High-redshift DLA Research Articles

Multi-phase gas properties of extremely strong intervening DLAs towards quasars

We present the results of a spectroscopic analysis of extremely strong damped Lyα absorbers (ESDLAs; log N(H I) ≳ 21.7) observed with the medium resolution spectrograph, X-shooter at the Very Large Telescope (VLT). Recent studies in the literature indicate that ESDLAs probe gas from within the star-forming disk of the associated galaxies and thus ESDLAs provide a unique opportunity to study the interstellar medium of galaxies at high redshift. We report column densities (N), equivalent widths (w, for Mg II and Ca II transitions), and the kinematic spread (Δv90) of species from neutral (namely O I, Ar I, Cl I, N I, and Na I), singly ionised (Mg II, Ca II, S II, Ni II, Mn II, Ti II, and P II), and higher ionisation (C IV, Si IV, N V, and O VI) species. We estimate the dust-corrected metallicity measured using different singly ionised gas species such as P II, S II, Si II, Mn II and Cr II, and Zn II. We find that, using the dust correction prescription, the measured metallicities are consistent for all mentioned species in all ESDLAs within 3σ uncertainty. We further perform a quantitative comparison of column densities, equivalent widths, and kinematic spreads of ESDLAs with other samples that are associated with galaxies and detected in absorption along the line of sight towards high-redshift quasars (QSOs). We find that the distributions of the Ar I to H I column density ratio (N(Ar I)/N(H I)) in DLAs and ESDLAs are similar. We further report that ESDLAs do not show a strong deficiency of Ar I relative to other α-capture elements as is seen in DLAs. This supports the idea that the mentioned under-abundance of Ar I in DLAs is possibly caused by the presence of background UV photons that penetrate the low N(H I) clouds to ionise Ar I, but they cannot penetrate deep enough in the high N(H I) ESDLA environment. The w(Mg II λ2796) distribution in ESDLAs is found to be similar to that of metal-rich C I-selected absorbers, but the velocity spread of their Mg II profile is different. The dust content (measured by modelling the quasar extinction) and w(Ca II λ3934) distributions are similar in ESDLAs and Ca II-selected absorbers, yet we do not see any correlation between w(Ca II λ3934) and dust content. The Δv90 velocity spread of singly ionised species in ESDLAs is statistically smaller than that of DLAs. For higher ionisation species (such as C IV and Si IV) that trace the warm ionised medium, Δv90 is similar in the two populations. This suggests that the ESDLAs sample a different H I region of their associated galaxy compared to the general DLA population. We further study the N(Cl I) distribution in high-redshift DLA and ESDLA sightlines, as Cl I is a good tracer of H2 gas. The N(Cl I)−N(H2) correlation is followed by all the clouds (ESDLAs and otherwise) having log N(H2) < 22.

Physical conditions in the diffuse interstellar medium of local and high-redshift galaxies: measurements based on the excitation of H2 rotational and C i fine-structure levels

ABSTRACT We present the results of an analysis of the physical conditions (number density, intensity of UV field, kinetic temperature) in the cold H2-bearing interstellar medium of local and high-redshift galaxies. Our measurements are based on the fit to the observed population of H2 rotational levels and C i fine-structure levels with the help of grids of numerical models calculated with the photon-dominated region (PDR) Meudon code. A joint analysis of low H2 rotational levels and C i fine-structure levels breaks the degeneracy in the IUV−nH plane and provides significantly tighter constraints on the number density and intensity of the UV field. Using archive data from the VLT/UVES, KECK/HIRES, HST/STIS and FUSE telescopes, we selected 12 high-redshift damped Lyα systems (DLAs) in quasar spectra and 14 H2 absorption systems along the lines of sight towards stars in the Milky Way and the Magellanic Cloud galaxies. These systems have strong H2 components, with a column density log N(H2)/[cm−2] > 18 and associated C i absorptions. We find that H2-bearing media in high-redshift DLAs and in local galaxies have similar values of the kinetic temperatures Tkin ∼ 100 K and number density 10−500 cm−3. However, the intensity of incident UV radiation in DLAs varies in a wide range (0.1−100 units of the Mathis field), while it is ∼0.1−3 units of the Mathis field for H2 systems in the Milky Way and Large and Small Magellanic Cloud galaxies. The large dispersion of measured UV flux in DLAs is probably a consequence of the fact that the DLA sample probes galaxies selected from the overall galaxy population at high redshift, and therefore corresponds to a wide range of physical conditions.

