Observations of damped Ly-alpha absorbers (DLA) indicate that the fraction of hydrogen in its neutral form (HI) is significant by mass at all redshifts. This gas represents the reservoir of material that is available for star formation at late times. As a result, observational identification of the systems in which this neutral hydrogen resides is an important missing ingredient in models of galaxy formation. Precise identification of DLA host mass via traditional clustering studies is not practical owing to the small numbers of known systems being spread across sparsely distributed sight lines. However following the completion of reionization, 21cm surface brightness fluctuations will be dominated by neutral hydrogen in DLAs. Observations of these fluctuations will measure the combined clustering signal from all DLAs within a large volume. We show that measurement of the spherically averaged power-spectrum of 21cm intensity fluctuations due to DLAs could be used to measure the galaxy bias for DLA host galaxies when combined with an independent measurement of the cosmological HI mass density from quasar absorption studies. Utilising this technique, the low frequency arrays now under construction could measure the characteristic DLA host mass with a statistical precision as low as 0.3 dex at z~4. In addition, high signal-to-noise observations of the peculiar-motion induced anisotropy of the power-spectrum would facilitate measurement of both the DLA host mass and the cosmic HI density directly from 21cm fluctuations. By exploiting this anisotropy, a second generation of low frequency arrays with an order of magnitude increase in collecting area could measure the values of cosmic HI density and DLA host mass, with uncertainties of a few percent and a few tens of percent respectively.
We present spectroscopic observations of the damped Ly-alpha absorber at redshift z=1.9342 seen in the spectrum of the quasar Q0151+048A. The redshift of the absorber is greater than the redshift of the quasar, so the system resembles the z(abs) ~ z(em) damped absorber at z=2.81 towards the quasar PKS0528-250. We have previously reported the detection of Ly-alpha emission from the latter absorber, one of only two damped absorbers for which Ly-alpha emission has unambiguously been detected. The resemblance between the PKS0528-250 and Q0151+048A systems is made closer by the detection of a weak …
In 2000, we started the program ``Building the Bridge between Damped Ly-alpha Absorbers and Lyman-Break Galaxies: Ly-alpha Selection of Galaxies'' at the European Southern Observatory's Very Large Telescope. This project is an attempt to use Ly-alpha selection of high-z galaxies to bridge the gap between absorption- and emission-selected galaxies by creating a large database of z=3 galaxies belonging to the abundant population of faint (R>25.5) galaxies probed by the Damped Ly-alpha Absorbers (DLAs). Here we present the first results of our program, namely the results from a deep Ly-alpha stud…
We present arguments based on the measured abundances in individual damped Lyman alpha systems (DLAs) and sub-damped Lyman alpha systems (sub-DLAs), and also the average abundances inferred in large samples of QSO absorption line systems, to suggest that the amount of dust in intervening QSO absorbers is small and is not responsible for missing many QSOs in magnitude limited QSO surveys. While we can not totally rule out a bimodal dust distribution with a population of very dusty, metal rich, absorbers which push the background QSOs below the observational threshold of current optical spectros…
Damped Lyman-alpha absorbers are believed to be associated with galactic disks. We show that gravitational lensing can therefore affect the statistics of these systems. First, the magnification bias due to lensing raises faint QSOs above a given magnitude threshold and thereby enhances the probability for observing damped absorption systems. Second, the bending of light rays from the source effectively limits the minimum impact parameter of the line-of-sight relative to the center of the absorber, thus providing an upper cut-off to the observed neutral hydrogen (HI) column density. The combina…
We present a Very Large Array (VLA) HI 21cm map and optical observations of the region around one of the nearest damped Lyman-alpha absorbers beyond the local group, SBS 1543+593. Two previously uncataloged galaxies have been discovered and a redshift has been determined for a third. All three of these galaxies are at the redshift of SBS 1543+593 and are ~185 kpc from the damped Lyman-alpha absorber. We discuss the HI and optical properties of SBS 1543+593 and its newly identified neighbors. Both SBS 1543+593 and Dwarf 1 have baryonic components that are dominated by neutral gas -- unusual for…
We report a deep search for 21 cm emission/absorption from the $z \sim 0.101$ 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 $3.3 σ$ level --- which yields a lower limit of 730 K on the spin temperature of the system. No HI emission was detected: the $3σ$ upper limit on the HI mass of the absorber is $2.25 \times 10^9 M_{\odot}$, for a velocity spread of $\sim 70$ km s$^{-1}$. The low HI mass and the high spin temperature seem to rule out the possibility that the absorber is a l…
We have used the SEST 15-metre and Onsala 20-metre telescopes to perform deep (r.m.s. >~ 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 approaching the sensitivities required to detect new systems and we present a small number of candidate systems which we believe warrant further observation.
