Damped Lyα Absorbers Research Articles

Evidence for Rotation in the Galaxy atz= 3.15 Responsible for a Damped Lyman‐Alpha Absorption System in the Spectrum of Q2233+1310

Proof of the existence of a significant population of normal disk galaxies at redshift z > 2 would have profound implications for theories of structure formation and evolution. We present evidence based on Keck HIRES spectra that the damped Lyα absorber at z = 3.15 toward the quasar Q2233+1310 may well be such an example. Djorgovski et al. have recently detected the Lyα emission from the absorber, which we assume is at the systemic redshift of the absorbing galaxy. By examining the profiles of the metal absorption lines arising from the absorbing galaxy in relation to its systemic redshift, we find strong kinematical evidence for rotation. Therefore the absorber is likely to be a disk galaxy. The inferred circular velocity for the galaxy is greater than or equal to 200 km s-1. With a separation of 17 kpc (q0 = 0.1, H0 = 75) between the galaxy and the quasar sight line, the implied dynamic mass for the galaxy is greater than or equal to 1.6 × 1011 M☉. The metallicity of the galaxy is found to be [Fe/H] = -1.4, typical of damped Lyα galaxies at such redshifts. However, in another damped Lyα absorber at z = 2.81 toward Q0528-2505, no kinematical evidence for galactic rotation is evident. In the latter case, the damped Lyα absorber occurs near the background quasar in redshift, so its properties may be influenced by the background quasar. These represent the only two cases at present for which the technique used here may be applied. Future applications of the same technique to a large sample of damped Lyα galaxies may allow us to determine if a significant population of disk galaxies already existed only a few billion years after the big bang.

The Population of Damped Lyα and Lyman Limit Systems in the Cold Dark Matter Model

Lyman limit and damped Lyα absorption systems probe the distribution of collapsed, cold gas at high redshift. Numerical simulations that incorporate gravity and gasdynamics can predict the abundance of such absorbers in cosmological models. We develop a semianalytical method to correct the numerical predictions for the contribution of unresolved low-mass halos, and we apply this method to the Katz et al. simulation of the standard cold dark matter model (Ω = 1, h = 0.5, Ωb = 0.05, and σ8 = 0.7). Using this simulation and higher resolution simulations of individual low-mass systems, we determine the relation between a halo's circular velocity vc and its cross section for producing Lyman limit or damped Lyα absorption. We combine this relation with the Press-Schechter formula for the abundance of halos—itself calibrated against the simulated halo population—in order to compute the number of absorbers per unit redshift. The resolution correction increases the predicted abundances by about a factor of 2 at z = 2, 3, and 4, bringing the predicted number of damped Lyα absorbers into quite good agreement with observations. Roughly half of these systems reside in halos with circular velocities vc ≥ 100 km s-1, and half in halos with 35 km s-1 ≤ vc ≤ 100 km s-1. Halos with vc > 150 km s-1 typically harbor two or more systems capable of producing damped absorption. Even with the resolution correction, the predicted abundance of Lyman limit systems is a factor of 3 below observational estimates, signifying either a failure of the standard cold dark matter model or a failure of these simulations to resolve most of the systems responsible for Lyman limit absorption. By comparing simulations with and without star formation, we find that the depletion of the gas supply by star formation affects absorption-line statistics at z ≥ 2 for column densities exceeding NH I = 1022 cm-2 only, even though half of the cold, collapsed gas has been converted to stars by z = 2.

