the DLA galaxy population. This could explain the difficulties of detecting high-redshift DLA galaxies in emission.

The first detection of [O III] emission from high-redshift damped Lyman- galaxies

We present the detection of [O iii] emission lines from the galaxies responsible for two high-redshift z > 1.75 damped Lyman-α (DLA) absorption lines. We find two sources of [O iii] emission corresponding to the z= 1.92 DLA absorber towards the quasar Q 2206−1958, and we also detect [O iii] emission from the galaxy responsible for the z= 3.10 DLA absorber towards the quasar 2233.9+1381. These are the first detections of rest-frame optical emission lines from high-redshift DLA galaxies. Unlike the Lyα line, the [O iii] line provides a measure of the systemic velocity of the galaxy. We compare the [O iii] redshifts with the velocity profile of the low-ionization metal lines in these two absorbers, with the goal of distinguishing between the model of Prochaska and Wolfe of DLA absorbers as large rapidly rotating cold thick discs, and the standard hierarchical cold dark matter model of structure formation, in which DLAs arise in protogalactic fragments. We find some discrepancies with the predictions of the former model. Furthermore, the image of the DLA galaxy towards Q 2206−1958 shows a complex disturbed morphology, which is more in accord with the hierarchical picture. We use the properties of the rest-frame optical emission lines to further explore the question posed by Moller et al.: are high-redshift DLA galaxies Lyman-break galaxies (LBGs) selected by gas cross-section? The measured velocity dispersions of the DLA galaxies are in agreement with this picture, while the data on the [O iii] luminosities and the velocity differences between the Lyα and [O iii] lines are inconclusive, as there are insufficient LBG measurements overlapping in luminosity. Finally, we estimate the star formation rates in these two DLA galaxies, using a variety of diagnostics, and include a discussion of the extent to which the [O iii] line is useful for this purpose.