Damped Lyman-α Systems Research Articles

H2-Bearing Damped Lyman-α Systems as Tracers of Cosmological Chemical Evolution

The chemical abundances in damped Lyman-α systems (DLAs) show more than 2 orders of magnitude variation at a given epoch, possibly because DLAs arise in a wide variety of host galaxies. This could significantly bias estimates of chemical evolution. We explore the possibility that DLAs in which H2 absorption is detected may trace cosmological chemical evolution more reliably since they may comprise a narrower set of physical conditions. The 9 known H2 absorption systems support this hypothesis: metallicity exhibits a faster, more well-defined evolution with redshift than in the general DLA population. The dust-depletion factor and, particularly, H2 molecular fraction also show rapid increases with decreasing redshift. We comment on possible observational selection effects which may bias this evolution. Larger samples of H2-bearing DLAs are clearly required and may constrain evolution of the UV background and DLA galaxy host type with redshift.

Perspectives in galactic chemical evolution studies

In this review I focus on a few selected topics, where recent theoretical and/or obervational progress has been made and important developments are expected in the future. They include: 1) Evolution of isotopic ratios, 2) Mixing processes and dispersion in abundance ratios, 3) Abundance gradients in the Galactic disk (and abundance patterns in the inner Galaxy), 4) The question of primary Nitrogen and 5) Abundance patterns in extragalactic damped Lyman-α systems (DLAs).

Do the fundamental constants vary in the course of cosmological evolution?

The possible cosmological variation of the proton-to-electron mass ratio μ = mp/me was estimated by measuring the H2 wavelengths in the spectra of distant quasars. We analyze high-resolution (FWHM≈7 km s−1) spectra of the two damped Lyman-α systems at redshifts zabs=2.3377 and 3.0249 observed in the spectra of the quasars Q 1232+082 and Q 0347−382, respectively. Our analysis yielded the most conservative estimate for the possible variation of μ in the past ∼ 10 Gyr, Δμ/μ = (5.7 ± 3.8) × 10−5. Since the significance of this result does not exceed 1.5σ, further observations are needed to increase the statistical significance. This is the most stringent limit on the possible cosmological variation of μ to date.

Molecular Hydrogen in high redshift Damped Lyman-α systems

I describe briefly the status of an ongoing mini-survey for molecular hydrogen in highredshift Damped Lyman-a systems using UVES at the VLT. H2 is detected in about 30% of the cases. When H2 is not detected the molecular fraction f = 2N(H2)/(2N(H2) + N(H1)) is smaller than 10−5. Therefore, most of the DLA systems arise in warm (T > 3000 K) and diffuse neutral gas embedded in a strong UV flux. The very recent detection of HD molecules in a Damped Lyman-α system at z abs = 2.337 demonstrates the possibility to discuss the high redshift chemistry.

On the Link between Disk-Galaxies and Damped Lyman-α Systems

We model the chemical and spectrophotometric evolution of the Milky Way, spiral galaxies and Low Surface Brightness Galaxies in a consistent way. Some consequences with respect to the nature of Damped Lyman-α systems (DLAs) are investigated.

Using Gravitational Lenses to Probe HI at High z

We investigate the possibility of detecting damped Lyman-α systems at high redshifts using GMRT if these clouds are magnified by intervening cluster lenses. We show that if the HI mass of the damped clouds ≥ 2 × 1010 M⊙ then they can be detected with a probability ≥ 0.2 using GMRT.

Damped Lyman alpha systems and galaxy formation models - I. The radial distribution of cold gas at highz

We investigate the properties of damped Lyman α systems (DLAS) in semi-analytic models, focusing on whether the models can reproduce the kinematic properties of low-ionization metal lines described by Prochaska & Wolfe. We explore a variety of approaches for modelling the radial distribution of the cold neutral gas associated with the galaxies in our models, and find that our results are very sensitive to this ingredient. If we use an approach based on work by Fall & Efstathiou, in which the sizes of the discs are determined by conservation of angular momentum, we find that the majority of the DLAS correspond to a single galactic disc. These models generically fail to reproduce the observed distribution of velocity widths. In alternative models in which the gas discs are considerably more extended, a significant fraction of DLAS arise from lines of sight intersecting multiple gas discs in a common halo. These models produce kinematics that fit the observational data, and also seem to agree well with the results of recent hydrodynamical simulations. Thus we conclude that cold dark matter based models of galaxy formation can be reconciled with the kinematic data, but only at the expense of the standard assumption that DLAS are produced by rotationally supported gas discs whose sizes are determined by conservation of angular momentum. We suggest that the distribution of cold gas at high redshift may be dominated by another process, such as tidal streaming caused by mergers.