Linking gas and galaxies at high redshift: MUSE surveys the environments of six damped Lyα systems at z ≈ 3

ABSTRACT We present results from a survey of galaxies in the fields of six z ≥ 3 damped Lyman α (Lyα) systems (DLAs) using the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT). We report a high detection rate of up to ${\approx } 80{{\ \rm per\ cent}}$ of galaxies within 1000 km s−1 from DLAs and with impact parameters between 25 and 280 kpc. In particular, we discovered five high-confidence Lyα emitters associated with three DLAs, plus up to nine additional detections across five of the six fields. The majority of the detections are at relatively large impact parameters (>50 kpc) with two detections being plausible host galaxies. Among our detections, we report four galaxies associated with the most metal-poor DLA in our sample (Z/Z⊙ = −2.33 ± 0.22), which trace an overdense structure resembling a filament. By comparing our detections with predictions from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological simulations and a semi-analytic model designed to reproduce the observed bias of DLAs at z > 2, we conclude that our observations are consistent with a scenario in which a significant fraction of DLAs trace the neutral regions within haloes with a characteristic mass of $M_{\rm h} \approx 10^{11}-10^{12}~\rm M_\odot$, in agreement with the inference made from the large-scale clustering of DLAs. We finally show how larger surveys targeting ≈25 absorbers have the potential of constraining the characteristic masses of haloes hosting high-redshift DLAs with sufficient accuracy to discriminate between different models.

Revealing H i gas in emission and absorption on pc to kpc scales in a galaxy at z ∼ 0.017

We present a detailed study of the quasar-galaxy pair: J1243+4043 - UGC07904. The sight line of the background quasar ( $z_q$= 1.5266) passes through a region of the galaxy ($z_g$=0.0169) at an impact parameter of 6.9 kpc with high metallicity (0.5 Z$_\odot$) and negligible dust extinction. We detect HI 21-cm absorption from the foreground galaxy at arcsecond and milliarcsecond scales. For typical cold neutral medium (CNM) temperatures in the Milky Way, this 21-cm absorber can be classified as a damped Ly$\alpha$ absorber (DLA). We infer the harmonic mean spin temperature of the gas to be $\sim$400 K and for a simple two-phase medium we estimate the CNM-fraction to be $f_{\rm CNM}$ = 0.27. This is remarkably consistent with the CNM fraction observed in the Galaxy and less than that of high-redshift DLAs. The quasar exhibits a core-jet morphology on milliarcsecond scales, corresponding to an overall extent of $\sim$9 pc at $z_g$. We show that the size of CNM absorbing clouds associated with the foreground galaxy is $>$5 pc and they may be part of cold gas structures that extend beyond $\sim$35 pc. Interestingly, the rotation measure of quasar J1243+4043 is higher than any other source in samples of quasars with high-$z$ DLAs. However, we do not find any detectable differences in RMs and polarization fraction of sight lines with or without high-$z$ ($z\ge2$) DLAs or low-$z$ ($z\le0.3$) 21-cm absorbers. Finally, the foreground galaxy UGC07904 is also part of a galaxy group. We serendipitously detect HI 21-cm emission from four members of the group, and a $\sim$80 kpc long HI bridge connecting two of the other members. The latter, together with the properties of the group members, suggests that the group is a highly interactive environment.

The role of binaries in the enrichment of the early Galactic halo

Detailed spectroscopic studies of metal-poor halo stars have highlighted the important role of carbon-enhanced metal-poor (CEMP) stars in understanding the early production and ejection of carbon in the Galaxy and in identifying the progenitors of the CEMP stars among the first stars formed after the Big Bang. Recent work has also classified the CEMP stars by absolute carbon abundance, A(C), into high- and low-C bands, mostly populated by binary and single stars, respectively. Our aim is to determine the frequency and orbital parameters of binary systems among the CEMP-s stars, which exhibit strong enhancements of neutron-capture elements associated with the s-process. This allows us to test whether local mass transfer from a binary companion is necessary and sufficient to explain their dramatic carbon excesses. Eighteen of the 22 stars exhibit clear orbital motion, yielding a binary frequency of 82+-10%, while four stars appear to be single (18+-10%). We thus confirm that the binary frequency of CEMP-s stars is much higher than for normal metal-poor giants, but not 100% as previously claimed. Secure orbits are determined for 11 of the binaries and provisional orbits for six long-period systems (P > 3,000 days), and orbital circularisation time scales are discussed. The conventional scenario of local mass transfer from a former AGB binary companion does appear to account for the chemical composition of most CEMP-s stars. However, the excess of C and s-process elements in some single CEMP-s stars was apparently transferred to their natal clouds by an external (distant) source. This finding has important implications for our understanding of carbon enrichment in the early Galactic halo and some high-redshift DLA systems, and of the mass loss from extremely metal-poor AGB stars. Abridged.

