Damped Lyα Systems Research Articles

Dust and hydrogen molecules in the extremely metal-poor dwarf galaxy SBS 0335–052

During the early stages of galaxy evolution, the metallicity is generally low and nearby metal-poor star-forming galaxies may provide templates for primordial star formation. In particular, the dust content of such objects is of great importance, because early molecular formation can take place on grains. To gain insight into primeval galaxies at high redshift, we examine the dust content of the nearby extremely low-metallicity galaxy SBS 0335–052 which hosts a very young starburst (≲107yr). In young galaxies, the dust formation rate in Type II supernovae governs the amount of dust, and by incorporating recent results on dust production in Type II supernovae we model the evolution of dust content. If the star-forming region is compact (≲100 pc), as suggested by observations of SBS 0335–052, our models consistently explain the quantity of dust, far-infrared luminosity, and dust temperature in this low-metallicity object. We also discuss the H2 abundance. The compactness of the region is important to H2 formation, because the optical depth of dust for UV photons becomes large and H2 dissociation is suppressed. We finally focus on implications for damped Lyα systems.

HI with the GMRT

The Giant Metrewave Radio Telescope (GMRT) has recently come into operation. This paper reports on observations of the HI line made using the GMRT during its commissioning phase. These include observations of the nearby Low Surface brightness galaxy Cepheus 1, as well as of a number of low and medium redshift damped Ly-α systems. The derived spin temperatures for these systems is typically higher than that characteristic of the gas in the disk of the Galaxy. Higher spin temperatures were known to be characteristic of high redshift damped Ly-α absorbers, and had been earlier attributed to evolutionary effects. Our observations suggest that high spin temperatures are not a consequence of evolutionary effects since the look back time to these low z absorbers is modest. We also report on the short time scale (days) variability of the absorption profile of the z = 0.3127 system in front of B1127-145. This is the second damped Ly-α system to show such variation (the other being the z = 0.524 absorber in front of AO 0235+164).

Dust formation in damped Lyα systems

A one-zone spectrophotometric and chemical evolution scenario of spiral galaxies incorporating dust formation is briefly described, and a self-consistent closed box chemical model of the gaseous disks (corrected on dust depletion) is presented. Comparison of the calculated abundances with the observed in damped Lyα systems shows first, that at least a fraction of DLA absorptions can be attributed to spiral galaxies, and that the dust is a nessasary physical ingredient of galaxy evolution model strongly affecting the observed abundance pattern and photometric properties.

ChandraDetection of X‐Ray Absorption Associated with a Damped Lyα System

We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray absorption associated with intervening 21-cm and damped Ly$\alpha$ absorbers. For PKS 1127-145, we detect absorption which, if associated with an intervening z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption is not at z_{abs}=0.312, then the metallicity is still constrained to be less than 23% solar. The advantage of the X-ray measurement is that the derived metallicity is insensitive to ionization, inclusion of an atom in a molecule, or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is consistent with the oxygen abundance of 30% solar derived from optical nebular emission lines in a foreground galaxy at the redshift of the absorber. For Q1331+171 and Q 0054+144, only upper limits were obtained, although the exposure times were intentionally short, since for these two objects we were interested primarily in measuring flux levels to plan for future observations. The imaging results are presented in a companion paper.

The UCSD HIRES/Keck I Damped Lyα Abundance Database. I. The Data

We present new chemical abundance measurements of 16 damped Lyα systems at z > 1.5 and update our previous abundance analyses. The entire database presented here was derived from HIRES observations on the Keck I telescope, reduced with the same software package, and analyzed with identical techniques. Altogether, we present a large, homogeneous database of chemical abundance measurements for protogalaxies in the early universe, ideal for studying a number of important aspects of galaxy formation. In addition, we have established an on-line directory for this database and will continuously update the results.

A New Measurement of the Primordial Abundance of Deuterium: Toward Convergence with the Baryon Density from the Cosmic Microwave Background?

