Damped Ly-alpha Research Articles

A Metal-poor Damped Lyα System at Redshift 6.4

Abstract We identify a strong Lyα damping wing profile in the spectrum of the quasar P183+05 at z = 6.4386. Given the detection of several narrow metal absorption lines at z = 6.40392, the most likely explanation for the absorption profile is that it is due to a damped Lyα system. However, in order to match the data a contribution of an intergalactic medium 5%–38% neutral or additional weaker absorbers near the quasar is also required. The absorption system presented here is the most distant damped Lyα system currently known. We estimate an H i column density of 1020.68±0.25 cm−2, metallicity [O/H] = −2.92 ± 0.32, and relative chemical abundances of a system consistent with a low-mass galaxy during the first Gyr of the universe. This object is among the most metal-poor damped Lyα systems known and, even though it is observed only ∼850 Myr after the big bang, its relative abundances do not show signatures of chemical enrichment by Population III stars.

ALMA observations of a metal-rich damped Ly α absorber at z = 2.5832: evidence for strong galactic winds in a galaxy group

We report on the results of a search for CO(3-2) emission from the galaxy counterpart of a high-metallicity Damped Ly-alpha Absorber (DLA) at z=2.5832 towards the quasar Q0918+1636. We do not detect CO emission from the previously identified DLA galaxy counterpart. The limit we infer on M_gas / M_star is in the low end of the range found for DLA galaxies, but is still consistent with what is found for other star-forming galaxies at similar redshifts. Instead we detect CO(3-2) emission from another intensely star-forming galaxy at an impact parameter of 117 kpc from the line-of-sight to the quasar and 131 km s^-1 redshifted relative to the velocity centroid of the DLA in the quasar spectrum. In the velocity profile of the low- and high-ionisation absorption lines of the DLA there is an absorption component consistent with the redshift of this CO-emitting galaxy. It is plausible that this component is physically associated with a strong outflow in the plane of the sky from the CO-emitting galaxy. If true, this would be further evidence, in addition to what is already known from studies of Lyman-break galaxies, that galactic outflows can be traced beyond 100 kpc from star-forming galaxies. The case of this z=2.583 structure is an illustration of this in a group environment.

A study of the circumgalactic medium at z ∼ 0.6 using damped Lyman α galaxies

We present the study of a sample of nine QSO fields, with damped-Ly-alpha (DLA) or sub-DLA systems at z~0.6, observed with the X-Shooter spectrograph at the Very Large Telescope. By suitably positioning the X-Shooter slit based on high spatial resolution images of HST/ACS we are able to detect absorbing galaxies in 7 out of 9 fields (~ 78\% success rate) at impact parameters from 10 to 30 kpc. In 5 out of 7 fields the absorbing galaxies are confirmed via detection of multiple emission lines at the redshift of DLAs where only 1 out of 5 also emits a faint continuum. In 2 out of these 5 fields we detect a second galaxy at the DLA redshift. Extinction corrected star formation rates (SFR) of these DLA-galaxies, estimated using their H-alpha fluxes, are in the range 0.3-6.7 M_sun yr^-1. The emission metallicities of these five DLA-galaxies are estimated to be from 0.2 to 0.9 Z_sun. Based on the Voigt profile fits to absorption lines we find the metallicity of the absorbing neutral gas to be in a range of 0.05--0.6 Z_sun. The two remaining DLA-galaxies are quiescent galaxies with SFR < 0.4 M_sun yr^-1 (3-sigma) presenting continuum emission but weak or no emission lines. Using X-Shooter spectrum we estimate i-band absolute magnitude of -19.5+/-0.2 for both these DLA-galaxies that indicates they are sub-L* galaxies. Comparing our results with that of other surveys in the literature we find a possible redshift evolution of the SFR of DLA-galaxies.