The role of binaries in the enrichment of the early Galactic halo

The detailed composition of most metal-poor halo stars has been found to be very uniform. However, a fraction of 20-70% (increasing with decreasing metallicity) exhibit dramatic enhancements in their abundances of carbon - the so-called carbon-enhanced metal-poor (CEMP) stars. A key question for Galactic chemical evolution models is whether this non-standard composition reflects that of the stellar natal clouds, or is due to local, post-birth mass transfer of chemically processed material from a binary companion; CEMP stars should then all be members of binary systems. Our aim is to determine the frequency and orbital parameters of binaries among CEMP stars with and without over-abundances of neutron-capture elements - CEMP-s and CEMP-no stars, respectively - as a test of this local mass-transfer scenario. This paper discusses a sample of 24 CEMP-no stars, while a subsequent paper will consider a similar sample of CEMP-s stars. Most programme stars exhibit no statistically significant radial-velocit variation over this period and appear to be single, while four are found to be binaries with orbital periods of 300-2,000 days and normal eccentricity; the binary frequency for the sample is 17+-9%. The single stars mostly belong to the recently-identified ``low-C band'', while the binaries have higher absolute carbon abundances. We conclude that the nucleosynthetic process responsible for the strong carbon excess in these ancient stars is unrelated to their binary status; the carbon was imprinted on their natal molecular clouds in the early Galactic ISM by an even earlier, external source, strongly indicating that the CEMP-no stars are likely bona fide second-generation stars. We discuss potential production sites for carbon and its transfer across interstellar distances in the early ISM, and implications for the composition of high-redshift DLA systems. Abridged.

The evolution of neutral gas in damped Lyman α systems from the XQ-100 survey

We present a sample of 38 intervening Damped Lyman $\alpha$ (DLA) systems identified towards 100 $z>3.5$ quasars, observed during the XQ-100 survey. The XQ-100 DLA sample is combined with major DLA surveys in the literature. The final combined sample consists of 742 DLAs over a redshift range approximately $1.6 < z_{\rm abs} < 5.0$. We develop a novel technique for computing $\Omega_{\rm HI}^{\rm DLA}$ as a continuous function of redshift, and we thoroughly assess and quantify the sources of error therein, including fitting errors and incomplete sampling of the high column density end of the column density distribution function. There is a statistically significant redshift evolution in $\Omega_{\rm HI}^{\rm DLA}$ ($\geq 3 \sigma$) from $z \sim 2$ to $z \sim$ 5. In order to make a complete assessment of the redshift evolution of $\Omega_{\rm HI}$, we combine our high redshift DLA sample with absorption surveys at intermediate redshift and 21cm emission line surveys of the local universe. Although $\Omega_{\rm HI}^{\rm DLA}$, and hence its redshift evolution, remains uncertain in the intermediate redshift regime ($0.1 < z_{\rm abs} < 1.6$), we find that the combination of high redshift data with 21cm surveys of the local universe all yield a statistically significant evolution in $\Omega_{\rm HI}$ from $z \sim 0$ to $z \sim 5$ ($\geq 3 \sigma$). Despite its statistical significance, the magnitude of the evolution is small: a linear regression fit between $\Omega_{\rm HI}$ and $z$ yields a typical slope of $\sim$0.17$\times 10^{-3}$, corresponding to a factor of $\sim$ 4 decrease in $\Omega_{\rm HI}$ between $z=5$ and $z=0$.

A search for boron in damped Lyα systems

We present the first systematic study of boron beyond the Local Group. This analysis is performed on a sample of 30 damped Ly{\alpha} systems (DLAs) with strong metal-lines, which are expected to trace the interstellar medium of high z galaxies. We report on two boron detections at > 3{\sigma} significance; one new detection and one confirmation. The ratios of B/O and, for the first time, B/S are compared with previous stellar and interstellar measurements in the Milky Way and Small Magellanic Cloud. The novel comparison with sulphur, which tracks oxygen's abundance, alleviates the uncertainty associated with stellar oxygen measurements. For both detections, the inferred B/S ratio is in excess of the prediction of primary boron production from spallation processes. Possible sources of contamination are discussed, as well as physical effects that could impact the observed ratios. However taken at face value, the implication of these measurements suggest potentially higher cosmic ray fluxes in DLAs. The prospects for future boron detections in other high redshift DLAs to confirm our results is also discussed.

