Damped Lyman-alpha Absorption Systems Research Articles

Multiple Absorption Systems in the Lines of Sight to Quadruply Lensed Quasar H1413+1143 As a Probe of the Circumgalactic medium around Dwarf Galaxy

AbstractWe present the first measurement of differences in MgII absorption strength in multiple intervening absorbers, which are also identified as (sub-)Damped Lyman alpha absorption systems, in the four spectra of the quadruply lensed quasar H1413+1143, often referred to the “Cloverleaf”, from highly spatial resolution and high signal-to-noise spectroscopy with an optical multi-mode spectrograph, the Kyoto tridimentional spectrograph II on board the Subaru telescope. The detection of significant MgII absorptions in multiple components in the spatially-resolved spectra suggests that chemical enrichment differs at least on scale of about 10 kpc within the separation of sightlines. For, a DLA system at redshift zabs = 1.66, the rest equivalent widths of MgII absorption lines change by factors up to 6, which is similar to those of HI absorption lines. This suggests that (inhomogeneous) cold absorbers which give rise to strong HI/MgII absorptions dwell on a scale within 10 kpc in the circumgalactic medium (CGM).

A FOURTH H I 21 cm ABSORPTION SYSTEM IN THE SIGHT LINE OF MG J0414+0534: A RECORD FOR INTERVENING ABSORBERS

We report the detection of a strong HI 21-cm absorption system at z=0.5344, as well as a candidate system at z=0.3389, in the sight-line towards the z=2.64 quasar MG J0414+0534. This, in addition to the absorption at the host redshift and the other two intervening absorbers, takes the total to four (possibly five). The previous maximum number of 21-cm absorbers detected along a single sight-line is two and so we suspect that this number of gas-rich absorbers is in some way related to the very red colour of the background source. Despite this, no molecular gas (through OH absorption) has yet been detected at any of the 21-cm redshifts, although, from the population of 21-cm absorbers as a whole, there is evidence for a weak correlation between the atomic line strength and the optical--near-infrared colour. In either case, the fact that so many gas-rich galaxies (likely to be damped Lyman-alpha absorption systems) have been found along a single sight-line towards a highly obscured source may have far reaching implications for the population of faint galaxies not detected in optical surveys, a possibility which could be addressed through future wide-field absorption line surveys with the Square Kilometre Array.

THE GEOMETRY EFFECTS OF AN EXPANDING UNIVERSE ON THE DETECTION OF COOL NEUTRAL GAS AT HIGH REDSHIFT

Recent high redshift surveys for 21-cm absorption in damped Lyman-alpha absorption systems (DLAs) take the number of published searches at z > 2 to 25, the same number as at z < 2, although the detection rate at high redshift remains significantly lower (20% cf. 60%). Using the known properties of the DLAs to estimate the unknown profile widths of the 21-cm non-detections and including the limits via a survival analysis, we show that the mean spin temperature/covering factor degeneracy at high redshift is, on average, double that of the low redshift sample. This value is significantly lower than the previous factor of eight for the spin temperatures and is about the same factor as in the angular diameter distance ratios between the low and high redshift samples. That is, without the need for the several pivotal assumptions, which lead to an evolution in the spin temperature, we show that the observed distribution of 21-cm detections in DLAs can be accounted for by the geometry effects of an expanding Universe. That is, as yet there is no evidence of the spin temperature of gas rich galaxies evolving with redshift.

The most metal-poor damped Lyα systems: insights into chemical evolution in the very metal-poor regime★

We present a high spectral resolution survey of the most metal-poor damped Lyman-alpha absorption systems (DLAs) aimed at probing the nature and nucleosynthesis of the earliest generations of stars. Our survey comprises 22 systems with iron abundance less than 1/100 solar; observations of seven of these are reported here for the first time. Together with recent measures of the abundances of C and O in Galactic metal-poor stars, we reinvestigate the trend of C/O in the very metal-poor regime and we compare, for the first time, the O/Fe ratios in the most metal-poor DLAs and in halo stars. We confirm the near-solar values of C/O in DLAs at the lowest metallicities probed, and find that their distribution is in agreement with that seen in Galactic halo stars. We find that the O/Fe ratio in very metal-poor (VMP) DLAs is essentially constant, and shows very little dispersion, with a mean [<O/Fe>] = +0.39 +/- 0.12, in good agreement with the values measured in Galactic halo stars when the oxygen abundance is measured from the [O I] 6300 line. We speculate that such good agreement in the observed abundance trends points to a universal origin for these metals. In view of this agreement, we construct the abundance pattern for a typical very metal-poor DLA and compare it to model calculations of Population II and Population III nucleosynthesis to determine the origin of the metals in VMP DLAs. Our results suggest that the most metal-poor DLAs may have been enriched by a generation of metal-free stars; however, given that abundance measurements are currently available for only a few elements, we cannot yet rule out an additional contribution from Population II stars.