The warm, the excited, and the molecular gas: GRB 121024A shining through its star-forming galaxy★

We present the first reported case of the simultaneous metallicity determination of a gamma-ray burst (GRB) host galaxy, from both afterglow absorption lines as well as strong emission-line diagnostics. Using spectroscopic and imaging observations of the afterglow and host of the long-duration Swift GRB121024A at z = 2.30, we give one of the most complete views of a GRB host/environment to date. We observe a strong damped Ly-alpha absorber (DLA) with a hydrogen column density of log N(HI) = 21.88 +/- 0.10, H2 absorption in the Lyman-Werner bands (molecular fraction of log(f)~ -1.4; fourth solid detection of molecular hydrogen in a GRB-DLA), the nebular emission lines H-alpha, H-beta, [O II], [O III] and [N II], as well as metal absorption lines. We find a GRB host galaxy that is highly star-forming (SFR ~ 40 solar masses/yr ), with a dust-corrected metallicity along the line of sight of [Zn/H]corr = -0.6 +/- 0.2 ([O/H] ~ -0.3 from emission lines), and a depletion factor [Zn/Fe] = 0.85 +/- 0.04. The molecular gas is separated by 400 km/s (and 1-3 kpc) from the gas that is photoexcited by the GRB. This implies a fairly massive host, in agreement with the derived stellar mass of log(M/M_solar ) = 9.9+/- 0.2. We dissect the host galaxy by characterising its molecular component, the excited gas, and the line-emitting star-forming regions. The extinction curve for the line of sight is found to be unusually flat (Rv ~15). We discuss the possibility of an anomalous grain size distributions. We furthermore discuss the different metallicity determinations from both absorption and emission lines, which gives consistent results for the line of sight to GRB 121024A.

Clues on the Galactic evolution of sulphur from star clusters

(Abridged) The abundances of alpha-elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy. Sulphur, being moderately volatile, can be reliably measured in the interstellar medium (ISM) of damped Ly-alpha galaxies and extragalactic HII regions. Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies. Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of O or Mg. The measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur. We find <A(S)>NLTE=6.11+/-0.04 for M 4, <A(S)>NLTE=7.17+/-0.02 for NGC 2477, and <A(S)>NLTE=7.13+/-0.06 for NGC 5822. For the only star studied in Trumpler 5 we find A(S)NLTE=6.43+/-0.03 and A(S)LTE=6.94+/-0.05. Our measurements show that, by and large, the S abundances in Galactic clusters trace reliably those in field stars. The only possible exception is Trumpler 5, for which the NLTE sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than observed in field stars of comparable metallicity, even though its LTE sulphur abundance is in line with abundances of field stars. Moreover the LTE sulphur abundance is consistent only with the abundance of another alpha-element, Mg, in the same star, while the low NLTE value is consistent with Si and Ca. The S abundances in our sample of stars in clusters imply that the clusters are chemically homogeneous for S within 0.05 dex.

THE RAPID DECLINE IN METALLICITY OF DAMPED Lyα SYSTEMS AT z ∼ 5

We present evidence that the cosmological mean metallicity of neutral atomic hydrogen gas shows a sudden decrease at z>4.7 down to <Z>=-2.03(+0.09-0.11), which is 6 sigma deviant from that predicted by a linear fit to the data at lower redshifts. This measurement is made possible by the chemical abundance measurements of 8 new damped Ly-alpha (DLA) systems at z>4.7 observed with the Echellette Spectrograph and Imager on the Keck II telescope, doubling the number of measurements at z>4.7 to 16. Possible explanations for this sudden decrease in metallicity include a change in the physical processes that enrich the neutral gas within disks, or an increase of the covering factor of neutral gas outside disks due to a lower ultra-violet radiation field and higher density at high redshift. The later possibility would result in a new population of presumably lower metallicity DLAs, with an increased contribution to the DLA population at higher redshifts resulting in a reduced mean metallicity. Furthermore, we provide evidence of a possible decrease at z>4.7 in the comoving metal mass density of DLAs, which is flat out to z~4.3. Such a decrease is expected, as otherwise most of the metals from star-forming galaxies would reside in DLAs by z~6. While the metallicity is decreasing at high redshift, the contribution of DLAs to the total metal budget of the universe increases with redshift, with DLAs at z~4.3 accounting for ~20% as many metals as produced by Lyman break galaxies.