From the analysis of the near-UV spectrum of the quasar (QSO)2206-199, obtained with a long series of exposures with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we deduce a value D/H = (1.65 ± 0.35) × 10-5 (1 σ error) for the abundance of deuterium in the zabs = 2.0762 damped Lyα system (DLA) along this sight line. The velocity structure of this absorber is very simple, and its neutral hydrogen column density N(H I) is accurately known; the error in D/H is mostly due to the limited signal-to-noise ratio of the spectrum. Since this is also one of the most metal-poor DLAs, with metal abundances ~1/200 of solar, the correction due to the astration of D is expected to be insignificant, and the value we deduce should be essentially the primordial abundance of deuterium. When all (six) available measurements of D/H in high-redshift QSO absorbers are considered, we find that the three DLAs, N(H I) is measured most reliably, give consistently lower values than the three Lyman limit systems. We point out that the weighted mean of the DLA measurements, D/H = (2.2 ± 0.2) × 10-5, yields a baryon density ΩBh2 = 0.025 ± 0.001, which is within ~1 σ of the value deduced from the analysis of the cosmic microwave background angular power spectrum, and is still consistent with the present-day D/H and models of Galactic chemical evolution. Future observations of D I absorption in other DLAs are needed to establish whether our finding reflects a real advantage of DLAs over other classes of QSO absorbers for the measurement of D or if it is just a statistical fluctuation.

On the Relation between High-Redshift Starburst Galaxies and Damped L[CLC]y[/CLC]α Systems

Essentially all high-redshift galaxies show evidence for strong large-scale outflows that should have a profound effect on the structure and kinematics of both the galaxies themselves and their environment. The interstellar absorption spectra of both Lyman break galaxies (LBGs) and local starburst galaxies are remarkably similar to those of damped Lyα (DLA) systems in QSO spectra. Their rest-frame UV spectra typically show broad, blueshifted Lyα absorption accompanied by low-ionization metallic absorption lines with complex profiles. Hydrodynamical simulations of galactic winds suggest that the absorption arises in a collection of dense clouds entrained in the hot wind and/or in shells swept up by the outflow. It seems likely that outflows would also give rise to DLA absorption when seen in absorption against a background QSO. It is emphasized that some differences are expected between the properties of DLA systems in the emission-weighted galaxy spectra, which always probe the centers of star formation, and those in random sight lines through the outflow. The observed LBGs alone can account for all DLA absorption at z ~ 3 if the cross section for DLA absorption is πr2 with r = 19 h-1 kpc. If the cross section is smaller than this, then the fraction of DLA systems arising in outflows could still be significant if there are many wind-driving galaxies below the current detection limits. This is certainly possible since the z = 3 luminosity function is still rising down to the detection limit of 0.1L* and observations and simulations of local dwarf galaxies indicate that many of these fainter galaxies drive winds as well.

Galaxy Number Counts in the Hubble Deep Field as a Strong Constraint on a Hierarchical Galaxy Formation Model

Number counts of galaxies are reanalyzed using a semianalytic model (SAM) of galaxy formation based on the hierarchical clustering scenario. We have determined the astrophysical parameters in the SAM that reproduce observations of nearby galaxies and used them to predict the number counts and redshifts of faint galaxies for three cosmological models: (1) the standard cold dark matter (CDM) universe, (2) a low-density flat universe with nonzero cosmological constant, and (3) a low-density open universe with zero cosmological constant. The novelty of our SAM analysis is the inclusion of selection effects arising from the cosmological dimming of the surface brightness of high-redshift galaxies and also from the absorption of visible light by internal dust and intergalactic H I clouds. Contrary to previous SAM analyses that do not take into account such selection effects, we find from comparison with observed counts and redshifts of faint galaxies in the Hubble Deep Field (HDF) that the standard CDM universe is not preferred, and a low-density universe either with or without a cosmological constant is favorable, as suggested by other recent studies. Moreover, we find that a simple prescription for the timescale of star formation (SF), being proportional to the dynamical timescale of the formation of the galactic disk, is unable to reproduce the observed number-redshift relation for HDF galaxies, and that the SF timescale should be nearly independent of redshift, as suggested by other SAM analyses for the formation of quasars and the evolution of damped Lyα systems.

