Quasar absorption lines have long been recognised to be a sensitive probe of the abundances, physical conditions, and kinematics of gas in a wide variety of environments including low-density intergalactic regions that probably cannot be studied by any other means. While some pre-Hubble Space Telescope (HST) observations indicated that Mg II absorption lines arise in gaseous galactic halos with a large covering factor, many early QSO absorber studies were hampered by a lack of information about the context of the absorbers and their connections with galaxies. By providing access to crucial ultraviolet resonance lines at low redshifts, deployment of HST and the Far Ultraviolet Spectroscopic Explorer enabled detailed studies of the relationships between QSO absorbers and galaxies. The advent of large surveys such as the Sloan Digital Sky Survey (SDSS) has also advanced the topic by greatly improving the size of absorber and galaxy samples. This paper briefly reviews some observational results on absorber-galaxy connections that have been obtained in the HST/SDSS era, including Mg II absorbers, the low$-z$ Ly$\alpha$ forest, Lyman limit and damped Ly$\alpha$ absorbers, and O VI systems.

The current status and remaining questions of MgII absorbers are reviewed with an eye toward new results incorporating high quality Hubble Space Telescope images of the absorbing galaxies. In the end, we find that our current picture of extended gaseous regions around galaxies at earlier epochs is in need of some revision; MgII absorbing “halos” appear to be patchier and their geometry less regular than previously inferred. We also find that the so-called “weak” MgII absorbers are associated with normal galaxies over a wide range of impact parameters, suggesting that this class of absorber does not strictly select low surface brightness, dwarf galaxies, or IGM material. We emphasise the need for a complete survey of the galaxies in quasar fields, and the importance of obtaining rotation curves of confirmed absorbing galaxies.

For a long time it has been usual to search for the objects that host the gas from which absorption line systems arise. However, the spatial distribution of metals around galaxies and more generally in the intergalactic medium is complex, and it is often very difficult to associate one absorption system with one galaxy. Except for possibly in the special case of DLA systems, it may be more appropriate to discuss the clustering properties of different classes of objects and to measure their correlation functions. I illustrate this approach with three examples: the distribution of metals around galaxies at intermediate redshift, the modelling of the clustering of C IV systems, and the determination of the transverse correlation function of the Lyman-$\alpha$ forest.

Using 138 low column density (N$_{\rm HI}=10^{12.5-16.5}$ cm$^{-2}$) Ly$\alpha$ “forest” absorbers discovered by HST at $z \leq 0.15$ and nearly one million low-$z$ galaxy redshifts from on-going surveys, we find that:

1. Although nearly 80% of Ly$\alpha$ forest absorbers are found within galaxy filaments, very few of them can be identified plausibly with bright ($\geq\,0.1\;{\rm L}^*$) galaxies. Either these absorbers are not related to any one individual galaxy or the individual galaxy to which they are related is fainter than $0.1\;{\rm L}^*$ (M$_B \geq -17$). O VI absorbers (H I+O VI) are found exclusively in galaxy filaments at median distances half as far away from the nearest galaxy as the Ly$\alpha$ absorber population from which they were selected (N$_{\rm HI}=10^{13.5-16.5}$ cm$^{-2}$).

2. Our work on individual absorber-galaxy associations at the lowest redshifts finds cases where low column density (N$_{\rm HI} = 10^{13.5-16.5}$ cm$^{-2}$) metal-enriched absorbers appear to be due to unbound winds from dwarf galaxies while higher column density (N$_{\rm HI}=10^{17.3-20}$ cm$^{-2}$) absorbers are found in the bound halos of massive galaxies. Therefore, both the statistics and a few well-studied examples lead us to propose that the low-$z$ IGM (and the Ly$\alpha$ forest absorbers in particular) are enriched in metals by outflowing winds from dwarf galaxies.

The spectra of the Sloan Digital Sky Survey (SDSS) are being used to construct a catalogue of QSO absorption lines, for use in studies of abundances, relevant radiation fields, number counts as a function of redshift, and other matters, including the evolution of these parameters. The catalogue includes intervening, associated, and BAL absorbers, in order to allow a clearer definition of the relationships between these three classes. We describe the motivation for and the data products of the project to build the SDSS QSO absorption line catalogue.