THE NATURE OF DAMPED Lyα SYSTEMS AND THEIR HOSTS IN THE STANDARD COLD DARK MATTER UNIVERSE

Using adaptive mesh-refinement cosmological hydrodynamic simulations with a physically motivated supernova feedback prescription we show that the standard cold dark matter model can account for extant observed properties of damped Lyman alpha systems (DLAs). We then examine the properties of DLA host galaxies. We find: (1) While DLA hosts roughly trace the overall population of galaxies at all redshifts, they are always gas rich. (2) The history of DLA evolution reflects primarily the evolution of the underlying cosmic density, galaxy size and galaxy interactions. With higher density and more interactions at high redshift DLAs are larger in both absolute terms and in relative terms with respect to virial radii of halos. (3) The variety of DLAs at high redshift is richer with a large contribution coming from galactic filaments, created through close galaxy interactions. The portion of gaseous disks of galaxies where most stars reside makes relatively small contribution to DLA incidence at z=3-4. (4) The vast majority of DLAs arise in halos of mass M_h=10^10-10^12 Msun at z=1.6-4. At z=3-4, 20-30% of DLA hosts are Lyman Break Galaxies (LBGs). (5) Galactic winds play an indispensable role in shaping the kinematic properties of DLAs. Specifically, the high velocity width DLAs are a mixture of those arising in high mass, high velocity dispersion halos and those arising in smaller mass systems where cold gas clouds are entrained to high velocities by galactic winds. (6) In agreement with observations, we see a weak but noticeable evolution in DLA metallicity. The metallicity distribution centers at [Z/H]=-1.5 to -1 at z=3-4, with the peak moving to [Z/H]=-0.75 at z=1.6 and [Z/H]=-0.5 by z=0. (7) The star formation rate of DLA hosts is concentrated in the range 0.3-30Msun/yr at z=3-4, gradually shifting lower to peak at ~0.5-1 Msun/yr by z=0.

DIRECT EVIDENCE OF COLD GAS IN DLA 0812+32B

We present the first direct evidence for cold gas in a high redshift DLA galaxy. We measured several multiplets of weak neutral carbon (CI) transitions in order to perform a curve of growth analysis. A delta chi-squared test constrains the best fit Doppler parameter, b = 0.33_{-0.04}^{+0.05} km/s, and logN(CI) = 13.30 +- 0.2 cm^-2. This Doppler parameter constrains the kinetic temperature of the gas to T <= 78 K (T <= 115 K, 2 sigma). We used the associated CI fine structure lines to constrain the volume density of the gas, n(HI) ~ 40 - 200 cm^-3 (2 sigma), resulting in a lower limit on the cloud size of approximately 0.1 - 1 parsec. While it is difficult to determine the metallicity of the cold component, the absence of Cr II indicates that the cold cloud suffers a high level of dust depletion. Additionally, the large amount of Lyman and Werner-band molecular hydrogen absorption (log N(H2)_{total} = 19.88 cm^-2, f_{H_2} >= 0.06) with an excitation temperature of T_{ex} = 46 K as determined by the rotational J = 0 and J = 1 states, is consistent with the presence of cold gas. We propose that this cloud may be gravitationally confined and may represent a transition gas-phase from primarily neutral atomic gas, to a colder, denser molecular phase that will eventually host star formation.

C, N, O abundances in the most metal-poor damped Lyman alpha systems

This study focuses on some of the most metal-poor damped Lyα (DLA) absorbers known in the spectra of high-redshift QSOs, using new and archival observations obtained with ultraviolet-sensitive echelle spectrographs on the Keck and VLT telescopes. The weakness and simple velocity structure of the absorption lines in these systems allow us to measure the abundances of several elements, and in particular those of C, N and O, a group that is difficult to study in DLAs of more typical metallicities. We find that when the oxygen abundance is less than ∼1/100 of solar, the C/O ratio in high-redshift DLAs and sub-DLAs matches that of halo stars of similar metallicity and shows higher values than expected from galactic chemical evolution models based on conventional stellar yields. Furthermore, there are indications that at these low metallicities the N/O ratio may also be above simple expectations and may exhibit a minimum value, as proposed by Centurion and her collaborators in 2003. Both results can be interpreted as evidence for enhanced production of C and N by massive stars in the first few episodes of star formation, in our Galaxy and in the distant protogalaxies seen as QSO absorbers. The higher stellar yields implied may have an origin in stellar rotation which promotes mixing in the stars' interiors, as considered in some recent model calculations. We briefly discuss the relevance of these results to current ideas on the origin of metals in the intergalactic medium and the universality of the stellar initial mass function.