We study the formation of damped \lya and Lyman limit absorbers in a hierarchical clustering scenario using a gas dynamical simulation of an $Ω= 1$, cold dark matter universe. In the simulation, these high column density systems are associated with forming galaxies. Damped \lya absorption, $N_{HI} \simgt 10^{20.2}\cm^{-2}$, arises along lines of sight that pass near the centers of relatively massive, dense protogalaxies. Lyman limit absorption, $10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}$, develops on lines of sight that pass through the outer parts of such objects or near the cent…
The galaxy giving rise to the damped Ly$α$ absorbing system in the QSO OI 363 with z=0.221 has been found. A galaxy which is probably associated with the second DLA in this same QSO at z=0.0912 has also been found. Neither galaxy is very luminous, and neither galaxy shows signs of extensive current star formation, a massive disk or lots of gas. The impact parameters for each of the two galaxies with respect to the QSO are reasonable. If most DLA absorbers arise in such low luminosity galaxies, it will be difficult to pick out the correct galaxy giving rise to DLA systems at high redshift withi…
We have obtained high signal:to:noise optical spectroscopy at 5Å resolution of 27 quasars from the APM z$>$4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-$α$ absorbers. These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with $\log$N(HI)$\ge17.5$, over the redshift range 0.01 $<$ z $<$ 5. The main results are: \begin{itemize} \item Lyman limit systems: The data are well fit by a power law $N(z) = N_0(1 + z)^γ$ for the number den…
We combine predictions for several hierarchical cosmogonies with observational evidence on damped Lyman alpha systems to establish a correspondence between the high redshift galaxy population and the properties of damped Lyman alpha systems. We assume that high redshift galaxies and damped Lyman alpha systems are hosted by the same dark matter halos and require consistency between the predicted halo space density, the rate of incidence and the velocity width distribution of damped Lyman alpha systems, and the observed galaxy luminosity function at the bright end. We arrive at the following res…
We present a study of HI-rich galaxies in the local universe selected from blind emission-line surveys. These galaxies represent the emission-line counterparts of local damped Lyman-alpha systems. We find that the HI cross-section of galaxies is drawn from a large range of galaxy masses below M_star, 66% of the area comes from galaxies in the range 8.5 < Log M_star < 9.7. Both because of the low mass galaxy contribution, and because of the range of galaxy types and luminosities at any given HI mass, the galaxies contributing to the HI cross-section are not exclusively L_star spirals, as is oft…
Damped Lyman-alpha absorbers (DLAs), seen in absorption against a background quasar, provide the most detailed probes available of element abundances in the Universe over > 90 % of its age. DLAs can be used to observationally measure the global mean metallicity in the Universe and its evolution with time. Paradoxically, these observations are more difficult at lower redshifts, where the absorber rest-frame UV spectra are cut-off due to the atmospheric absorption. We present here high-resolution VLT/UVES observations of several elements contained in three DLAs and one sub-DLA with 0.6<z_abs<0.9…
Eighteen candidates for emission line galaxies were discovered in a narrow-band infrared survey that targeted the redshifts of damped Lyman-alpha or metal lines in the spectra of quasars. The presence of emission lines is inferred from the photometric magnitudes in narrow and broad band interference filters, corresponding to H-alpha at redshifts of 0.89 (6 objects) and 2.4 (10 objects), and [OII] at a redshift of 2.3 (2 objects). Most of the candidates are small resolved objects, compatible with galaxies at the redshifts of the absorbers. Because a similar survey targeted at the redshifts of q…
We report a detection of the 9.7 micrometer silicate absorption feature in a damped Lyman-alpha (DLA) system at z_{abs} = 0.524 toward AO0235+164, using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The feature shows a broad shallow profile over about 8-12 micrometers in the absorber rest frame and appears to be > 15 sigma significant in equivalent width. The feature is fit reasonably well by the silicate absorption profiles for laboratory amorphous olivine or diffuse Galactic interstellar clouds. To our knowledge, this is the first indication of 9.7 micrometer silicate …