Faint resonantly scattered Lyα emission from the absorption troughs of damped Lyα systems at z∼ 3

We demonstrate that the Lyman alpha emission in the absorption troughs of a large sample of stacked damped Lyman alpha absorption systems (DLAS) presented by Rahmani et al (2010) is consistent with the spectral profiles and luminosities of a recently detected population of faint Lyman alpha emitters at z ~ 3. This result supports the suggestion that the faint emitters are to be identified with the host galaxies of DLAS at these redshifts.

New searches for H i 21 cm in damped Lyman α absorption systems

We present the results of three separate searches for HI 21-cm absorption in a total of twelve damped Lyman-alpha absorption systems (DLAs) and sub-DLAs over the redshift range z = 0.86-3.37. We find no absorption in the five systems for which we obtain reasonable sensitivities and add the results to those of other recent surveys in order to investigate factors which could have an effect on the detection rate: We provide evidence that the mix of spin temperature/covering factor ratios seen at low redshift may also exist at high redshift, with a correlation between the 21-cm line strength and the total neutral hydrogen column density, indicating a roughly constant spin temperature/covering factor ratio for all of the DLAs searched. Also, by considering the geometry of a flat expanding Universe together with the projected sizes of the background radio emission regions, we find, for the detections, that the 21-cm line strength is correlated with the size of the absorber. For the non-detections it is apparent that larger absorbers (covering factors) are required in order to exhibit 21-cm absorption, particularly if these DLAs do not arise in spiral galaxies. We also suggest that the recent z = 2.3 detection towards TXS 0311+430 arises in a spiral galaxy, but on the basis of a large absorption cross-section and high metallicity, rather than a low spin temperature

Reconciling the local galaxy population with damped Lyman α cross-sections and metal abundances

A comprehensive analysis of 355 high-quality Westerbork Synthesis Radio Telescope (WSRT) HI 21-cm line maps of nearby galaxies shows that the properties and incident rate of damped Lyman alpha absorption systems (DLAs) observed in the spectra of high-redshift QSOs are in good agreement with DLAs originating in gas discs of galaxies like those in the z approximate to 0 population. Comparison of low-z DLA statistics with the HI incidence rate and column density distribution f(N (HI)) for the local galaxy sample shows no evidence for evolution in the integral 'cross-section density' = l(-1) (l = mean free path between absorbers) below z approximate to 1.5, implying that there is no need for a hidden population of galaxies or H I clouds to contribute significantly to the DLA cross-section. Compared with z approximate to 4, our data indicate evolution of a factor of 2 in the comoving density along a line of sight. We find that dN/dz(z = 0) = 0.045 +/- 0.006. The idea that the local galaxy population can explain the DLAs is further strengthened by comparing the properties of DLAs and DLA galaxies with the expectations based on our analysis of local galaxies. The distribution of luminosities of DLA host galaxies, and of impact parameters between QSOs and the centres of DLA galaxies, is in good agreement with what is expected from local galaxies. Approximately 87 per cent of low-z DLA galaxies are expected to be fainter than L-*, and 37 per cent have impact parameters less than 1 arcsec at z = 0.5. The analysis shows that some host galaxies with very low impact parameters and low luminosities are expected to be missed in optical follow-up surveys. The well-known metallicity-luminosity relation in galaxies, in combination with metallicity gradients in galaxy discs, causes the expected median metallicity of low-z DLAs to be low (similar to 1/7 solar), which is also in good agreement with observations of low-z DLAs. We find that f(N H I) can be fitted satisfactorily with a gamma distribution, a single power law is not a good fit at the highest column densities N H I > 10(21) cm(-2). The vast majority (approximate to 81 per cent) of the H I gas in the local Universe resides in column densities above the classical DLA limit (N (H I) > 2 x 10(20) cm(-2)), with N (H I) similar to 10(21) cm(-2) dominating the cosmic H I mass density.