Ionization Properties and Elemental Abundances in Damped Lyα Systems

We analyze extant data of Al+2, Al+, and other low ions with the aim of studying the ionization properties of damped Lyα systems (DLAs) from the analysis of the ratio R(Al+2/Al+) ≡ N(Al+2)/N(Al+). We find the good correlations log N(Al+)-log N(Si+) and log N(Al+)-log N(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way, we determine the ratio R(Al+2/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that the gas of the intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe and R(Al+2/Al+) indicates that abundances are not severely influenced by ionization effects. We find a log R(Al+2/Al+)-log N(H0) anticorrelation that we use in conjunction with idealized photoionization equilibrium calculations to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of the low- and intermediate-ionization state: (1) neutral regions devoid of Al+2 and/or (2) partially ionized, Al+2-bearing regions. The log R(Al+2/Al+)-log N(H0) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements, the corrections are generally below measurements errors, which is contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies in the Al recombination rates used in the photoionization calculations.

The Implications of the NewZ = 0 Stellar Models and Yields on the Early Metal Pollution of the Intergalactic Medium

Motivated by the recent detection of metals in different components of the high-redshift universe and by the abundance ratios measured in the extremely metal-poor stars of our Galaxy, we study the nucleosynthesis constraints that this imposes on an early generation of stars (Population III). To do so we take into account the chemical yields obtained from homogeneous evolutionary calculations of zero-metal stars in the mass range 3 m/M☉ 40 (Limongi et al.; Chieffi et al.). We also consider the role played by metal-free very massive objects (m > 100 M☉). Using both analytical and numerical chemical evolution models, we confront model predictions from the different choices of the mass function proposed for Population III with the observational constraints. We show that low values of star formation efficiency (<1%) are required so as not to exceed the minimum metallicity ([C/H] ≈ -2.4) measured in the high-redshift systems for any of the initial mass functions (IMFs) proposed. We also show that the observational constraints require Ωsr < 3 × 10-3Ωb, confirming previous claims that the possible contribution of the stellar remnants from Population III to the baryonic dark matter is insignificant. At present, however, the scarcity of abundance measurements for high-redshift systems does not permit us to put severe limitations on the nature of the initial mass function for Population III. In fact, overabundances of α-elements with respect to iron of the order of those measured in damped Lyα systems are obtained for any of the IMFs tested. Nevertheless, to account for the very large [C, N/Fe] ratios found in a considerable number of extremely metal-poor stars of our Galaxy, an IMF peaking at the intermediate stellar mass range (4-8 M☉) is needed.

Are Simulations of Cold Dark Matter Consistent with Galactic-Scale Observations at High Redshift?

We compare new observations on the kinematic characteristics of the damped Lyα systems against results from numerical smoothed particle hydrodynamics (SPH) simulations to test the predictions of hierarchical galaxy formation. This exercise is particularly motivated by recent numerical results on the cross section of damped Lyα systems. Our analysis focuses on the velocity widths Δv of ≈50 low-ion absorption profiles from our sample of z > 1.5 damped Lyα systems. The results indicate that current numerical simulations fail to match the damped Lyα observations at high confidence levels (>99.9%). Although we do not believe that our results present an insurmountable challenge to the paradigm of hierarchical cosmology, the damped Lyα observations suggest that current numerical SPH simulations overlook an integral aspect of galaxy formation.