We report the first observations of the clustering environment of damped Lyman alpha absorption systems at z=4. Color selection and photometric redshifts were used to select 44 candidate Lyman-break galaxies brighter than I_AB=25.5 from deep BRI images of the 35 sq. arcmin field containing the quasar BR 0951-04. Multislit spectroscopy of 35 candidate galaxies was performed and 8 of these candidates have been confirmed as z>3.5 Lyman-break galaxies. With only BRI photometry, the photometric redshifts are quite accurate for the spectroscopically confirmed galaxies but have a high rate of misclas…
We have carried out a high angular resolution near-infrared imaging study of the fields of 6 quasars with 7 strong absorption line systems at z < 0.5, using the Hokupa'a adaptive optics system and the QUIRC near-infrared camera on the Gemini-North telescope. These absorption systems include 4 classical damped Lyman-alpha absorbers (DLAs), 2 sub-DLAs, and one Lyman-limit system. Images were obtained in the H or K' filters with FWHM between 0.2"-0.5" with the goal of detecting the absorbing galaxies and identifying their morphologies. Features are seen at projected separations of 0.5"-16.0" from…
We detect 21-cm emission from the Low Surface Brightness (LSB) galaxy SBS 1543+593, which gives rise to a Damped Lyman-alpha (DLA) absorption line in the spectrum of the background QSO HS 1543+5921 (z=0.807). We obtain an accurate measure of the velocity of the H I gas in the LSB galaxy, v=2868 km/s, and derive a mass of 1.3e9 solar masses. We compare this value with limits obtained towards two other z~0.1 DLA systems, and show that SBS 1543+593 would not have been detected. Hence LSB galaxies similar to SBS 1543+593 can be responsible for DLA systems at even modest redshifts without being det…
We present a study of the expected properties of the low redshift damped Lyman-alpha absorber population determined from a sample of HI-selected galaxies in the local universe. Because of a tight correlation between the HI mass and HI cross-section, which we demonstrate spans all galaxy types, we can use our HI-selected sample to predict the properties of the absorption line systems. We use measurements of the number density and HI cross-section of galaxies to show that the total HI cross-section at column densities sufficient to produce damped Lyman-alpha absorption is consistent with no evol…
[abridged] Using stacked Sloan Digital Sky Survey spectra, we present the detection of [OII]3727,3730 nebular emission from galaxies hosting CaII and MgII absorption line systems. Both samples of absorbers, 345 CaII systems and 3461 MgII systems, span the redshift interval 0.4 < z < 1.3; all of the former and half the latter sample are expected to be bona-fide damped Lyman-alpha (DLA) absorbers. The measured star formation rate (SFR) per absorber from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^-1 kpc) is 0.11-0.14 Msol/yr for the MgII-selected DLAs …
We present initial results from a Magellan telescope program to image galaxies that give rise to damped Lyman absorbers (DLAs) at 1.63 < z_DLA < 2.37. Our program differs from previous efforts in that we target quasars with intervening Lyman limit systems (LLS) along the line of sight at redshift z_LLS > 3.5. The higher-redshift LLS is applied as a blocking filter to remove the glare of the background quasar at the rest-frame ultraviolet wavelengths of the foreground galaxy. The complete absence of quasar light offers an unimpeded view along the sightline to the redshift of the LLS, allowing a…
[Abridged] We present out results on the cross-correlation of Lyman break galaxies (LBGs) around three damped \Ly absorbers (DLAs) at redshift three from deep multi-band MOSAIC images obtained at the KPNO 4m. The large area of the MOSAIC images, 0.31 $°^2$ allows us to probe the clustering of LBGs on scales up to 20 Mpc co-moving to test whether DLA halos are more or less massive than LBG halos. . We present our survey that covers a total of 1 deg$^2$ and contains $\sim$3,000 LBGs with photometric redshifts between 2.8 and 3.5. Within a redshift slice of width $W_z=0.15$, we find that the DLA-…
In this work, observational evidence for the connection between low column density Lyman-alpha absorbers and large-scale structure traced by gas-rich galaxies is investigated. The HI Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density Lyman-alpha absorbers from the literature. The absorber-galaxy cross-correlation function shows that on scales from 1 to 10 h^-1 Mpc, absorbers are imbedded in halos with masses similar to that of galaxy groups. This statistical evidence suggests that galaxy groups could be the dominant environment of lo…
The effects of the dust on the determination of elemental abundances in damped Ly alpha (DLA) absorbers are investigated. Relations between the observed abundances measured in the gas phase and the overall abundances (gas plus dust) are derived as a function of dust-to-gas ratio, metallicity, element-to-element abundance pattern, average extinction coefficient of dust grains, and chemical composition of dust grains. A method is presented for determining dust-to-gas ratios, dust-to-metals ratios, and dust-corrected relative abundances in DLA absorbers by assuming dust of Galactic type and const…