Molecular fraction limits in damped Lyman α absorption systems

We have used the Green Bank Telescope (GBT) and Berkeley-Illinois-Maryland Association (BIMA) array to search for redshifted millimetre absorption in a sample of damped Lymanalpha absorption systems (DLAs). This brings the number of published systems searched from 18 to 30. In 17 cases we reach 3σ limits of τ 6 0.1, which is a significant improvement over the previous searches and more than sufficient to detect the 4 known redshifted millimetre absorbers (τ > 1). While the CO rotational (millimetre) column density limits obtained are weaker than the electronic (optical) limits, they may provide useful limits below the atmospheric cut-off for the Lyman and Werner H2-bands in the UV (zabs < 1.8). Using a model for the DLA metallicity evolution combined with assumed HCO + /H2 and CO/H2 conversion ratios, we use the molecular column density limits to calculate plausible H2 molecular fraction limits. Finally, we use these results to discuss the fea sibility of detecting rotational CO transitions in DLAs with the next generation of large radio telescopes.

Intergalactic HiiRegions Discovered in SINGG

A number of very small isolated HII regions have been discovered at projected distances up to 30 kpc from their nearest galaxy. These HII regions appear as tiny emission line objects in narrow band images obtained by the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG). We present spectroscopic confirmation of four isolated HII regions in two systems, both systems have tidal HI features. The results are consistent with stars forming in interactive debris due to cloud-cloud collisions. The H-alpha luminosities of the isolated HII regions are equivalent to the ionizing flux of only a few O stars each. They are most likely ionized by stars formed in situ, and represent atypical star formation in the low density environment of the outer parts of galaxies. A small but finite intergalactic star formation rate will enrich and ionize the surrounding medium. In one system, NGC 1533, we calculate a star formation rate of 1.5e-3 msun/yr, resulting in a metal enrichment of ~1e-3 solar for the continuous formation of stars. Such systems may have been more common in the past and a similar enrichment level is measured for the `metallicity floor' in damped Lyman-alpha absorption systems.

A 22-GHz search for molecular absorption at z 3 with the upgraded ATCA

We report a λ≳ 1 cm search for rotational molecular absorption towards quasars, now possible with the upgraded Australia Telescope Compact Array (ATCA). The targets were PKS 0201+113, PKS 0336—017 and Q 0537—286, where known damped Lyman-alpha absorption systems (DLAs) could cause redshifted molecular absorption in the 12-mm band of the telescope. We place 3σ upper limits on any HCO+ 0 → 1 absorption features of <30 mJy per 3.4 km s−1 channel. The non-detections could be attributed to the inherent low metallicities in DLAs leading to generally low H2, and thus HCO+, column densities. In general, the detection of molecular rotational transitions in DLAs could be further hindered by a lower than expected CO-to-H2 conversion ratio, whether due either to photoionization of carbon or to its relative under-abundance at high redshift.

A search for molecules in damped Lyman-alpha absorbers occulting millimetre-loud quasars

We have used the SEST 15-metre and Onsala 20-metre telescopes to perform deep (rms> 30 mJy) integrations of various molecular rotational transitions towards damped Lyman-alpha absorption systems (DLAs) known to occult millimetre- loud quasars. We have observed 6 new systems and improved the existing limits for 11 transitions. These limits may be ap- proaching the sensitivities required to detect new systems and we present a small number of candidate systems which we believe warrant further observation.

Star Formation and Chemical Evolution of Lyman-Break Galaxies

The number density and clustering properties of Lyman-break galaxies (LBGs) observed at redshift z ~ 3 are best explained by assuming that they are associated with the most massive haloes at z ~ 3 predicted in hierarchical models of structure formation. In this paper we study, under the same assumption, how star formation and chemical enrichment may have proceeded in the LBG population. A consistent model, in which the amount of cold gas available for star formation must be regulated, is suggested. It is found that gas cooling in dark haloes provides a natural regulation process. In this model, the star formation rate in an LBG host halo is roughly constant over about 1 Gyr. The predicted star formation rates and effective radii are consistent with observations. The metallicity of the gas associated with an LBG is roughly equal to the chemical yield, or about the order of 1Z⊙ for a Salpeter IMF. The contribution to the total metals of LBGs is roughly consistent with that obtained from the observed cosmic star formation history. The model predicts a marked radial metallicity gradient in a galaxy, with the gas in the outer region having much lower metallicity. As a result, the metallicities for the damped Lyman-alpha absorption systems expected from the LBG population are low. Since LBG halos are filled with hot gas in this model, their contributions to the soft X-ray background and to the UV ionization background are calculated and discussed.