Deep galaxy redshift surveys in the field around sight lines that probe OVI absorbers is a powerful method to investigate the nature of these absorbers and their correlation with galaxies. Here we present preliminary results of a study of fields observed with the Gemini/GMOS spectrograph centred on three low redshift QSOs known to probe a variety of OVI systems. Notably, some OVI absorbers appear to be associated with isolated, individual galaxies. Others are clearly found within large-scale structures but at substantial distances from any individual galaxy.

I review current results from searching for galaxies giving rise to damped Ly$\alpha$ absorbers (DLAs) at $z<1$. Using 14 confirmed DLA galaxies, I further show that intermediate-redshift galaxies possess a large H I envelope out to $24-30\ h^{-1}$ kpc in radius. The photometric and spectral properties of these galaxies confirm that DLA galaxies are drawn from the typical field population, and not from a separate population of low surface brightness or dwarf galaxies. The spatial distribution of metals in the cold ISM of intermediate-redshift galaxies is characterised by a radial gradient of $-0.041\pm 0.012$ dex per kiloparsec (or equivalently a scale length of $10.6\ h^{-1}$ kpc) to $30\ h^{-1}$ kpc radius based on an ensemble of six galaxy-DLA pairs. Adopting this abundance gradient and known $N({\rm H I})$ profiles of nearby galaxies, I show that the observed low metal content of the DLA population can arise naturally as a combination of gas cross-section selection and metallicity gradients commonly observed in local disk galaxies.

Using integral field spectroscopy (IFS) we have observed QSOs which were previously known to have intervening DLAs, to search for emission lines in the field surrounding the QSOs. The sample consists of 13 DLAs plus 7 sub-DLAs at $z>2$, from which we find emission line candidates at the expected wavelengths for 8 systems, implying a detection frequency of 40%. Emission lines detected at the DLA redshifts are interpreted as being due to Ly$\alpha$ lines from galaxies associated with the DLA clouds. Candidates are detected at $3-4\sigma$ levels in both narrow-band images and spectra created from the data cubes. We find no preference for candidate detection associated with either high or low metallicity DLAs. In comparison with the properties of previously known DLA galaxies, the candidates show similar distributions of line fluxes and line widths, but their impact parameters are slightly different, suggesting H I cross sections larger than 10 kpc.

We studied a sample of 38 intermediate redshift MgII absorption-selected galaxies using (1) Keck/HIRES and VLT/UVES quasar spectra to measure the halo gas kinematics from MgII absorption profiles and (2) HST/WFPC–2 images to study the absorbing galaxy morphologies. We have searched for correlations between quantified gas absorption properties, and host galaxy impact parameters, inclinations, position angles, and quantified morphological parameters. We report a $3.2\sigma$ correlation between asymmetric perturbations in the host galaxy morphology and the MgII absorption equivalent width. We suggest that this correlation may indicate a connection between past merging and/or interaction events in MgII absorption-selected galaxies and the velocity dispersion and quantity of gas surrounding these galaxies.

Using a large sample of MgII absorbers with $0.4<z<2.2$ detected by Nestor et al. (2005) in the Early Data Release of the SDSS, we present new constraints on the physical properties of these systems based on two statistical analyses: (i) by computing the ratio between the composite spectra of quasars with and without absorbers, we measure the reddening effects induced by these intervening systems; and (ii) by stacking SDSS images centred on quasars with strong MgII absorption lines and isolating the excess light around the PSF, we measure the mean luminosity and colours of the absorbing galaxies. This statistical approach does not require any spectroscopic follow up and allows us to constrain the photometric properties of absorber systems.