The Damped Lyman-alpha systems (DLAs), seen in absorption in the spectrum of quasars, are believed to contain a large fraction of the neutral gas in the Universe. Paradoxically, these systems are more difficult to observe at z_abs<1.7, since they are rare and their HI feature then falls in UV spectra. Rao & Turnshek (2000) pioneered a method based on MgII-selected DLAs, that is absorbers discovered thanks to our knowledge of their MgII feature in optical spectra. We use new observations undertaken at the TNG as well as a careful literature & archival search to build samples of low redshift abs…
We present elemental abundance measurements for 9 damped Ly-alpha systems (DLAs) and 1 sub-DLA at 0.1 < z < 1.5 from recent observations with the Multiple Mirror Telescope. Most of these absorbers are found to be metal-poor, while 2 are found to have ~ 30-50 solar metallicities. Combining our data with other data from the literature, we find that the systems with higher [Zn/H] also have stronger depletion as measured by [Cr/Zn] and [Fe/Zn]. The relationship between the metallicity and H I column density is also investigated. Together with our previous MMT survey (Khare et al. 2004) we have dis…
We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted HI 21cm absorption in two low redshift (z=0.2212, z=0.0912) damped Lyman-alpha systems seen towards the gigahertz peaked source OI 363 (z_em = 0.630). The object at z=0.0912 is the lowest redshift damped Lyman-alpha system known to date. Ground based imaging (Rao & Turnshek, 1998) shows that at neither redshift is there a large spiral galaxy at low impact parameter to the line of sight to OI 363, in contradiction with the suggestion that these systems are large proto-disks. Since OI 363 is a highly compact, core domin…
The discovery of a z_abs = 0.0912 damped Lyman-alpha absorption-line system in the HST-FOS ultraviolet spectrum of the quasar OI 363 (0738+313) is reported. This is the lowest redshift quasar damped Lyman-alpha system known. Its neutral hydrogen column density is N(HI) = 1.5(+/- 0.2) E21 atoms/cm^2, which easily exceeds the classical criterion for damped Lyman-alpha of N(HI) greater than or equal to 2E20 atoms/cm^2. Remarkably, a z_abs = 0.2212 damped system with N(HI) = 7.9(+/- 1.4) E20 atoms/cm^2 has also been discovered in the same spectrum. In the past, the standard paradigm for damped Lym…
Lyman alpha absorber correlations contain information about the underlying density distribution associated with a particular class of absorbers. As such, they provide an opportunity to independently measure the "bias" of the Lyman alpha forest, i.e. the relationship between HI column density and underlying dark matter density. In these proceedings we use hydrodynamic simulations to investigate whether the evolution of this bias is measurable from observable correlations. Unfortunately, the increasingly complex physics in the IGM at z<1 makes a direct measurement of the bias difficult. Neverthe…
We report the discovery of two low redshift HI 21cm absorbers, one at z = 0.2212 towards the z_{em} = 0.630 quasar OI 363 (B0738+313), and the other at z = 0.3127 towards PKS B1127-145 (z_{em} = 1.187). Both were found during a survey of MgII selected systems at redshifts 0.2 < z < 1 using the new UHF-high system at the Westerbork Synthesis Radio Telescope (WSRT). New HST/FOS observations also identify both systems as damped Ly-alpha (DLa) absorbers. By comparing the column density from the DLa line with that from the HI 21cm line, we calculate the spin temperature, and find that T_s is approx…
Evidence is presented that the damped Lyman alpha absorption systems are the high-redshift (z > 3) progenitors of galaxy disks. I discuss kinematic evidence that the damped Lyman Alpha systems are rotating disks. I also discuss implications of the lack of metal-poor damped Lyman alpha systems with line width Delta v > 100 {\kms}. I then present new evidence stemming from correlations between element-abundance ratios and [Fe/H], which connects damped systems to the thick stellar disk of the Galaxy. I discuss the connections between damped Lyman alpha systems and Lyman break galaxies, and how [C…
Damped Lyman-alpha systems (DLAs) and sub-DLAs seen toward background quasars provide the most detailed probes of elemental abundances. Somewhat paradoxically these measurements are more difficult at lower redshifts due to the atmospheric cut-off, and so a few years ago our group began a programme to study abundances at z < 1.5 in quasar absorbers. In this paper, we present new UVES observations of six additional quasar absorption line systems at z < 1.5, five of which are sub-DLAs. We find solar or above solar metallicity, as measured by the abundance of zinc, assumed not to be affected by du…