Damped Lya systems at high redshift and models of protogalactic discs

We employ observationally determined intrinsic velocity widths and column densities of damped Lyman alpha (Lyα) systems at high redshift to investigate the distribution of baryons in protogalaxies within the context of a standard cold dark matter (CDM) model. We proceed under the assumption that damped Lyα systems represent a population of cold, rotationally supported, protogalactic discs, and that the abundance of dark matter haloes is well approximated by a CDM model with critical density and vanishing cosmological constant. Using conditional cross-sections to observe a damped system with a given velocity width and column density, we compare observationally inferred velocity width and column density distributions to the corresponding theoretically determined distributions for a variety of disc parameters and CDM normalizations. In general, we find that the observations cannot be reproduced by the models for most disc parameters and CDM normalizations. Whereas the column density distribution favours small discs with large neutral gas fraction, the velocity width distribution favours large and thick discs with small neutral gas fraction. The possible resolutions of this problem in the context of this CDM model may be (1) an increased contribution of rapidly rotating discs within massive dark matter haloes to damped Lya absorption, or (2) the abandoning of simple disc models within this CDM model for damped Lya systems at high redshift. Here the first possibility may be achieved by supposing that damped Lya system formation occurs only in haloes with fairly large circular velocities, and the second possibility may result from a large contribution of mergers and double discs to damped Lya absorption at high redshift.

Damped Lyα Absorber at High Redshift: Large Disks or Galactic Building Blocks?

The nature of the physical structures giving rise to damped Lyman alpha absorption systems (DLAS) at high redshifts is investigated. The proposal that rapidly rotating large disks are the only viable explanation for the observed asymmetric profiles of low ionization absorption lines is examined. Using hydrodynamic simulations of galaxy formation, it is demonstated that irregular protogalactic clumps can reproduce the observed velocity width distribution and asymmetries of the absorption profiles equally well. The velocity broadening in the simulated clumps is due to a mixture of rotation, random motions, infall and merging. The observed velocity width correlates with the virial velocity for the dark matter halo of the forming protogalactic clump. The typical virial velocity of the halos required to lead to the DLAS population is approximately 100 km/s. It is concluded that the evidence that DLAS at high redshift are related to large, rapidly rotating disks, is not compelling.

Gravitational lensing by damped Ly-α absorbers

Assuming that (i) damped Ly-α absorbers (DLAs) arise in present-day-like spiral galaxies which are immersed in isothermal dark matter halos, (ii) that these galaxies obey the Tully-Fisher σ σ ∗ = ( L L ∗ ) 1 α TF and the Holmberg R L R ∗ = ( L L ∗ ) α H relations, and (iii) that they follow the Schechter luminosity distribution, we describe how their observed number density ( dN dz ), distribution of column density ( f( N)) as well as inferred cosmological density of HI ( ω HI derived from DLA surveys are affected by gravitational lensing (GL). The ‘by-pass’ effect causes the lines-of-sight (LOSs) towards background QSOs to avoid the central parts of galaxies and reduces their effective cross-section for absorption; the ‘amplification bias’ leads observers to select QSOs whose LOSs preferentially cross galaxies close to their Einstein radius. As a consequence, the determination of the quantities dN dz , f( N) and ω HI from DLA surveys does not only depend on the redshift z and luminosity L of galaxies responsible for the absorbers but also on the column density profile of HI within the galaxies and on the redshift z q and magnitude b q of the background QSOs. For most of the existing surveys using b q ≲ 19 QSOs, the amplification bias dominates the combined effect resulting in a slight overestimate of dN dz , f( N) and ω HI. We mainly find that observational strategies presently used to produce high- z DLA surveys result in avoiding the signature of significant GL effects: following our model, we determine that an overestimate of ω HI by more than 10% is unlikely for the z ⋍ 1.7 existing surveys, but may reach ⋍ 35% for the low redshift ones. However, we show that, in the absence of extinction by dust and micro-lensing effects, surveys ideally designed to enhance GL effects, i.e. to search for DLAs at z ∼ 0.5 in front of very bright ( b q ⋍ 16 ), high- z ( z q > 1) QSOs, may lead 1) to overestimate by up to ⋍90% the number of DLAs per unit redshift; 2) to bias the survey towards high HI column density systems so that it could contain up to 4 times as many such systems, thus 3) to overestimate by up to ⋍170% the cosmological density of gas associated with those DLAs. Identification of the galaxies responsible for the DLAs may be severely biased towards luminous galaxies if 2 α TF − α H > 0 ; this latter effect is greatly increased for log N HI > 21 DLAs.