Simulating extremely metal-poor gas and DLA metal content at redshift z ≃ 7

We present the first theoretical study of metals in damped Ly-alpha (DLA) systems at redshift z=7. The features of cold, primordial gas are studied by means of N-body, hydro, chemistry simulations, including atomic and molecular non-equilibrium chemistry, cooling, star formation for population III and population II-I regimes, stellar evolution, cosmic metal spreading according to proper yields (for He, C, O, Si, Fe, Mg, S, etc.) and lifetimes, and feedback effects. Theoretical expectations are then compared to recently available constraints from DLA observations. We find that DLA galaxies at z=7 account for 10 per cent of the whole galaxy population and for most of the metal-poor galaxies at these epochs. About 7 per cent of these DLA galaxies contain purely pristine material and 34 per cent of them consist of very weakly polluted gas, being, therefore, suitable candidate s as population III sites. The remaining 59 per cent are enriched above ~10^{-4} Zsun. Additionally, DLA candidates appear to have: gas masses <~2x10^8 Msun; very low star formation rate, ~10^{-3} - 10^{-2} Msun/yr (significantly weaker than late-time counterparts); mean molecular fractions covering a fairly wide range, x_mol~10^{-3}- 10^{-6}; typical metallicities Z<~3x10^{-3}Zsun and HI column densities NHI>~3x10^{20} cm^{-2}. They present no or weak correlations between their gas mass and Z, NHI, or x_mol; a moderate correlation between x_mol and Z, linked to the ongoing molecular-driven star formation and metal pollution processes; a mild anti-correlation between NHI and x_mol, due to H depletion into molecules; and a chemical content that is subject to environmental dependencies.

Probing the extended gaseous regions of M31 with quasar absorption lines★

We present HST-COS spectra of ten quasars located behind M31, selected to investigate the properties of gas associated with its extended disk and high velocity clouds (HVCs). The sightlines have impact parameters ranging between b= 13 kpc and 112 kpc. No absorption is detected in the four sightlines beyond b=57 kpc. Of the six remaining sightlines, all of which lie at b<32 kpc and within the N(HI)= 2E18 cm^{-2} boundary of the HI disk of M31, we detect low-ionization absorption at M31 velocities along four of them (three of which include MgII absorption). We also detect MgII absorption from an HVC. We find that along sightlines where both are detected, the velocity location of the low-ion gas tracks the peak in 21 cm emission. High-ionization absorption is detected along the three inner sightlines, but not along the three outer sightlines, for which CIV data exist. As inferred from 21 cm emission line maps, only one sightline may have a damped Ly-alpha system. This sightline has b= 17.5 kpc, and we detect both low- and high-ion absorption lines associated with it. The impact parameters through M31 are similar to the impact parameters of galaxies identified with MgII absorbers at redshifts 0.1<z<1.0 in a 2011 study by Rao et al. However, the M31 MgII2796 rest equivalent width values are significantly smaller. In comparison, moderate-to-strong MgII absorption from Milky Way gas is detected along all ten sightlines. Thus, this study indicates that M31 does not present itself as an absorbing galaxy which is typical of higher-redshift galaxies inferred to give rise to moderate-strength quasar absorption lines. M31 also appears not to possess an extensive large gaseous cross section, at least not along the direction of its major axis. (Abridged.)

Mg II ABSORPTION AT 2 &lt;z&lt; 6 WITH MAGELLAN/FIRE. II. A LONGITUDINAL STUDY OF H I, METALS, AND IONIZATION IN GALACTIC HALOS