The damped Lyman alpha absorbers (DLAs) in quasar spectra are believed to be the progenitors of present-day disk galaxies. We examine the probability for microlensing of background quasars by stars in their DLAs. Microlensing by an individual star should magnify the continuum but not the broad emission lines of the quasars. Consequently, the equivalent width distribution of microlensed quasars would be distorted. We model a representative spiral galaxy as a closed system composed of a bulge, a disk, and a halo, and evolve the mass fraction of stars in the disk based on the observed metallicity…
We present a detailed study of the HI 21cm absorption system at z=0.0912 towards the radio quasar B0738+313. The uncommonly narrow main absorption line and weak secondary line are resolved for the first time. In addition we find it necessary to add a third, broader shallow component to obtain a good fit to the spectrum. Although the harmonic mean spin temperature calculated by comparison of the 21cm lines to the damped Ly-alpha line is T_s = 775 K, the thermal kinetic temperatures of the two narrow components, calculated from their widths, are much lower: T_k \leq 297 and \leq 103 K respective…
[Abridged] We present results on the clustering og Lyman break galaxies (LBGs) around a probable damped Ly-alpha absorption (DLA) line cloud at z_DLA=2.974 from deep UBVI images of the field containing the quasar APM 08279+5255 (z=3.91). The large area covered by our images, 0.31 deg^2 or 40x40 Mpc co-moving at z=3, and their depth (27.6 mag arcsec^{-2}), allow us to identify 450 LBG candidates brighter than I(AB)=24.80 at 2.75<z<3.25. LBG candidates were selected using photometric redshift techniques that include priors within a redshift slice of width Wz=0.15 centered at z_DLA. Within that r…
A remarkably small scatter in the N/O ratios for the HII regions in low-metallicity blue compact galaxies (BCG) has been found recently. It lead to the conclusion that N and O are both produced by massive stars. Conversely, the N/Si ratios in damped Lya absorbers (DLA) show a large scatter. This result provides support for the time-delay model of nitrogen production in intermediate-mass stars, nitrogen production in massive stars being not required. The goal of this study is to test whether these observational data are compatible with each other and with the existing ideas on the chemical evol…
We calculate the number of damped Lyman-alpha absorbers expected in various popular cosmological models as a function of redshift and compare our predictions with observed abundances. The Press-Schechter formalism is used to obtain the distribution of halos with circular velocity in different cosmologies, and we calibrate the relation between circular velocity and absorption cross-section using detailed gas dynamical simulations of a ``standard'' cold dark matter (CDM) model. Because of this calibration, our approach makes more realistic assumptions about the absorption properties of collapsed…
The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at z < 1.5, an epoch spanning \~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5. Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise…
We study the distribution of star formation rate (SFR) and metallicity of damped Lyman-alpha absorbers (DLAs) using cosmological smoothed particle hydrodynamics (SPH) simulations of the Lambda cold dark matter model. Our simulations include a phenomenological model for feedback by galactic winds which allows us to examine the effect of galactic outflows on the distribution of SFR and metallicity of DLAs. For models with strong galactic winds, we obtain good agreement with recent observations with respect to total neutral hydrogen mass density, N_HI column-density distribution, abundance of DLA…
We report results on the incidence and cosmological mass density of damped Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an efficient non-traditional (but unbiased) survey technique to discover DLA systems at redshifts z<1.65, where we observe the Lyman-alpha line in known MgII absorption-line systems. We uncovered 14 DLA lines including 2 serendipitously. We find that (1) The DLA absorbers are drawn almost exclusively from the population of MgII absorbers which have rest equivalent widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift, n(DLA), is obs…
Studies of damped Lyman-alpha absorption systems in quasar spectra are yielding very interesting results regarding the chemical evolution of these galaxies. We present some preliminary results from such a program.