Lyα Absorption‐Line Systems in the Gravitational Lens Q0957+561

Far-ultraviolet spectra of the gravitational lens components Q0957+561A and B were obtained with the Hubble Space Telescope Faint Object Spectrograph (HST FOS). Two previously known absorption-line systems were detected at redshifts zdamped = 1.3911 and zLyα = 1.1249. Their prominent absorption features are superposed on intense QSO continuum emission between λλ900-1400 in the quasar rest frame. Strong O VI λ1033, Lyα λ1216, and N V λ1240 line emission found at the QSO redshift (zQSO = 1.41) accompany the absorption-line systems. Lyα through Ly associated with the damped absorption system were found in both lensed components, together with other ionic species of N I, N III, C II, C III, Si II, Si III, and O I. We tentatively identify O VI λλ1033, 1037 absorption at the damped Lyα redshift, which, if confirmed, would be the highest ionization species yet detected in such systems. The equivalent widths of the Lyman series in Q0957+561A are measurably greater compared with absorption in 0957+561B, consistent with the narrower and shallower depth of the Lyman series line profiles in image B. The differences of the damped Lyman series absorption in the lensed components are the only significant spectral characteristic that distinguishes the far-ultraviolet spectra of 0957+561A and B. These results indicate that the damped Lyα absorber is inhomogeneous over scale lengths of ~200 pc, which corresponds to the beam separation at the damped Lyα redshift. However, the equivalent widths of neutral and ionized metals in lens components A and B are correlated, which suggests these spectral features arise in an extended region. The metal line-absorption strength is consistent with lower column densities compared with the hydrogen line-forming region. Thus, the small coherence length scale indicated by the difference in hydrogen line absorption between the lensed components suggests the geometric ray paths intercept different regions of a galactic disk that is viewed pole-on, while the metal absorption occurs in the halo.

Damped Lyα Absorbers as Tracers of the Evolution of the Mass Spectrum of Interstellar Matter in the Galaxy Population

A generally accepted approach to the interpretation of the damped Lyα absorption systems seen in QSO spectra is to attribute them to cases in which the observer's sight line passes through high column density gaseous disks of galaxies, i.e., spiral galaxies or the progenitors of spirals. Here we consider an alternative natural possibility, consistent with available observational data, namely that the absorption is simply associated with neutral gas in giant hydrogen clouds that could be associated with any type of gaseous galaxy or protogalaxy. At high redshift such galaxies could be the progenitors of different types of galaxies (e.g., ellipticals, spirals, etc.) that are observed at the present epoch. We show that the observational data, which include low to moderate redshift data recently obtained with HST and moderate to high-redshift data compiled using large ground-based telescopes over the last decade, coupled with some reasonable assumptions about the properties of giant hydrogen clouds in galaxies, can be used to form the two-dimensional distribution for the number of damped Lyα systems in redshift and column density, d2/dz dNH I, over the redshift interval 0.1 < z < 3.5. This can be further extended to redshift z = 0 using information provided by 21 cm observations of nearby galaxies. By combining this result with the assumption that damped Lyα absorbers behave like the giant hydrogen clouds in our Galaxy and neighboring galaxies, we show that the mass spectrum of clouds responsible for the damped Lyα absorption systems can be derived. This mass spectrum can be used to study the evolution of the giant hydrogen cloud population associated with galaxies over cosmic time. An interesting feature of the mass spectrum is that its steepness increases systematically as the redshift decreases over the interval 3 < z < 0.75. This effect is likely to be related to intensive star formation processes, which lead to the destruction of high-mass clouds and the formation of numerous low-mass clouds.