First Detection of NaiD Lines in High‐Redshift Damped Lyα Systems

An NIR (1.18–1.35 � m) high-resolution spectrum of the gravitationally lensed QSO APM 08279+5255 was obtained with the Subaru Telescope IRCS using the AO system. We have detected strong Na i D kk5891, 5897 doublet absorption in high-redshift DLAs at zabs ¼ 1:062 and 1.181, confirming the presence of Na i ,w hich was firstreportedfortherest-frameUVNaikk3303.3,3303.9doubletbyPetitjeanetal.ThisisthefirstdetectionofNa i D absorption in a high-redshift (z > 1)DLA. We have also detected a new Nai component in thezabs ¼ 1:062DLA and four new components in the zabs ¼ 1:181 DLA. Using an empirical relationship between Na i and H i column density,wefoundthatallcomponentshavelargeHicolumndensity( logNH i ½cm � 2 � k20:3),soeachcomponentis classified as DLA absorption. We also detected strong Na i D absorption associated with a Mg ii system at zabs ¼ 1:173. Because no other metal absorption lines were detected in this system at the velocity of the Na i absorption in previously reported optical spectra, we infer that this Na i absorption cloud probably appeared in the line of sight toward the QSO after the optical observation. This newly found cloud is likely to be a DLA based on its large estimated H i column density. We found that the NNa i/NCa ii ratios in these DLAs are systematically smaller than those observed in the Galaxy; they are more consistent with the ratios seen in the LMC. This is consistent with dust depletion generally being smaller in lower metallicity environments. However, all five clouds of the zabs ¼ 1:181 system have a high NNa i/NCa ii ratio, which is characteristic of cold dense gas. We tentatively suggest that the host galaxy of this system may be the most significant contributor to the gravitational lens toward APM 08279+5255. Subject headings: galaxies: formation — gravitational lensing — intergalactic medium — quasars: absorption lines — quasars: individual (APM 08279+5255)

A Comparison of Absorption‐ and Emission‐Line Abundances in the Nearby Damped Lyα Galaxy SBS 1543+593

We have used STIS aboard HST to measure a sulfur abundance of [S/H] = -0.41 ± 0.06 in the ISM of the nearby DLA galaxy SBS 1543+593. A direct comparison between this QSO absorption line abundance and abundances measured from H II region emission line diagnostics yields the same result: the abundance of sulfur in the neutral ISM is in good agreement with that of oxygen measured in an H II region 3 kpc away. Our result contrasts with those of other recent studies that have claimed order-of-magnitude differences between H I (absorption) and H II (emission) region abundances. We also derive a nickel abundance of [Ni/H] < -0.81, some 3 times less than that of sulfur, and suggest that the depletion is due to dust, although we cannot rule out an overabundance of α-elements as the cause of the lower metallicity. It is possible that our measure of [S/H] is overestimated if some S II arises in ionized gas; adopting a plausible star formation rate for the galaxy along the line of sight and a measurement of the C II* λ1335.7 absorption line detected from SBS 1543+593, we determine that the metallicity is unlikely to be smaller than we derive by more than 0.25 dex. We estimate that the cooling rate of the cool neutral medium is log[lc(ergs s-1 H atom-1)] ≈ -27.0, the same value as that seen in the high-redshift DLA population.

Emission-Line Spectroscopy of Damped Lyα Systems: The Case of SBS 1543+593/HS 1543+5921

We report HST/STIS spectroscopy and Gemini/GMOS-N imaging of the Damped Lyα (DLA) system toward HS 1543+5921 caused by the host star-forming galaxy (SFG) SBS 1543+593. The Gemini image shows new morphological details of this well-resolved DLA galaxy. In combination with previous optical spectra, the new UV spectra enable us to compare, for the first time, ionized and neutral gas-phase α-element abundances derived from emission- and absorption-line spectroscopy, in a bona fide DLA galaxy. The abundances we determine using emission-line diagnostics agree with those from absorption-line diagnostics. We present our results on a metallicity versus redshift diagram that combines local H II regions and SFGs with high-redshift DLAs, and discuss implications for the chemical evolution of galaxies.