We present a detailed study of HI and metals for 110 MgII absorption systems discovered at 1.98 <= z <= 5.33 in the infrared spectra of high redshift QSOs. Using new measurements of rest-frame UV lines from optical spectra of the same targets, we compare the high redshift sample with carefully constructed low redshift control samples from the literature to study evolutionary trends from z=0 --> 5.33 (>12 Gyr). We observe a significant strengthening in the characteristic N(HI) for fixed MgII equivalent width as one moves toward higher redshift. Indeed at our sample's mean zbar=3.402, all MgII systems are either damped Ly-alpha absorbers or sub-DLAs, with 40.7% of systems exceeding the DLA threshold (compared to 16.7% at zbar=0.927). We set lower limits on the metallicity of the MgII systems where we can measure HI; these results are consistent with the full DLA population. The classical MgII systems (W(2796)=0.3-1.0 Ang), which preferentially associate with sub-DLAs, are quite metal rich at ~0.1 Solar. We applied quantitative classification metrics to our absorbers to compare with low redshift populations, finding that weak systems are similar to classic MgII absorbers at low redshift. The strong systems either have very large MgII and FeII velocity spreads implying non-virialized dynamics, or are more quiescent DLAs. There is tentative evidence that the kinetically complex systems evolve in similar fashion to the global star formation rate. We speculate that if weaker MgII systems represent accreting gas as suggested by recent studies of galaxy-absorber inclinations, then their high metal abundance suggests re-accretion of recently ejected material rather than first-time infall from the metal-poor IGM, even at early times.

Evidence of Gunn–Peterson damping wings in high-z quasar spectra: strengthening the case for incomplete reionization at z ∼ 6–7

The spectra of several high-redshift (z>6) quasars have shown evidence for a Gunn-Peterson (GP) damping wing, indicating a substantial mean neutral hydrogen fraction (x_HI > 0.03) in the z ~ 6 intergalactic medium (IGM). However, previous analyses assumed that the IGM was uniformly ionized outside of the quasar's HII region. Here we relax this assumption and model patchy reionization scenarios for a range of IGM and quasar parameters. We quantify the impact of these differences on the inferred x_HI, by fitting the spectra of three quasars: SDSS J1148+5251 (z=6.419), J1030+0524 (z=6.308), and J1623+3112 (z=6.247). We find that the best-fit values of x_HI in the patchy models agree well with the uniform case. More importantly, we confirm that the observed spectra favor the presence of a GP damping wing, with peak likelihoods decreasing by factors of > few - 10 when the spectra are modeled without a damping wing. We also find that the Ly alpha absorption spectra, by themselves, cannot distinguish the damping wing in a relatively neutral IGM from a damping wing in a highly ionized IGM, caused either by an isolated neutral patch, or by a damped Ly alpha absorber (DLA). However, neutral patches in a highly ionized universe (x_HI < 0.01), and DLAs with the large required column densities (N_HI > few x 10^{20} cm^{-2}) are both rare. As a result, when we include reasonable prior probabilities for the line of sight (LOS) to intercept either a neutral patch or a DLA at the required distance of ~ 40-60 comoving Mpc away from the quasar, we find strong lower limits on the neutral fraction in the IGM, x_HI > 0.1 (at 95% confidence). This strengthens earlier claims that a substantial global fraction of hydrogen in the z~6 IGM is in neutral form.

THE ALFALFA H I ABSORPTION PILOT SURVEY: A WIDE-AREA BLIND DAMPED Lyα SYSTEM SURVEY OF THE LOCAL UNIVERSE

We present the results of a pilot survey for neutral hydrogen (HI) 21 cm absorption in the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA) Survey. This project is a wide-area "blind" search for HI absorption in the local universe, spanning -650 km/s < cz < 17,500 km/s and covering 517.0 square degrees (7% of the full ALFALFA survey). The survey is sensitive to HI absorption lines stronger than 7.7 mJy (8983 radio sources) and is 90% complete for lines stronger than 11.0 mJy (7296 sources). The total redshift interval sensitive to all damped Lyman alpha (DLA) systems (N_HI >= 2x10^20 cm^-2) is Delta z = 7.0 (129 objects, assuming T_s = 100 K and covering fraction unity); for super-DLAs (N_HI >= 2x10^21 cm^-2) it is Delta z= 128.2 (2353 objects). We re-detect the intrinsic HI absorption line in UGC 6081 but detect no intervening absorption line systems. We compute a 95% confidence upper limit on the column density frequency distribution function f(N_HI,X) spanning four orders of magnitude in column density, 10^19 (T_s/100 K)(1/f) cm^-2 < N_HI < 10^23 (T_s/100 K)(1/f) cm^-2, that is consistent with previous redshifted optical damped Ly alpha surveys and the aggregate HI 21 cm emission in the local universe. The detection rate is in agreement with extant observations. This pilot survey suggests that an absorption line search of the complete ALFALFA survey --- or any higher redshift, larger bandwidth, or more sensitive survey, such as those planned for Square Kilometer Array pathfinders or a low frequency lunar array --- will either make numerous detections or will set a strong statistical lower limit on the typical spin temperature of neutral hydrogen gas.