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.

Constraints on models of galaxy formation from the evolution of damped Lyman alpha absorption systems

There is accumulating observational evidence suggesting that damped Ly$\alpha$ absorption systems systems are the progenitors of present-day spiral galaxies. We use the observed properties of these systems to place constraints on the history of star formation in galactic disks, and on cosmological theories of structure formation in the universe. We show that the observed increase in $\Omega_{HI}$ contributed by damped Ly$\alpha$ systems at high redshift implies that star formation must have been considerably less efficient in the past. We also show that the data can constrain cosmological models in which structure forms at late epochs. A mixed dark matter (MDM) model with $\Omega_{\nu}=0.3$ is unable to reproduce the mass densities of cold gas seen at high redshift, even in the absence of any star formation. We show that at redshifts greater than 3, this model predicts that the total baryonic mass contained in dark matter halos with circular velocities $V_c > 35$ km s$^{-1}$ is less than the observed mass of HI in damped systems. At these redshifts, the photo-ionizing background would prevent gas from dissipating and collapsing to form high column density systems in halos smaller than 35 km s$^{-1}$. MDM models are thus ruled out by the observations.

Erratum: "Superclustering of Damped Lyman-Alpha Absorption Systems at Redshifts Above Two" [A. J. 105,1633(1993)

Erratum: "Superclustering of Damped Lyman-Alpha Absorption Systems at Redshifts Above Two" [A. J. 105,1633(1993)

Superclustering of damped Lyman-alpha absorption systems at redshifts above two

Observations of two high redshift quasi-stellar objects (QSOs) (z=3.23 and z=3.18) separated by 8 arcmin on the sky are presented. Two strong Lyman-α absorption systems are seen in each QSO, and in both QSOs, one absorber is at a redshift of 2. 380 and the other is at a redshift of 2.853. The velocities of the absorption systems differ between the lines of sight by only ∼300 km s −1 in both cases. To cover both lines of sight, the absorbers would need a transverse comoving size of ∼6h 100 −1 Mpc (for q 0 = 0. 5). The neutral hydrogen column densities inferred from the profiles lie between 1.2 × 10 18 and 1.2 × 10 20 cm −2 , and low ionization metal lines are associated with at least one of the absorption systems. The a posteriori joint probability of seeing these redshift coincidences is very low ∼3 × 10 −4

New high-redshift damped Lyman-alpha absorption systems and the redshift evolution of damped absorbers

We have discovered two high-redshift (z abs =3.388 and 2.908) damped Lyα absorption systems and a third possible damped system (z abs =3.080) while obtaining spectra of optical counterparts from a sample of flat spectrum radio sources. These three systems all have higher redshifts than any previously known damped Lyα absorption systems in the spectra of radio-loud quasars, and they are among the highest redshift damped Lyα systems known. It is unlikely that we would have found as many as two or three damped Lyα absorption systems among our sample unless there is a significant increase in the area-weighted comoving density of damped systems for z≥3

Molecules at high redshift. V - Improved limits on the column density of CO in two damped Lyman alpha absorption systems

view Abstract Citations (10) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecules at High Redshift. V. Improved Limits on the Column Density of CO in Two Damped Lyman-Alpha Absorption Systems Levshakov, Sergei A. ; Foltz, Craig B. ; Chaffee, Frederic H. ; Black, John H. Abstract We present improved limits on the ratio of the column density of carbon monoxide to that of neutral hydrogen in the high neutral hydrogen column density damped Lyman α absorption systems toward the QSOs PHL 957 and MC 1331 + 170. The ratio is more than two orders of magnitude lower than that found in diffuse clouds in the interstellar medium of the Galaxy. Publication: The Astronomical Journal Pub Date: December 1989 DOI: 10.1086/115277 Bibcode: 1989AJ.....98.2052L Keywords: Absorption Spectra; Carbon Monoxide; Lyman Alpha Radiation; Quasars; Red Shift; Hydrogen; Interstellar Matter; Molecular Clouds; Spectral Line Width; Astrophysics; QUASARS full text sources ADS | data products SIMBAD (2) NED (2)