We discuss issues concerning the physical nature of intervening MgII quasar absorption systems in light of results from our recent surveys using SDSS EDR QSO spectra and data obtained at the MMT. These surveys indicate an excess of weak ($W_0^{\lambda2796} \lesssim 0.3$Å) systems relative to the exponential $\partial N/\partial W_0$ distribution of stronger systems. The incidence of intermediate-strength lines shows remarkably little evolution with redshift, thereby constraining models for the nature of the clouds comprising these absorbers. The total distribution does evolve, with the incidence decreasing with decreasing redshift in a $W_0^{\lambda2796}$-dependent rate (the strongest systems evolve the fastest). This suggests that multiple populations that evolve at different rates contribute to the incidence in a $W_0^{\lambda2796}$-dependent manner. We also present two images of fields containing unprecedented “ultra-strong” ($W_0^{\lambda2796} \ge 4.0$Å) MgII absorbers.

We present results from two projects in which we have used the HI 21cm emission line as a tracer of gas-rich galaxy populations in the vicinity of Lyman-$\alpha$ absorbers. In the first case, we examine the HI environment of SBS 1543+593, the nearest damped Lyman-$\alpha$ absorber. We use a VLA map of the region around this LSB galaxy which itself shows an extended HI disk to identify 3 gas-rich neighbours within 185 kpc. While it is not clear whether we should expect local damped Lyman-$\alpha$ systems to reside in such gas-rich regions, we would expect this kind of environment to be more prevalent at higher redshifts where less of the gas is in the dense inner regions of galaxies or has been consumed by star formation. This local galaxy is the only system in which we can study the gaseous environment in this kind of detail. In the second case, we examine the HI environment surrounding 16 Lyman-$\alpha$ forest absorbers along 4 QSO sight-lines. We do not detect any gas-rich galaxies at the absorber positions indicating that, at least down to our sensitivity limits, these absorption lines do not seem to be associated with galaxy halos. For half of the Lyman-$\alpha$ absorption systems there is a galaxy within 500 kpc, but for the other half there is not. In two cases there is no galaxy within 2 Mpc of the Lyman-$\alpha$ absorption systems indicating that absorbers do, in some cases, reside in voids.

Sloan Digital Sky Survey (SDSS) quasar spectroscopy is yielding a database of strong low-ionisation MgII absorbers over the redshift interval $0.36<z<2.28$ which is over two orders of magnitude larger than anything previously assembled. Hubble Space Telescope (HST) UV spectroscopy has been used to measure neutral hydrogen column densities for a small subset of them. These data empirically show that MgII absorbers with rest equivalent widths $W^{\lambda2796}_0 \ge 0.6$ Å have a mean neutral hydrogen column density that is roughly constant at $N(HI) \approx 4\times10^{20}$ atoms cm$^{-2}$, with individual systems lying in the damped Ly$\alpha$ (DLA) and sub-DLA regimes. Since the MgII doublets generally exhibit saturation, the $W^{\lambda2796}_0$ values are an indication of the absorbers' velocity spreads. Thus, we can study neutral-gas-phase metallicities as a function of kinematics by forming SDSS composite spectra and measuring weak unsaturated metal lines that form in neutral gas (e.g., CrII, FeII, MnII, SiII, ZnII) as a function of $W^{\lambda2796}_0$. We use this method on SDSS composite spectra to show how metallicity and kinematics are positively correlated for large-$N(HI)$ absorbers, including trends related to dust depletion and the enhancement of $\alpha$-elements. We also discuss the need to account for selection effects in DLA surveys, and we make inferences about models for DLA absorption and their contribution to cosmic star formation.

We calculate in detail the expected properties of low redshift DLAs under the assumption that they arise in the gaseous disks of galaxies like those in the $z\approx 0$ population. A sample of 355 nearby galaxies were analysed, for which high quality H I 21-cm emission line maps are available as part of an extensive survey with the Westerbork telescope (WHISP). We find that expected luminosities, impact parameters between quasars and DLA host galaxies, and metal abundances are in good agreement with the observed properties of DLAs and DLA galaxies. The measured redshift number density of $z=0$ gas above the DLA limit is $dN/dz=0.045\pm 0.006$, which compared to higher $z$ measurements implies that there is no evolution in the co-moving density of DLAs along a line of sight between $z\sim 1.5$ and $z=0$, and a decrease of only a factor of two from $z\sim 4$ to the present time. We conclude that the local galaxy population can explain all properties of low redshift DLAs.