EFFECTS OF ULTRAVIOLET BACKGROUND AND LOCAL STELLAR RADIATION ON THE H I COLUMN DENSITY DISTRIBUTION

We study the impact of ultraviolet background (UVB) radiation field and the local stellar radiation on the H_I column density distribution f(N_HI) of damped Ly-alpha systems (DLAs) and sub-DLAs at z=3 using cosmological smoothed particle hydrodynamics simulations. We find that, in the previous simulations with an optically thin approximation, the UVB was sinking into the H_I cloud too deeply, and therefore we underestimated the f(N_HI) at 19 < log(N_HI) < 21.2 compared to the observations. If the UVB is shut off in the high-density regions with n_gas > 6 x 10^{-3} cm^{-3}, then we reproduce the observed f(N_HI) at z=3 very well. We also investigate the effect of local stellar radiation by post-processing our simulation with a radiative transfer code, and find that the local stellar radiation does not change the f(N_HI) very much. Our results show that the shape of f(N_HI) is determined primarily by the UVB with a much weaker effect by the local stellar radiation and that the optically thin approximation often used in cosmological simulation is inadequate to properly treat the ionization structure of neutral gas in and out of DLAs. Our result also indicates that the DLA gas is closely related to the transition region from optically-thick neutral gas to optically-thin ionized gas within dark matter halos.

The halo occupation distribution of H i from 21-cm intensity mapping at moderate redshifts

The spatial clustering properties of HI galaxies can be studied using the formalism of the halo occupation distribution (HOD). The resulting parameter constraints describe properties like gas richness verses environment. Unfortunately, clustering studies based on individual HI galaxies will be restricted to the local Universe for the foreseeable future, even with the deepest HI surveys. Here we discuss how clustering studies of the HI HOD could be extended to moderate redshift, through observations of fluctuations in the combined 21cm intensity of unresolved galaxies. In particular we make an analytic estimate for the clustering of HI in the HOD. Our joint goals are to estimate i) the amplitude of the signal, and ii) the sensitivity of telescopes like the Australian SKA Pathfinder to HOD parameters. We find that the power spectrum of redshifted 21cm intensity could be used to study the distribution of HI within dark matter halos at z>0.5 where individual galaxies cannot be detected. In addition to the HOD of HI, the amplitude of the 21cm power spectrum would also yield estimates of the cosmic HI content at epochs between the local Universe, and redshifts probed by damped Ly-alpha absorbers.

Faint extended Lyα emission due to star formation at the centre of high column density QSO absorption systems

We use detailed Lyalpha radiative transfer calculations to further test the claim of Rauch et al. (2008) that they have detected spatially extended faint Lyalpha emission from the elusive host population of Damped Lyalpha Absorption systems (DLAs) in their recent ultra-deep spectroscopic survey. We investigate the spatial and spectral distribution of Lyalpha emission due to star-formation at the centre of DLAs, and its dependence on the spatial and velocity structure of the gas. Our model simultaneously reproduces the observed properties of DLAs and the faint Lyalpha emitters, including the velocity width and column density distribution of DLAs and the large spatial extent of the emission of the faint emitters. Our modelling confirms previous suggestions that DLAs are predominately hosted by Dark Matter (DM) halos in the mass range 10^{9.5}-10^{12} M_sun, and are thus of significantly lower mass than those inferred for L_* Lyman Break Galaxies (LBGs). Our modelling suggests that DM halos hosting DLAs retain up to 20% of the cosmic baryon fraction in the form of neutral hydrogen, and that star formation at the centre of the halos is responsible for the faint Lyalpha emission. The scattering of a significant fraction of the Lyalpha emission to the observed radii, which can be as large as 50 kpc or more, requires the amplitude of the bulk motions of the gas at the centre of the halos to be moderate. The observed space density and size distribution of the emitters together with the incidence rate of DLAs suggests that the Lyalpha emission due to star formation has a duty cycle of ~ 25%.