Spin temperatures and covering factors for H i 21-cm absorption in damped Lymanαsystems

We investigate the practice of assigning high spin temperatures to damped Lyman a absorption systems (DLAs) not detected in H 121-cm absorption. In particular, Kanekar & Chengalur have attributed the mix of 21-cm detections and non-detections in low-redshift (z abs ≤ 2.04) DLAs to a mix of spin temperatures, while the non-detections at high redshift were attributed to high spin temperatures. Below z abs = 0.9, where some of the DLA host galaxy morphologies are known, we find that 21-cm absorption is normally detected towards large radio sources when the absorber is known to be associated with a large intermediate (spiral) galaxy. Furthermore, at these redshifts, only one of the six 21-cm non-detections has an optical identification and these DLAs tend to lie along the sight-lines to the largest background radio continuum sources. For these and many of the high-redshift DLAs occulting large radio continua, we therefore expect covering factors of less than the assumed/estimated value of unity. This would have the effect of introducing a range of spin temperatures considerably narrower than the current range of ΔT s ≥ 9000 K, while still supporting the hypothesis that the high-redshift DLA sample comprises a larger proportion of compact galaxies than the low-redshift sample.

Abundances and kinematics of a candidate sub-damped Lymanα galaxy toward PHL 1226

The spectrum of the quasar PHL 1226 is known to have a strong Mg II and sub-damped Lyman-alpha (sub-DLA) absorption line system with N(HI)=(5+/-2)*10^19 cm^-2 at z=0.1602. Using integral field spectra from the Potsdam Multi Aperture Spectrophotometer (PMAS) we investigate a galaxy at an impact parameter of 6".4 which is most probably responsible for the absorption lines. A fainter galaxy at a similar redshift and a slightly larger distance from the QSO is known to exist, but we assume that the absorption is caused by the more nearby galaxy. From optical Balmer lines we estimate an intrinsic reddening consistent with 0, and a moderate star formation rate of 0.5 M_sun yr^-1, is inferred from the Halpha luminosity. Using nebular emission line ratios we find a solar oxygen abundance 12+log(O/H)=8.7+/-0.1 and a solar nitrogen to oxygen abundance ratio log(N/O)=-1.0+/-0.2. This abundance is larger than those of all known sub-DLA systems derived from analyses of metal absorption lines in quasar spectra. On the other hand, the properties are compatible with the most metal rich galaxies responsible for strong Mg II absorption systems. These two categories can be reconciled if we assume an abundance gradient similar to local galaxies. Under that assumption we predict abundances 12+log(O/H)=7.1 and log(N/O)=-1.9 for the sub-DLA cloud, which is similar to high redshift DLA and sub-DLA systems. We find evidence for a rotational velocity of ~200 km s^-1 over a length of ~7 kpc. From the geometry and kinematics of the galaxy we estimate that the absorbing cloud does not belong to a rotating disk, but could originate in a rotating halo.

Detection of Lyman-α emission from a DLA galaxy: Possible implications for a luminosity-metallicity relation at z = 2–3

In an ongoing programme to identify a sample of high z DLA galaxies we have found the long sought for of aL yα emitter seen in the centre of a broad DLA trough. This is the predicted textbook case of an intervening DLA galaxy if DLA galaxies are small, but would not be expected if intervening high redshift DLA galaxies have large gaseous disks. The Lyα flux is 5.4 +2 −0.8 × 10 −17 erg s −1 cm −2 similar to what has been found in previously known high z DLA galaxies. The impact parameter is found to be 0. �� 3 ± 0. �� 3. This is smaller than what was found in previous cases but still consistent with random sight- lines through absorbers with mean impact parameter ≈1 �� . Of the 24 DLAs targeted in the NICMOS imaging survey five have now been identified as Lyα emitters. The DLA galaxies with detected Lyα emission tend to have higher interstellar metallicities than those with undetected Lyα emission. This is plausibly explained as a consequence of a positive correlation between the Lyα line luminosities of the galaxies and their metallicities, although the present sample is too small for a definitive conclusion. The available observations of high-redshift DLA galaxies are also consistent with a negative correlation between Lyα equiv- alent widths and metallicities, as seen in nearby star-forming galaxies and usually attributed to the preferential absorption of Lyα photons by dust grains.