APM z ≳ 4 survey: distribution and evolution of high column density H i absorbers

Eleven candidate damped Lya absorption systems were identified in 27 spectra of the quasars from the APM z>4 survey covering the redshift range 2.8<z(abs)<4.4 (8 with z(abs)>3.5). High resolution echelle spectra (0.8A FWHM) have been obtained for three quasars, including 2 of the highest redshift objects in the survey. Two damped systems have confirmed HI column densities of N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold. We have discovered the highest redshift damped Lya absorber known at z=4.383 in QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. We combine this high redshift sample with other quasar samples covering the redshift range 0.008 < z < 4.7 to study the redshift evolution and the column density distribution function for absorbers with log N(HI)>=17.2. In the HI column density distribution f(N)=kN^-beta we find evidence for breaks in the power law, flattening for 17.2< log N(HI)<21 and steepening for log N(HI)>21.2. The column density distribution function for the data with log N(HI)>=20.3 is better fit with the form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the number density per unit redshift is evident in the higher column density systems with an apparent decline in N(z) for z>3.5.

Evolution of neutral gas at high redshift: implications for the epoch of galaxy formation

Though observationally rare, damped Lya absorption systems dominate the mass density of neutral gas in the Universe. Eleven high redshift damped Lya systems covering 2.8<z<4.4 were discovered in 26 QSOs from the APM z>4 QSO Survey, extending these absorption system surveys to the highest redshifts currently possible. Combining our new data set with previous surveys we find that the cosmological mass density in neutral gas, omega_g, does not rise as steeply prior to z~2 as indicated by previous studies. There is evidence in the observed omega_g for a flattening at z~2 and a possible turnover at z~3. When combined with the decline at z>3.5 in number density per unit redshift of damped systems with column densities log N(HI)>21 atoms cm^-2, these results point to an epoch at z>3 prior to which the highest column density damped systems are still forming. We find that over the redshift range 2<z<4 the total mass in neutral gas is marginally comparable with the total visible mass in stars in present day galaxies. However, if one considers the total mass visible in stellar disks alone, ie excluding galactic bulges, the two values are comparable. We are observing a mass of neutral gas comparable to the mass of visible disk stars. Lanzetta, Wolfe & Turnshek (1995) found that omega_g(z~3.5) was twice omega_g(z~2), implying a much larger amount of star formation must have taken place between z=3.5 and z=2 than is indicated by metallicity studies. This created a `cosmic G-dwarf problem'. The more gradual evolution of omega_g we find alleviates this. These results have profound implications for theories of galaxy formation.

Disc galaxies at z = 0 and at high redshift: an explanation of the observed evolution of damped Lya absorption systems

The analysis of disk formation is based on the White & Rees (1978) picture, in which disk galaxies form by continuous cooling and accretion of gas within a merging hierarchy of dark matter halos. A simple Kennicutt law of star formation for disks, based on a single- fluid gravitational instability model is introduced. Since the gas supply in the disk is regulated by infall from the surrounding halo, the gas is always maintained at a critical threshold surface density. Chemical enrichment of the disks occurs when the surrounding hot halo gas is enriched with heavy elements ejected during surpernova explosions. This gas then cools onto the disk producing a new generation of metal-rich stars. I first show that models of this type can reproduce many of the observed properties of a typical spiral galaxy like the Milky Way, including its gas and stellar surface density profiles and the observed relationship between the ages and metallicities of solar neighbourhood stars. I then use the models to make inferences about the properties of disk galaxies at high redshift. The total neutral hydrogen density Omega(HI) increases at higher z. The predicted increase is mild, but is roughly consistent with the latest derivation of Omega(HI) as a function of z by Storrie-Lombardi & MacMahon (1995). The models are also able to account for some of the other trends other trends seen in the high-redshift data, including the increase in the number of high column-density systems at high redshift, as well as the metallicity distribution of damped Lyman-alpha systems at redshifts 2-3.