H I-SELECTED GALAXIES AS A PROBE OF QUASAR ABSORPTION SYSTEMS

We investigate the properties of HI-rich galaxies detected in blind radio surveys within the hierarchical structure formation scenario using a semi-analytic model of galaxy formation. By drawing a detailed comparison between the properties of HI-selected galaxies and HI absorption systems, we argue a link between the local galaxy population and quasar absorption systems, particularly for Damped Ly-alpha absorption (DLA) systems and sub-DLA systems. First, we evaluate how many HI-selected galaxies exhibit HI column densities as high as those of DLA systems. We find that HI-selected galaxies with HI masses M(HI) > 10^8 solar masses have gaseous disks that produce HI column densities comparable to those of DLA systems. We conclude that DLA galaxies where the HI column densities are as high as those of DLA systems, contribute significantly to the population of HI-selected galaxies at M(HI) > 10^8 solar masses. Second, we find that star formation rates (SFRs) correlate tightly with HI masses rather than B- (and J-) band luminosities. In the low-mass range M(HI) < 10^8 solar masses, sub-DLA galaxies replace DLA galaxies as the dominant population. The number fraction of sub-DLA galaxies relative to galaxies reaches 40%-60% at HI masses 10^8 solar masses and 30%-80% at 10^7 solar masses. The HI-selected galaxies at 10^7 solar masses are a strong probe of sub-DLA systems that place stringent constraints on galaxy formation and evolution.

WAVELENGTH ACCURACY OF THE KECK HIRES SPECTROGRAPH AND MEASURING CHANGES IN THE FINE STRUCTURE CONSTANT

We report on an attempt to accurately wavelength calibrate four nights of data taken with the Keck HIRES spectrograph on QSO PHL957, for the purpose of determining whether the fine structure constant was different in the past. Using new software and techniques, we measured the redshifts of various Ni II, Fe II, Si II, etc. lines in a damped Lyalpha system at z = 2.309. Roughly half the data were taken through the Keck iodine cell which contains thousands of well calibrated iodine lines. Using these iodine exposures to calibrate the normal Th-Ar Keck data pipeline output, we found absolute wavelength offsets of 500 m s{sup -1} to 1000 m s{sup -1} with drifts of more than 500 m s{sup -1} over a single night, and drifts of nearly 2000 m s{sup -1} over several nights. These offsets correspond to an absolute redshift of uncertainty of about DELTAz approx 10{sup -5}(DELTAlambda approx 0.02 A), with daily drifts of around DELTAz approx 5 x 10{sup -6} (DELTAlambda approx 0.01 A), and multiday drifts of nearly DELTAz approx 2 x 10{sup -5}(approx0.04 A). The causes of the wavelength offsets are not known, but since claimed shifts in the fine structure constant wouldmore » result in velocity shifts of less than 100 m s{sup -1}, this level of systematic uncertainty may make it difficult to use Keck HIRES data to constrain the change in the fine structure constant. Using our calibrated data, we applied both our own fitting software and standard fitting software to measure DELTAalpha/alpha, but discovered that we could obtain results ranging from significant detection of either sign, to strong null limits, depending upon which sets of lines and which fitting method were used. We thus speculate that the discrepant results on DELTAalpha/alpha reported in the literature may be due to random fluctuations coming from underestimated systematic errors in wavelength calibration and fitting procedure.« less