Galactic Chemical Abundances atz &gt; 3. I. First Results from the Echellette Spectrograph and Imager

We present the first results from an ongoing survey to discover and measure the metallicity of z > 3 damped Lyα systems with the Echellette Spectrograph and Imager (ESI) on the Keck II telescope. Our motivation arises from a recent study on damped Lyα systems suggesting only mild evolution in the cosmic metallicity from z ~ 2 to 4. The Echellette Spectrograph and Imager, which provides two complementary spectroscopic modes, is the ideal instrument for a z > 3 damped Lyα survey. We describe our observing strategy and report on the discovery and analysis of five new z > 3 damped Lyα systems acquired in a single night of observing. These observations further support the principal conclusions of the previous study: (1) the cosmic metallicity in neutral gas inferred from the damped Lyα systems does not evolve significantly from z ~ 2 to 4; (2) the unweighted metallicity exhibits a statistically significant decrease with increasing redshift; and (3) not a single damped Lyα system has a metallicity below [Fe/H] = -3. We discuss the implications of these results and comment on recent theoretical studies that attempt to explain the observations.

Globular Cluster Systems. II. On the Formation of Old Globular Clusters and Their Sites of Formation

We studied the metal-poor globular cluster populations of a large variety of galaxies and compared their mean metallicity with the properties of the host galaxies. For this purpose, we constructed a comprehensive database of old metal-poor globular cluster populations, hosted by 47 galaxies, spanning about 10 mag in absolute brightness. The mean metallicities of the systems are found to be very similar and lie in the -1.65 < [Fe/H] ≤ -1.20 range (74% of the population). Using only globular cluster systems with more than six objects detected, we find that 85% of the population are within -1.65 < [Fe/H] ≤ -1.20. The relation between the mean metallicity of the metal-poor globular cluster systems and the absolute V magnitude of their host galaxies presents a very low slope that includes zero. An analysis of the correlation of the mean metallicity of the populations with other galaxy properties (such as velocity dispersion, metallicity, and environment density) also leads to the conclusion that no strong correlation exists. The lack of correlation with galaxy properties suggests a formation of all metal-poor globular clusters in very similar gas fragments. A weak correlation (to be confirmed) might exist between the mean metallicity of the metal-poor clusters and the host galaxy metallicity. This would imply that at least some fragments in which metal-poor globular clusters formed were already embedded in the larger dark matter halo of the final galaxy (as opposed to being independent satellites that were accreted later). Our result suggests a homogeneous formation of metal-poor globular clusters in all galaxies in typical fragments of masses around 109–1010 M⊙ with very similar metallicities, compatible with hierarchical formation scenarios for galaxies. We further compare the mean metallicities of the metal-poor globular cluster populations with the typical metallicities of high-redshift objects. If we add the constraint that globular clusters need a high column density of gas to form, damped Lyα systems are the most likely sites among the known high-redshift objects for the formation of metal-poor globular cluster populations.

Absorption Systems in the Spectra of 66 [CLC]z[/CLC] ≳ 4 Quasars

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

Hα Imaging withHubble Space Telescope‐NICMOS of an Elusive Damped Lyα Cloud atz = 0.6

Despite previous intensive ground-based imaging and spectroscopic campaigns and wideband Hubble Space Telescope (HST) imaging of the z = 0.927 QSO 3C 336 field, the galaxy that hosts the damped Lyα system along this line of sight has eluded detection. We present a deep narrowband Hα image of the field of this zabs = 0.656 damped Lyα absorber, obtained through the F108N filter of NICMOS1 on board the HST. The goal of this project was to detect any Hα emission 10 times closer than previous studies to unveil the damped absorber. We do not detect Hα emission between 005 and 6'' (0.24 and 30 h-1 kpc) from the QSO, with a 3 σ flux limit of 3.70 × 10-17 h-2 ergs s-1 cm-2 for an unresolved source, corresponding to a star formation rate (SFR) of 0.3 h-2 M☉ yr-1. This leads to a 3 σ upper limit of 0.15 M☉ yr-1 kpc-2 on the SFR density, or a maximum SFR of 1.87 M☉ yr-1 assuming a disk of 4 kpc in diameter. This result adds to the number of low-redshift damped Lyα absorbers that are not associated with the central regions of Milky Way-like disks. Damped Lyα absorption can arise from high-density concentrations in a variety of galactic environments including some that, despite their high local H I densities, are not conducive to widespread star formation.