Damped Lyman alpha systems and galaxy formation models - I. The radial distribution of cold gas at high z

We investigate the properties of damped Lyman α systems (DLAS) in semi-analytic models, focusing on whether the models can reproduce the kinematic properties of low-ionization metal lines described by Prochaska & Wolfe. We explore a variety of approaches for modelling the radial distribution of the cold neutral gas associated with the galaxies in our models, and find that our results are very sensitive to this ingredient. If we use an approach based on work by Fall & Efstathiou, in which the sizes of the discs are determined by conservation of angular momentum, we find that the majority of the DLAS correspond to a single galactic disc. These models generically fail to reproduce the observed distribution of velocity widths. In alternative models in which the gas discs are considerably more extended, a significant fraction of DLAS arise from lines of sight intersecting multiple gas discs in a common halo. These models produce kinematics that fit the observational data, and also seem to agree well with the results of recent hydrodynamical simulations. Thus we conclude that cold dark matter based models of galaxy formation can be reconciled with the kinematic data, but only at the expense of the standard assumption that DLAS are produced by rotationally supported gas discs whose sizes are determined by conservation of angular momentum. We suggest that the distribution of cold gas at high redshift may be dominated by another process, such as tidal streaming caused by mergers.

HI 21 cm absorption in low $\vec z$ damped Lyman-αsystems

We report Giant Metrewave Radio Telescope (GMRT) 21cm observations of two confirmed and one candidate low redshift damped Lyman-α systems (DLAS). HI absorption was detected in the two confirmed systems, at z=0.2378 and z=0.5247, with spin temperatures (T s ) of 2050 ± 200 K and 330 ± 70 K respectively. The non-detection of absorption in the third (candidate) system places an upper limit on its optical depth, which in turn implies a 3σ lower limit of 850 K on its spin temperature. Ground based K-band imaging observations (Turnshek et al. 2000) have identified the low T s absorber with a spiral galaxy while the high T s system appears to be an LSB galaxy. This continues the trend noted earlier by Chengalur and Kanekar (2000) of low spin temperatures for DLAS associated with spiral galaxies and high spin temperatures for those with no such association. Combining these new results with 21cm observations from the published literature, we find that (1) The 21 cm equivalent width (EW 21 ) of DLAS increases with increasing (N HI ). Further, absorbers associated with spiral galaxies have systematically higher EW 21 at the same N HI than DLAS with no such association, (2)DLAS at high redshift (z ≳ 2) have small velocity spreads (ΔV), while, at low redshift (z ≲ 2), both large and small ΔV are observed. These trends are in qualitative agreement with hierarchical models of galaxy formation.

ATCA search for 21 cm emission from a candidate damped Ly-αabsorber at ${\vec z} = 0.101$

We report a deep search for 21 cm emission/absorption from the z ~ 0.101 candidate damped Lyman-α system towards PKS 0439-433, using the Australia Telescope Compact Array (ATCA). The spectrum shows a weak absorption feature -at the 3.3σ level -which yields a lower limit of 730 K on the spin temperature of the system. No H I emission was detected: the upper limit on the H I mass of the absorber is 2.25 10 9 M O , for a velocity spread of ~70 km s −1 . The low H I mass and the high spin temperature seem to rule out the possibility that the absorber is a large gas-rich spiral galaxy.

Metals at high redshifts

The amount of metals present in the Universe and its cosmological evolution is a key issue for our understanding of how star formation proceeds from the collapse of the first objects to the formation of present day galaxies. We discuss here recent results at the two extremes of the density scale.1. Part of the tenuous intergalactic medium (IGM) revealed by neutral hydrogen absorptions in the spectra of remote quasars (the so-called Lyman-α forest) contains metals. This is not surprising as there is a close interplay between the formation of galaxies and the evolution of the IGM. The IGM acts as the baryonic reservoir from which galaxies form, while star formation in the forming galaxies strongly influences the IGM by enrichment with metals and the emission of ionizing radiation. The spatial distribution of metals in the IGM is largely unknown however. The possibility remains that metals are associated with the filaments and sheets of the dark matter spatial distribution where stars are expected to form, whereas the space delineated by these features remains unpolluted.2. Damped Lyman-α (DLA) systems observed in the spectra of high-redshift quasars are considered as the progenitors of present-day galaxies. Indeed, the large neutral hydrogen column densities observed and the presence of metals imply that the gas is somehow closely associated with regions of star formation. The nature of the absorbing objects is unclear however. It is probable that very different objects contribute to this population of absorption systems. Here we concentrate on summarizing the properties of the gas: presence of dust in small amount; nucleosynthesis signature and lack of H2 molecules. The presence of H2 molecules has been investigated in the course of a mini-survey with UVES at the VLT. The upper limits on the molecular fraction,f = 2N(H2)/(2N(H2)+N(HI)),derived in eight systems are in the range 1.2 ×10-7 - 1.6 × 10-5. There is no evidence in this sample for any correlation between H2 abundance and relative heavy element depletion into dust grains. The molecular abundance in a few DLA systems (and in particular in the two systems where H2 is detected) is consistent with what is seen in the Magellanic clouds, but most of the DLA measurements are well below these values. This is probably partly due to small amounts of dust and/or high UV flux. We argue however that the lack of molecules is a direct consequence of high kinetic temperature (T > 3000 K) implying a low formation rate of H2 onto dust grains. The conclusion is that most of the DLA systems arise in warm and diffuse neutral gas.