Damped Lyman-Alpha and Lyman-Limit Absorbers in the Cold Dark Matter Model

We study the formation of damped Lyα and Lyman-limit absorbers in a hierarchical clustering scenario using a gasdynamical simulation of an Ω = 1 cold dark matter universe. In the simulation, these high column density systems are associated with forming galaxies. Damped Lyα absorption, NH I 1020.2 cm-2, arises along lines of sight that pass near the centers of relatively massive, dense protogalaxies. Lyman-limit absorption, 1017 cm-2 NH I 1020.2 cm-2, develops on lines of sight that pass through the outer parts of such objects or near the centers of smaller protogalaxies. The number of Lyman-limit systems is less than observed, while the number of damped Lyα systems is quite close to the observed abundance. Damped absorbers are typically ~10 kpc in radius, but the population has a large total cross section because the systems are much more numerous than present-day L* galaxies. Our results demonstrate that high column density systems like those observed arise naturally in a hierarchical theory of galaxy formation and that it is now possible to study these absorbers directly from numerical simulations.

Results from Searches for High-Redshift Galaxies around Damped Lyα Systems

There was not much said at this conference about the damped Lyα absorbers, yet locally and at high redshift nearly all the neutral hydrogen in the Universe lies in the damped systems, so they may have much to tell us about when and how galaxies formed. This talk is concerned with the results of narrow-band imaging searches for Lyα emission from, and around, damped Lyα absorbers. There have been many such searches, but very few detections. Here we concentrate on two groups of galaxies at high redshift, z &gt; 2, found by this technique. One of these is a recent discovery (Francis et al. 1995), and for the other we discuss some new observations (Warren and Møller 1995). A study of these two groups provides some clues to the nature of the damped absorbers at high redshift, and the rôle they play in the formation of galaxies.

A Redshift 4.38 Mg [CSC]ii[/CSC] Absorber Toward BR 1202−0725

We have detected a strong (EWobs = 20 A) redshift 4.38 Mg II absorber toward the redshift 4.68 quasar BR 1202-0725. The identification of this feature as Mg II λλ2796, 2803 is supported by the identification of a damped Lyα absorber and ultraviolet Si II λ1526 absorption at the same redshift. The strengths of Mg II λλ2796, 2803 and Si II λ1526 are consistent with those of strong low-redshift Mg II systems, damped Lyα absorbers, and diffuse clouds within the Milky Way. C IV λλ1548, 1550 is not detected, indicating a lower ionization state than is seen in lower redshift Mg II absorbers and within the interstellar medium of our Galaxy. The detection of a single strong Mg II absorber at z > 4.2, among the three quasars observed with sufficient signal-to-noise ratio to detect an absorber, is consistent with the statistics and evolution of lower redshift Mg II absorbers. Thus, despite being at nearly twice the redshift of any previously identified Mg II absorber, this system appears quite similar to lower redshift Mg II systems that are identified with normal field galaxies. Because of the saturated nature of the Si II and Mg II lines, the metallicity of the gas is uncertain, but it is clear that some enrichment of Si and Mg, probably amounting to more than 1% of solar, has occurred by a redshift of 4.38.

Molecular Hydrogen at z = 2.8108

The Lyman and Werner bands of H2 in interstellar gas provide information about gas temperature, density and the ultraviolet radiation field, and possibly about dust content. This is especially useful for high redshift QSO absorption systems, where usually the only data available arise from absorption lines. We present 25 – 50 km s−1 resolution data taken with the CTIO 4m telescope plus echelle spectrograph of the Lyα forest region of 0528–250, which has a damped Lyα absorption system at z = 2.81. Using a χ2 profile fitting routine (Lanzetta &amp; Bowen 1992. ApJ, 391, 48), we find an H2 fraction of ∼ 10−2, an order of magnitude below that of Galactic diffuse interstellar clouds. This may be caused by some combination of a less efficient H2 formation rate or an increased H2 dissociation rate. Using the relative populations of the J″ = 0, 1 rotational levels, we derive a kinetic temperature of TK = 136 ± 16 K. The total velocity spread as traced by sensitive metal transitions is 250 km s−1, consistent with a highly inclined, rotating ensemble of clouds associated with a luminous spiral galaxy. A representative section of the spectrum is shown below, binned at roughly the Nyquist rate with the χ2 fit to H2 of the z = 2.8108 absorption system.