Sharp HiEdges at Highz: The Gas Distribution from Damped Lyα to Lyman Limit Absorption Systems

We derive the distribution of neutral and ionized gas in high-redshift clouds, which are optically thick to hydrogen ionizing radiation, using published data on Lyman limit and damped Lyα absorption systems in the redshift range 1.75 ≤ z < 3.25. We assume that the distribution of the hydrogen total (H I+H II) column density in the absorbers, NH, follows a power law KN, whereas the observed H I column density distribution deviates from a pure power law as a result of ionization from a background radiation field. We use an accurate radiative transfer code for computing the rapidly varying ratio NH/N as a function of NH. Comparison of the models and observations gave excellent fits with maximum likelihood solutions for the exponent α and X, the value of log when the Lyman limit optical depth along the line of sight is τLL = 1. The slope of the total gas column density distribution with its relative 3 σ errors is α = 2.7 and X = 2.75 ± 0.35. This value of X is much lower than what would be obtained for a gaseous distribution in equilibrium under its own gravity. The ratio η0 of dark matter to gas density, however, is not well constrained since log (η0) = 1.1 ± 0.8. An extrapolation of our derived power-law distribution toward systems of lower column density, the Lyα forest, tends to favor models with log(η0) 1.1 and α ~ 2.7-3.3. With α appreciably larger than 2, Lyman limit systems contain more gas than damped Lyα systems and Lyα forest clouds even more. Estimates of the cosmological gas and dark matter density due to absorbers of different column density at z ~ 2.5 are also given.

On Ionization Effects and Abundance Ratios in Damped Lyα Systems

The similarity between observed velocity structures of Al III and singly ionised species in damped Lyα systems (DLAs) suggests the presence of ionised gas in the regions where most metal absorption lines are formed. To explore the possible implications of ionisation effects we construct a simplified two-region model for DLAs consisting of an ionisation bounded region with an internal radiation field and a neutral region with a lower metal content. Within this framework we find that ionisation effects are important. If taken into account, the element abundance ratios in DLAs are quite consistent with those observed in Milky Way stars and in metal-poor H II regions in blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. From our models no dust depletion of heavy elements needs to be invoked; little depletion is however not excluded.

An Imaging and Spectroscopic Study of thezabs = 3.38639 Damped Lyα System in Q0201+1120: Clues to Star Formation at High Redshift

We present the results of a series of imaging and spectroscopic observations aimed at identifying and studying the galaxy responsible for the z = 3.38639 damped lya system in the z = 3.61 QSO Q0201+1120. We find that the DLA is part of a concentration of matter which includes at least four galaxies (probably many more) over linear comoving dimensions, greater than 5h^-1Mpc. The absorber may be a 0.7 L* galaxy at an impact parameter of 15 h^-1 kpc, but follow-up spectroscopy is still required for positive identification. The gas is turbulent, with many absorption components distributed over approximately 270 km/s and a large spin temperature, T_s greater than 4000K. The metallicity is relatively high for this redshift, Z(DLA) approximately 1/20 Z(solar). From consideration of the relative ratios of elements which have different nucleosynthetic timescales, it would appear that the last major episode of star formation in this DLA occurred at z greater than 4.3, more than approximately 500 Myr prior to the time when we observe it.