Westerbork Radio Synthesis Telescope observations of the redshifted 21cm line absorber against the z_em = 0.871 double lobed quasar 3C196 show that the intervening absorber is an L approximately equal L* spiral galaxy (3C196-G1) and that the absorbing layer of cold gas extends to radii of at least 30 h_50^-1 kpc. The new data solve several long standing puzzles about this system by (1) discovering a second 21cm absorption feature, corresponding to absorption against the NE lobe of the background radio source and (2) spatially ``resolving'' the two absorption features to isolate the absorption …
We review the ongoing debate about the relationship between low redshift Lyman Alpha absorbers and luminous galaxies. In particular, we discuss the difficulty of `assigning' a particular absorber to a particular galaxy, and consider methods of circumventing this problem. We also provide a status report on an ongoing project collecting more data to address this issue, and show some results for a close together pair of QSOs providing two adjacent lines of sight through the inter-galactic medium.
Damped Lyman-alpha absorption systems found in the spectra of high redshift quasars are believed to trace the interstellar gas in high redshift galaxies. In this paper, we study the elemental abundances of C, N, O, Al, Si, S, Cr, Mn, Fe, Ni, and Zn in a sample of 14 damped Lyman-alpha systems using high quality echelle spectra of quasars obtained with the 10m Keck telescope. These abundances are combined with similar measurements in the literature in order to investigate the chemical evolution of damped Lyman-alpha galaxies in the redshift range 0.7<z<4.4. Among the things investigated are: th…
Damped Lyman-alpha systems seen in spectra of background quasars are generally thought to represent high redshift counterparts of present-day galaxies. We summarize observations of heavy element abundances in damped Lyman-alpha systems. The results of a systematic search for CO and C II* absorption in 17 damped Lyman-alpha systems are also presented using observations obtained with the 10m Keck telescopes. The latter provides a useful constraint on the expected strength of [C II] 158 micron emission from damped Lyman-alpha galaxies. It is hoped that these results will be useful for planning fu…
We describe recent discoveries of low column density [N(HI) < 10^14.5 cm^-2] H I Ly-alpha absorbers made with the Hubble Space Telescope (HST), which have allowed us a first look at gas in local intergalactic space between us and the "Great Wall". Despite the mere 2.4m aperture of HST, these new observations allow us to detect absorbers with column densities, N(HI) \approx 10^12.5 cm^-2, as low as those found using Keck/HIRES at high-z. Owing to the proximity of these absorbers to the Earth, the 197 absorbers in our combined GHRS + STIS sample (Penton, Stocke, & Shull 2001) provide our best vi…
Spectroscopy of ten quasars obtained with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) is presented. A clustering analysis reveals an excess of nearest neighbor line pairs on velocity scales of 250-750 km/s at a 95-98% confidence level. The hypothesis that the absorbers are randomly distributed in velocity space can be ruled out at the 99.8% confidence level. No two-point correlation power is detected (xi < 1 with 95% confidence). Lyman-alpha absorbers have correlation amplitudes on scales of 250-500 km/s at least 4-5 times smaller than the correlation am…
The local power law relationship between the surface densities of neutral hydrogen gas and star formation rate (SFR) can be used to explore the SFR properties of damped Lyman alpha (DLA) systems at higher redshift. We find that while the SFR densities for DLA systems are consistent with luminous star forming galaxies at redshifts below z~0.6, at higher redshifts their SFR density is too low for them to provide a significant contribution to the cosmic star formation history (SFH). This suggests that the majority of DLAs may be a distinct population from the Lyman break galaxies (LBGs) or submil…
We present high resolution spectroscopic observations of two neighbouring damped Lya systems (DLAs) along the same line of sight towards B2314-409. Due to their separation (v ~ 2000 km/s) and the high spectral resolution of the data, it is possible to fit not only the weak metal transitions, but also the separate HI absorption profiles. This has permitted, for the first time, a detailed study of metal abundances in two neighbouring galaxy-scale absorbers. The two DLAs have z_abs = 1.8573 and 1.8745 and have column densities log N(HI) = 20.9+/-0.1 and 20.1+/-0.2 respectively. We have determined…
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 observations that the damped Lyman-alpha absorber at z=3.15 toward the quasar Q2233+1310 may well be such an example. Djorgovski et al have recently detected the Lyman-alpha 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 syste…
We are exploring the connection between damped Ly-alpha absorbers (DLAs) and Lyman break galaxies (LBGs) using deep -- (5sigma)=26 m_{AB}-- broad band imaging (UBVI) of four wide fields (0.25 sq. deg. each) obtained at the Kitt Peak 4-m telescope with MOSAIC. Each field contains a damped system at z=3. We want to address the nature of DLAs at high-redshifts: (1) Are they embedded in much larger systems of galaxies? (2) How does the spatial distribution of LBGs in 3D (space and redshift) correlate with the absorber? Contrary to most previous DLA studies, we are not looking for the absorber, and…
We present HST/WFPC2 high-spatial resolution images in the R and B bands of the close environment of the sightlines to seven quasars which spectra show either a damped Ly-alpha absorption line, 21cm absorption, or a very strong MgII/FeII absorption system at intermediate redshifts (0.4 < z < 1). Objects down to about 0.3", or 2.0 kpc at z=0.6 (H0 = 50 kms/s/Mpc, q0=0), and to a limiting magnitude m(702, lim)=25.9 could be detected for seven fields comprising eight absorbers (one at higher redshift z=1.78 towards MC 1331+170) with high HI column densities of at least 1x10^20 cm^-2. In each case…
The distribution of element abundances with redshift in Damped Ly-alpha (DLA) systems can be adequately reproduced by the same model reproducing the halo and disk components of the Milky Way Galaxy at different galactocentric distances: DLA systems are well represented by normal spiral galaxies in their early evolutionary stages.