The Incidence of Damped LY alpha Systems in the Redshift Interval 0 &lt; Z &lt; 4

view Abstract Citations (57) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Incidence of Damped LY alpha Systems in the Redshift Interval 0 < Z < 4 Rao, Sandhya M. ; Turnshek, David A. ; Briggs, Franklin H. Abstract The determination of the incidence and column density distribution of damped Lyα systems from QSO absorption-line surveys in the redshift interval 0 < z < 1.65 is crucial for constraining the evolution of neutral gas in galaxies. This redshift interval corresponds to a time interval of ∼62% of the age of the universe for q0 = 0 (77% for q0 = 0.5). However, because damped Lyα systems are relatively rare in spectra, a sensitive UV spectroscopic survey would consume considerable telescope time. We have therefore undertaken a new approach that takes advantage of the fact that all damped Lyα systems studied so far are found to have corresponding Mg II metal-line absorption. For redshifts z < 1.65 we have used available UV data to study the Lyα absorption line corresponding to 43 Mg II systems. After accounting for possible biases, we find that at a mean redshift of <z> = 0.8 the number of damped Lyα absorption lines with neutral hydrogen column density N(H I) ≥ 2 × 1020 cm-2 per unit redshift is nDLyα(z = 0.8) ≤ 0.12. A similar analysis based on studying the Lyα line in Lyman-limit systems is consistent with this result. This limit on the observed number density at z ≍ 0.8 is consistent with the trend at higher redshift as well as the number density at the current epoch inferred from H I cross sections of nearby disk galaxies. The result that n(z) in the redshift interval 0 < z < 1.65 conforms to a trend established by three different observing techniques over three redshift regimes provides evidence that this trend actually traces the evolution of neutral gas in galaxies. The data at z > 1.65 combined with the constraint from nearby galaxies at redshift z = 0 result in a parameterization that is valid from redshift z ≍ 4 to the present epoch: nDLyα(z) = (0.015±0.004) (1 + z)2.27±0.25. The power-law index of 2.27±0.25 indicates that damped Lyα absorbers undergo evolution, either in size or in number density, from a redshift of about 4 to the present epoch. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/176073 Bibcode: 1995ApJ...449..488R Keywords: GALAXIES: EVOLUTION; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS | data products NED (95) SIMBAD (40) MAST (2) INES (1) ESA (1)

A Faraday rotation search for magnetic fields in quasar damped LY alpha absorption systems

We present the results of a Faraday rotation survey of 61 radio-bright QSOs conducted at the NRAO Very Large Array (VLA). The Galactic contribution to the Faraday rotation is estimated and subtracted to determine the extragalactic rotation measure (RRM) for each source. Eleven of these QSOs are known to exhibit damped Lyα absorption. The rate of incidence of significant Faraday rotation of these 11 sources is compared to the remaining 50 and is found to be higher at the 99.8% confidence level. However, as this is based upon only two detections of Faraday rotation in the damped Lyα sample, the result is only tentative. If the two detections in the damped Lyα sample are due to the absorbing systems, then the inferred rotation measure induced by these systems is roughly 250 rad m^-2^, The two detections were for the two lowest redshift absorbers in the sample. We find that a rotation measure of 250 rad m^-2^ would have gone undetected for any other absorber in the damped Lyα sample due to the (1 + z)^-2^ dilution of the observed RRM with redshift. Thus the data are consistent with, but do not prove, the hypothesis that Faraday rotation is a generic property of damped Lyα absorbers. We do not confirm the suggestion that the amplitude of RRMs increases with redshift. Rather, the data are consistent with no redshift evolution. We find that the uncertainty in the estimation of the Galactic rotation measure (GRM) is a more serious problem than previously realized for extragalactic Faraday rotation studies of QSO absorbers. A careful analysis of current methods for estimating GRM indicate that it can be determined to an accuracy of about 15-20 rad m^-2^. Previous studies underestimated this uncertainty by more than a factor of 2. Due to this uncertainty, rotation measures such as we suspect are associated with damped Lyα absorption systems can only be detected at redshifts less than z~1.