DAMPED LYα ABSORBER KINEMATICS AND OUTFLOWS FROM STARBURST GALAXIES

We present results from a numerical study of the multiphase interstellar medium in sub-Lyman-break galaxy protogalactic clumps. Such clumps are abundant at z=3 and are thought to be a major contributor to damped Ly-alpha absorption. We model the formation of winds from these clumps and show that during star formation episodes they feature outflows with neutral gas velocity widths up to several hundred km/s. Such outflows are consistent with the observed high-velocity dispersion in DLAs. In our models thermal energy feedback from winds and supernovae results in efficient outflows only when cold (~ 300 K), dense (> 100 msun/pc^3) clouds are resolved at grid resolution of 12 pc. At lower 24 pc resolution the first signs of the multiphase medium are spotted; however, at this low resolution thermal injection of feedback energy cannot yet create hot expanding bubbles around star-forming regions -- instead feedback tends to erase high-density peaks and suppress star formation. At 12 pc resolution feedback compresses cold clouds, often without disrupting the ongoing star formation; at the same time a larger fraction of feedback energy is channeled into low-density bubbles and winds. These winds often entrain compact neutral clumps which produce multi-component metal absorption lines.

GALEXDISCOVERY OF A DAMPED Lyα SYSTEM AT REDSHIFTz≈ 1

We report the first discovery of a QSO damped Ly-alpha (DLA) system by the GALEX satellite. The system was initially identified as an MgII absorption-line system (z_abs=1.028) in the spectrum of SDSS QSO J0203-0910 (z_em=1.58). The presence of unusually strong absorption due to metal lines of ZnII, CrII, MnII, and FeII clearly suggested that it might be a DLA system with N{HI} > 2 x 10^20 atoms cm^-2. Follow-up GALEX NUV grism spectroscopy confirms the system exhibits a DLA absorption line, with a measured HI column density of N{HI} = 1.50+/-0.45 x 10^21 atoms cm^-2. By combining the GALEX N{HI} determination with the SDSS spectrum measurements of unsaturated metal-line absorption due to ZnII, which is generally not depleted onto grains, we find that the system's neutral-gas-phase metal abundance is [Zn/H] = -0.69+/-0.22, or ~20% solar. By way of comparison, although this system has one of the largest Zn^+ column densities, its metal abundances are comparable to other DLAs at z~1. Measurements of the abundances of Cr, Fe, and Mn help to further pin down the evolutionary state of the absorber.

The 21-cm power spectrum after reionization

We discuss the 21cm power spectrum (PS) following the completion of reionization. In contrast to the reionization era, this PS is proportional to the PS of mass density fluctuations, with only a small modulation due to fluctuations in the ionization field on scales larger than the mean-free-path of ionizing photons. We derive the form of this modulation, and demonstrate that its effect on the 21cm PS will be smaller than 1% for physically plausible models of damped Ly-alpha systems. In contrast to the 21cm PS observed prior to reionization, in which HII regions dominate the ionization structure, the simplicity of the 21cm PS after reionization will enhance its utility as a cosmological probe by removing the need to separate the PS into physical and astrophysical components. As a demonstration, we consider the Alcock-Paczynski test and show that the next generation of low-frequency arrays could measure the angular distortion of the PS at the percent level for z~3-5.

ARE THE KINEMATICS OF DLAs IN AGREEMENT WITH THEIR ARISING IN THE GAS DISKS OF GALAXIES?

We demonstrate in this paper that the velocity widths of the neutral gas in Damped Ly Alpha (DLA) systems are inconsistent with these systems originating in gas disks of galaxies similar to those seen in the local Universe. We examine the gas kinematics of local galaxies using the high quality HI 21-cm data from the HI Nearby Galaxies Survey (THINGS) and make a comparison with the velocity profiles measured in the low-ionization metal lines observed in DLAs at high redshifts. The median velocity width of z=0 HI gas above the DLA column density limit of N=2x10^20 cm-2 is approximately 30 km/s, whereas the typical value in DLAs is a factor of two higher. We argue that the gas kinematics at higher redshifts are increasingly influenced by gas that is not participating in ordered rotation in cold disks, but is more likely associated with tidal gas related to galaxy interactions or processes such as superwinds and outflows. An analysis of the HI in the local interacting star-burst galaxy M82 shows that the velocity widths in this galaxy are indeed similar to what is seen in DLAs.