Although damped Lyman alpha (DLA) systems are usually considered metal-poor, it has been suggested that this could be due to observational bias against metal-enriched absorbers. I review recent surveys to quantify the particular issue of dust obscuration bias and demonstrate that there is currently no compelling observational evidence to support a widespread effect due to extinction. On the other hand, a small sub-set of DLAs may be metal-rich and I review some recent observations of these metal-rich absorbers and the detection of diffuse interstellar bands in one DLA at z ~ 0.5.
We report a quantitative study of the escape of Lyman-alpha photons from an inhomogeneous optically thick medium that mimics the structure of damped Lyman-alpha absorbers. Modeling the optically thick disk with 3 components (massive stars and HII regions, dust, and neutral hydrogen), we study the resulting emission line profile that may arise near the extended damped absorption profile.
Measurements of element abundances in damped Lyman alpha systems are providing new means to investigate the chemical evolution of galaxies, particularly at early times. We review progress in this area, concentrating on recent efforts to extend the range of existing surveys to both higher and lower redshifts.
[Abridged] We test the reliability of a method to measure the mean halo mass of Damped Ly-alpha absorbers (DLAs). The method is based on measuring the ratio of the cross-correlation between DLAs and galaxies to the auto-correlation of the galaxies themselves ($w_{\rm dg}/w_{\rm gg}$), which is (in linear theory) the ratio of their bias factor. This is shown to be true irrespective of the galaxy redshift distribution, provided that one uses the same galaxies for the two correlation functions. The method is applicable to all redshifts. Here, we focus on z=3 DLAs and we demonstrate that the metho…
We present new GHRS observations of Lyman alpha absorption lines associated with a group of galaxies towards the QSO 1545+2101. We have identified eight distinct Lyman alpha absorption features in the spectrum of QSO 1545+2101 at a mean redshift of z=0.2648 with a velocity dispersion of 163 km/s. A group of galaxies is detected in the vicinity of this QSO at a mean redshift of z=0.2645 and velocity dispersion 239 km/s. The identification of discrete absorption systems indicates that they arise in clouds of neutral hydrogen rather than in a diffuse intragroup medium. Our analysis suggests that …
While it is possible to explain Lyman alpha absorber counts at low redshift using gas which is associated with moderately extended galaxies (Linder 1997), absorbers are often observed to be associated with galaxies at larger impact parameters from quasar lines of sight than are expected from calculated galaxy absorption cross sections in such absorber-galaxy models. However, a large fraction of absorbers is expected to arise in lines sight through galaxies which are low in luminosity and/or surface brightness, so that they are unlikely to be detected in surveys for galaxies close to quasar lin…
We present a catalogue of the 322 damped Lyman alpha absorbers taken from the literature. All damped Lyman alpha absorbers are included, with no selection on redshift or quasar magnitude. Of these, 123 are candidates and await confirmation using high resolution spectroscopy. For all 322 objects we catalogue the radio properties of the background quasars, where known. Around 60 quasars have radio flux densities above 0.1 Jy and approximately half of these have optical magnitudes brighter than V = 18. This compilation should prove useful in several areas of extragalactic/cosmological research.