The IUE Survey for Damped Lyman- alpha and Lyman-Limit Absorption Systems: Evolution of the Gaseous Content of the Universe

view Abstract Citations (354) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The IUE Survey for Damped Lyman- alpha and Lyman-Limit Absorption Systems: Evolution of the Gaseous Content of the Universe Lanzetta, Kenneth M. ; Wolfe, Arthur M. ; Turnshek, David A. Abstract We present a spectroscopic survey for moderate-redshift absorption features from damped Lyα and Lyman- limit absorption systems. The survey is based on a set of optimally extracted and co-added International Ultraviolet Explorer spectra of QSOs, BL Lac objects, and Seyfert galaxies. Results of this survey are combined with those of previous surveys carried out at optical wavelengths in order to examine evolution of the gaseous content of the universe over the redshift interval 0.008 ≲ z ≲ 3.5. The main results of the survey are that (1) the product of the absorption cross section and the comoving spatial number density of damped Lyα absorption systems does not appear to evolve strongly over the redshift interval 0.008 ≲ z ≲ 3.5, (2) the cosmological mass density Ωg(z) of neutral gas associated with damped Lyα absorption systems appears to decrease significantly from z 3.5 to 0.008, and (3) the observed evolution of Ωg(z) results from a steady decrease in the incidence of high column density absorption systems with decreasing redshift. Together these results suggest that the average mass of neutral gas per absorption system decreases with time, which is qualitatively consistent with the hypothesis that the observed evolution of Ωg(z) results from the consumption of gas into stars at z ≲ 3.5. We present additional arguments supporting this hypothesis and assess the implications of the observations of Ωg(z) for scenarios of galaxy formation and evolution. Under a "closed box assumption" we determine a characteristic epoch zs of star formation in galaxies of zs = 2-3 and a characteristic timescale τs of star formation in galaxies of r = 0.5-1.0 × 109 h-1 yr at z ≳ 1.5. Considering recent chemical abundance measurements by Pettini et al. and others we conclude that the cosmic metallicity at z ≍ 2.5 is given by log Z(z ≍ 2.5)/Zsun = -1.0±0.5. Comparing the inferred cosmological mass density of metals in gas and stars at the epoch z with the nominal cosmological mass density of metals in nearby galaxies, we conclude that the characteristic epoch of metal production in galaxies occurred after the characteristic epoch of star formation in galaxies. The implication of this result is that the bulk of stars in nearby galaxies should be metal poor, whereas only a small fraction of the disk stars in the solar neighborhood are actually metal poor. This inconsistency represents a cosmic G-dwarf problem" in that a straightforward interpretation of our observations predicts many more metal-poor stars than are found in the solar neighborhood. We propose that the cosmic G-dwarf problem may be resolved if damped Lyα absorption systems trace not only galactic disk evolution, as suggested previously by Wolfe, but also galactic spheroid evolution. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175286 Bibcode: 1995ApJ...440..435L Keywords: COSMOLOGY: OBSERVATIONS; GALAXIES: ABUNDANCES; GALAXIES: EVOLUTION; GALAXIES: FORMATION; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS | data products NED (273) SIMBAD (239) MAST (1) INES (1)