The HiiRegions of the Damped Lyα Absorber SBS 1543+593

We report new imaging and spectroscopic observations of the damped Ly? absorber (DLA) galaxy SBS 1543+593, a nearby dwarf galaxy whose stellar disk is intersected by the sight line to the bright background QSO HS 1543+5921. Hubble Space Telescope imaging observations with WFPC2 in the F450W and F702W bands are used to measure the DLA galaxy's properties and compile a catalog of its (candidate) H II regions. Ground-based long-slit spectroscopy of the brightest H II region in the galaxy yields estimates of the star formation rate (SFR) and chemical abundances in the galaxy's interstellar medium. We find that SBS 1543+593 exhibits an SFR of ?0.006 h M? yr-1, or an SFR per unit area of ?1.4 ? 10-4 h M? yr-1 kpc-2. We derive gas-phase abundances in the ionized gas of 12 + log (O/H) = 8.2 ? 0.2, which is about of the solar value, and log (N/O) = -1.40. These values are consistent with the morphological appearance of SBS 1543+593, an Sm dwarf of MB - 5 log h70 = -16.8 ? 0.2 and intermediate surface brightness. SBS 1543+593 is the first bona fide DLA for which abundances have been measured using emission-line diagnostics. When compared with future high-resolution ultraviolet spectroscopy, our results should prove key for interpreting abundance determinations in high-redshift DLAs.

A deep search for 21-cm absorption in high redshift damped Lyman-αsystems

We present deep GMRT 21-cm absorption spectra of 10 damped Lyman- systems (DLAs), of which 8 are at redshifts z> 1:3. HI absorption was detected in only one DLA, the z= 0:5318 absorber toward PKS 1629+12. This absorber has been identified with a luminous spiral galaxy; the spin temperature limit (Ts 310 K) derived from our observations continues the trend of DLAs associated with bright spirals having low spin temperatures. In seven of the remaining 9 systems, the observations place strong lower limits on the spin temperature of the HI gas. We combine this sample with data taken from the literature to study the properties of all known DLAs with 21-cm absorption studies. The sample of DLAs which have been searched for 21-cm absorption now consists of 31 systems, with Ts estimates available in 24 cases; of these, 16 are at z 2, with 11 (all at z 1000 K) have been identified either with LSBs or dwarfs. Further, we find no correlation between impact parameter and spin temperature; it is thus unlikely that the high measured Ts values for DLAs arise from lines of sight passing through the outskirts of large disk galaxies. Instead, the spin temperature of DLAs appears to correlate with the host galaxy type. The trend (noted earlier by Chengalur & Kanekar 2000) that low z DLAs exhibit both high and low Ts values while high redshift (z> 3) DLAs only show high spin temperatures is present in this expanded data set. Based on this dierence in spin temperatures, the Gehan test rules out the hypothesis that DLAs at z> 2 and DLAs at z 3. For local spirals, fCNM 0:5; in contrast, we find that fCNM< 0: 3i n all 7h ighz absorbers, and fCNM< 0: 1i n 5o f the 7c ases.

Are High‐Redshift Damped Lyα Galaxies Lyman Break Galaxies?

We use deep HST STIS and NICMOS images of three spectroscopically confirmed galaxy counterparts of high-redshift damped Ly? (DLA) absorbers (one of which is a new discovery) to test the hypothesis that high-redshift DLA galaxies are Lyman break galaxies. If this hypothesis is correct, the emission properties of DLA galaxies must lie within the range of emission properties measured for Lyman break galaxies of similar absolute magnitude. This will be true regardless of selection biases in the sample of detected DLA galaxies. We test this prediction using several emission properties: half-light radius, radial profile (S?rsic n-parameter), optical to near-infrared color, morphology, Ly? emission equivalent width, and Ly? emission velocity structure. In all cases the measured values for the DLA galaxies lie within the range measured for the population of Lyman break galaxies. None of the measurements is in conflict with the prediction. We conclude that the measured emission properties of the three DLA galaxies studied here are consistent with the conjecture that high-redshift DLA galaxies are Lyman break galaxies. We show that this result does not conflict with the observation that the few high-redshift DLA galaxies discovered are mostly fainter than spectroscopically confirmed L* Lyman break galaxies.