Star Formation and Chemical Evolution of Damped Ly-α Systems

In this paper, we investigate the star formation and chemical evolution of damped Lyman-α systems (DLAs) based on the disk galaxy formation model developed by H. J. Mo, S. Mao, & S. D. M. White (1998, MNRAS, 295, 319). We propose that the DLAs are the central galaxies of less massive dark haloes present at redshifts z ~ 3, and that they should inhabit haloes of moderately low circular velocity. We adopt the empirical Schmidt law of star formation rates, and a closed box model of chemical evolution in which an approximation known as instantaneous recycling is assumed. In calculating the predicted distribution of metallicities for DLAs in our models, two cases are considered. One is that, using the closed box model, empirical Schmidt law, and star formation epoch, the distribution of metallicity can be directly calculated. The other is that, when the simple gravitational instability of a thin isothermal gas disk first discussed by A. Toomre (1964, ApJ, 139, 1217) is considered, star formation occurs only in the region where the surface density of gas satisfies the critical value — rather than everywhere in the gas disk. We first obtain the region where star formation can occur by assuming that the disk has a flat rotation curve and that the rotational velocity is equal to the circular velocity of the surrounding dark matter halo. We then calculate the metallicity distribution for case one. We assume that star formation in each DLA lasts for a period of 1 Gyr from redshifts z = 3. There is only one output parameter in our model, i.e., the stellar yield, which relates to the epoch of star formation. It is obtained by normalizing the predicted distribution of metallicity to the mean value of 1/13 Z⊙ as presented by M. Pettini, L. J. Smith, D. L. Kind, & R. W. Hunstead (1997, ApJ, 486, 665). The predicted metallicity distribution is consistent with current (rather limited) observational data. A random distribution of galactic disks is taken into account.

Damped Lyman-α Systems

Damped Lyman-α systems observed in the spectra of high-redshift quasars are considered as the progenitors of present-day galaxies.Indeed, the large neutral hydrogen column densities observed and the presence of metals imply that the gas is somehow closely associated with regions of star formation. The nature of the absorbing object is unclear however.The discussion of whether high-redshift damped Lyman-α systems are produced throughlarge, fast-rotating, thick disks or through building-blocks of galaxies is important since it is related to how present-day galaxies form, either through initial formation of large disks and subsequent accretion of gas or as a result of merging of pregalactic clumps.We argue here that damped Lyman-α systems are progenitors of any kind of galaxy (dwarfs and large spirals); they trace peaks in the spatial density distribution of the gas.

Identification of Two z ~ 3·8 QSOs in a Deep CCD Survey

AbstractWe present the identifications of two z ~ 3·8 quasars from a deep UBI imaging survey with the Palomar 5·1 m Telescope. The survey covers an area of 0·25 degree2 around a sample of ten z =0·2–0·3 luminous X-ray clusters. The QSOs were identified on the basis of their stellar morphologies, relatively blue optical and very red UV–optical colours. The two objects are Q 1322+5034, with total magnitudes of B =20·8, I =18·3 and (U–B) > 4·7, and Q 1722+3211, which has total magnitudes of B =21·8, I =19·7 and (U–B) > 3·2. Subsequent spectroscopic observations with the 4·2 m William Herschel Telescope have confirmed the identity of these two sources as QSOs at z =3·82 and z =3·73 respectively. Our spectroscopic observations identify a damped Lyman-α absorber in the spectrum of Q 1322+5034 at z =3·439, as well as a second absorption system at z =2·700, which may be either a single very high column density, damped Lyman-α system, or more likely a blend of a number of high column-density absorbers spread over a distance of ~10 Mpc.

Identification of a galaxy responsible for a high-redshift Lyman-α absorption system

DAMPED Lyman-α systems are high-column-density intergalactic clouds of hydrogen, the existence of which is inferred from absorption lines appearing in the emission spectra of distant quasars. The galaxies believed to be responsible for these absorption systems have been suggested as possible progenitors of the normal disk galaxies observed in the local Universe. Indeed, Lyman-α systems appear to contain a substantial fraction of the baryons known to exist in galaxies today. Here we report the optical detection of a galaxy (designated DLA2233 + 131) associated with a known damped Lyman-α absorption system at a redshift of z = 3.150. The properties of this galaxy correspond closely to those expected of a young disk galaxy in the early stages of formation, and show no evidence for an active nucleus. This finding gives strong support to the idea that damped Lyman-α systems represent a population of young galaxies at high red-shifts.