More confirmed damped Lyman-Alpha systems from the Las Campanas/Palomar survey

view Abstract Citations (27) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS More Confirmed Damped Lyman-Alpha Systems From the Las Campanas/Palomar Survey Lu, Limin ; Wolfe, Arthur M. Abstract We present moderate-resolution spectra of six high-redshift quasars which contain nine candidate damped Lyα absorption systems identified in the Las Campanas/Palomar survey of Lanzetta et al. [ApJS, 77, 1 (1991)]. Eight of the nine candidate damped Lyα absorption systems are confirmed (one tentative), including seven with N(H I) >= 2 x 10^20^ cm^- 2^. The remaining one turns out to be due to blending of many narrow absorption lines. Accurate absorption redshifts, HI column densities, and metal-line equivalent widths are provided for these systems in order to facilitate future studies of detailed physical condition and chemical abundances. We briefly discuss the 18 other metal-line systems identified along these sight lines. The z_abs_ = 2.6527 system toward Q 2231-00 is interesting in that it shows strong O VIλλ1031, 1037 absorption. The O VI ion is one of the best diagnostics of hot gas, and high resolution studies of the O VI absorption is likely to yield important clues regarding the ionization mechanism of the metal-line systems. Another system, at z_abs_ = 2.377l toward Q 2239-38, shows relatively strong Zn II and Cr II absorption, clearly indicating that it belongs to the same class absorbers as the damped Lyα systems. We have not obtained metal abundances for this system because the H I column density is not available. We also find a Lyman limit absorption system at z_abs_ = 3.5090 toward Q 2239-38 which does not show obvious metal absorption lines. More sensitive observations are required to check on this interesting possibility. Publication: The Astronomical Journal Pub Date: July 1994 DOI: 10.1086/117043 Bibcode: 1994AJ....108...44L Keywords: Astronomical Catalogs; Lyman Alpha Radiation; Lyman Spectra; Quasars; Red Shift; Galactic Evolution; Radio Astronomy; Astrophysics; QUASARS: ABSORPTION LINES; STARS: ABUNDANCES full text sources ADS | data products NED (25) SIMBAD (6)

Lick optical spectra of quasar HS 1946+7658 at 10 kilometers per second resolution Lyman-alpha forest and metal absorption systems

We present optical spectra of QSO HS~1946+7658 with either high resolution (FWHM=10 km/s) or high signal to noise ratio (SNR=40-100). We find 113 Lyman alpha and six metal line systems. The metal systems at Zabs=2.844 and 3.050 have complex velocity structures. We find that the system at 2.844 is a damped Ly-a absorption system, with neutral hydrogen column density of logN(HI)=20.2+/-0.4, and it is the cause of the Lyman limit break at 3520\AA. The metal abundance in the gas phase of the system is [M/H]=-2.6+/-0.3, with a best estimate of [M/H]= -2.8, with ionization parameter Gamma=-2.75, from a photoionization model. We show that the abundance ratios rule out appreciable dust in this DLA system. We do not see the enhancement of O over C reported in Nature by Reimers last years. We see CII*(1335) offset by 15 km/s with respect to CII(1334), presumably because the gas density varies from 2 - 8 cm(-3) with changing velocity in the DLA system. These densities imply that the damped component is 6 - 25 pc thick, which is reasonable for a single cloud in a cold spiral disk. We see a 2.6sigma lack of Ly-a forest lines well away from the QSO redshift, which may be a chance fluctuation. We also see a correlation between column density N(HI) and Doppler parameter b for 96 unsaturated Ly-a forest absorption lines, and although this correlation persists in the 36 Ly-a lines which lie in regions where SNR=8-16, we agree with Rauch etal (1993) that it is probably a bogus effect of low SNR. We show that there are few low-b Ly-a forest lines in moderate SNR data.

Conditions in the Z = 0.692 absorber toward 3CR 286

We present HST/FOS^1^ ultraviolet and ground-based optical spectra of the z = 0.692 21 cm absorption system in the QSO 3CR 286. The strength of the damped Lyα absorption implies an H I column density, N(H I)~2 x 10^21^ cm^-2^. We derive a high spin temperature for the H I gas, T_s_~&gt; 10^3^ K, as has been found for other high-redshift 21 cm absorbing systems; at least 80% of the H I is hotter than 1200 K. Curve- of growth analysis yields Mg^+^ and Fe^+^ abundances which are ~1-2 dex below solar values; the Ca^+^ abundance is even lower implying some depletion onto dust grains. The H_2_ fraction is not high. We speculate that the high inferred T_s_, for the gas may reflect continuing active star formation at the 5-8 Gyr look-back time to the absorbing galaxy.