UVES Observations of QSO 0000−2620: Argon and Phosphorus Abundances in the Dust‐free Damped Lyα System atzabs = 3.3901

The UV resonance transitions of neutral argon, Ar I j1066, and of singly ionized phosphorus, P II j963, originated in the damped Lya system (DLA) at toward QSO 0000[2620, have been z abs 3.3901 detected by means of the Ultraviolet-Visual Echelle Spectrograph (UVES) spectrograph at the 8.2 m ESO KUEYEN telescope. So far, this is the —rst measurement of Ar I and the second of P II ever performed in damped galaxies and in high-redshift objects. This DLA is well known for having one of the lowest metal abundances and dust content and the lowest fractional abundance of molecular hydrogen The measured Ar abundance is [Ar/H] \[ 1.91 ^ 0.09, which is equal to the abundances of the H 2 . other a-chain elements (O, S, and Si). The similarity of the Ar abundance with the other a-chain elements implies the absence of signi—cant photoionization by either UV background or stellar sources along the sight line throughout the damped Lya system. Both log (Ar/O) and log (Ar/S) ratios are found close to those measured in the extragalactic H II regions and in blue compact galaxies where O is more abundant by at least 1 order of magnitude. This strengthens the universality of the Ar/O and Ar/S ratios and lends support to the existence of a universal IMF. The abundance of the nonrefractory element phosphorus [P/H] \[ 2.31 ^ 0.10 con—rms the low amount of chemical evolution in the DLA. This is the measurement of P in the most metal-poor material and shows a subsolar [P/Fe] \[ 0.27 value. The measured ratios [P/Si] \[ 0.40 ^ 0.13 and [P/S] \[ 0.40 ^ 0.13 provide evidence for a mild odd-even eUect. Finally, a stringent upper limit to the population of the level in the ground state of O I is 3P 1 derived, which provides a lower limit to the physical dimensions of the system of L [ 7p c. z abs 3.3901

The Rest‐Frame Optical Spectra of Lyman Break Galaxies: Star Formation, Extinction, Abundances, and Kinematics

We present the first results of a spectroscopic survey of Lyman break galaxies (LBGs) in the near-infrared aimed at detecting the emission lines of [O II], [O III], and Hβ from the H II regions of normal star-forming galaxies at z 3. From observations of 15 objects with the Keck telescope and the Very Large Telescope augmented with data from the literature for an additional four objects, we reach the following main conclusions. The rest-frame optical properties of LBGs at the bright end of the luminosity function are remarkably uniform, their spectra are dominated by emission lines, [O III] is always stronger than Hβ and [O II], and projected velocity dispersions are between 50 and 115 km s-1. Contrary to expectations, the star formation rates deduced from the Hβ luminosity are on average no larger than those implied by the stellar continuum at 1500 A; presumably any differential extinction between rest-frame optical and UV wavelengths is small compared to the relative uncertainties in the calibrations of these two star formation tracers. For the galaxies in our sample, the abundance of oxygen can only be determined to within 1 order of magnitude without recourse to other emission lines ([N II] and Hα), which are generally not available. Even so, it seems well established that LBGs are the most metal-enriched structures at z 3, apart from quasi-stellar objects, with abundances greater than about 1/10 solar and generally higher than those of damped Lyα systems at the same epoch. They are also significantly overluminous for their metallicities; this is probably an indication that their mass-to-light ratios are low compared to present-day galaxies. At face value, the measured velocity dispersions imply virial masses of about 1010 M☉ within half-light radii of 2.5 kpc. The corresponding mass-to-light ratios, M/L ≈ 0.15 in solar units, are indicative of stellar populations with ages between 108 and 109 yr, consistent with the UV-optical spectral energy distributions. However, we are unable to establish conclusively whether or not the widths of the emission lines reflect the motions of the H II regions within the gravitational potential of the galaxies, even though in two cases we see hints of rotation curves. All 19 LBGs observed show evidence for galactic-scale superwinds; such outflows have important consequences for regulating star formation, distributing metals over large volumes, and allowing Lyman continuum photons to escape and ionize the intergalactic medium.