The discovery of the first damped Lyman alpha (DLA) system in the early 1970s followed by the recognition that DLAs arise in intervening galaxies opened up a new field of galaxy evolution research. These highest HI column density absorption-line systems trace the bulk of the observed neutral gas in the Universe, and therefore, have been used as powerful probes of galaxy formation and evolution back to the redshifts of the most distant quasars. The history and progress of DLA research over the past several decades is reviewed here. Larger datasets and deeper surveys, particularly over the last couple of years, have improved our knowledge of the neutral gas content and distribution in the Universe at all observable redshifts, including the present epoch. New results on the statistics of DLAs at $z<1.65$ from our HST-UV surveys are presented and discussed in the context of recent results at $z=0$ and at high redshift. We find that $\Omega_{DLA}(z>0)$ remains roughly constant to within the uncertainties; the $z=0$ value of the neutral gas mass density, $\Omega_{g}$, is a factor of $\approx 2$ less than $\Omega_{DLA}$. The DLA incidence, $n(z)$, undergoes rapid evolution between redshifts 5 and 2, but is consistent with the no-evolution curve in the current concordance cosmology for $z\lesssim 2$. We also show that if the local Schmidt law relating surface density of gas and star formation rate (SFR) is valid at the DLA redshifts, then the DLA SFR density is too low for them to provide a significant contribution to the cosmic star formation history (SFH) at $z\gtrsim 1$. This implies that the DLAs are unlikely to be the same population as the star forming galaxies (i.e. the Lyman break and sub-millimetre galaxies) that dominate the SFH of the high redshift Universe. We suggest that this discrepancy and the DLA “missing metals” problem could be the result of missing very high column density gas due to its very small absorption cross-section.

We determine star formation rates for a sample of 50 DLAs from properties of the absorbing gas alone. Assuming thermal balance, we determine the grain photoelectric heating rate of the neutral gas from the [C II] $158\,\mu$ m cooling rate per H atom, $\ell_{c}$, inferred from C II$^{*}$ 1335.7 and damped Ly$\alpha$ absorption lines. We deduce the star formation rate per unit area and the FUV luminosity per unit co-moving volume, ${\cal L}_{\nu}^{DLA}$. Comparison of ${\cal L}_{\nu}^{DLA}$ with the luminosity density of Lyman Break Galaxies, ${\cal L}_{\nu}^{LBG}$, shows that ${\cal L}_{\nu}^{DLA}>>{\cal L}_{\nu}^{LBG}$ for most models. These models are ruled out if our assumption that LBGs dominate the total FUV luminosity density of the Universe is correct. The only feasible models are those in which ${\cal L}_{\nu}^{DLA}\approx {\cal L}_{\nu}^{LBG}$. We conclude that DLAs in which $\ell_{c}>10^{-27.1}$ ergs s$^{-1}$H$^{-1}$ contain centrally located LBGs, while the gas in DLAs with $\ell_{c}<10^{-27.1}$ ergs s$^{-1}$H$^{-1}$ is heated by background radiation alone.

HI 21cm absorption spectroscopy provides an independent probe of the nature of damped Lyman-$\alpha$ systems (DLAs), yielding information on their ISM conditions, kinematics, and spatial extent. We find that the majority of DLAs have significantly higher spin temperatures than seen in the Milky Way and local spirals, indicating larger fractions of the warm phase of HI, especially at high redshifts. Present data suggests that the spin temperature might be used as a secondary indicator of the nature of the absorbing galaxy. We have succeeded in spatially resolving the 21cm absorption in two $z\sim 0.4$ DLAs, thus obtaining lower limits on the transverse extent of these absorbers.

We combine FUV spectra obtained with FUSE and STIS/E140M of HE 0226–4116 ($z=0.495$), PG 1116+215 ($z_{\rm em}=0.177$), and PG 1259+593 ($z_{\rm em}=0.478$) to determine the distribution of column density and Doppler width of the Ly$\alpha$ absorber population at $z<0.5$ detected in the spectra of these 3 quasars. The high spectral resolution UV spectra allow us to derive simultaneously the redshift, column density, and Doppler width of each absorber. Two different populations of H I absorbers appear to be present: narrow and broad H I absorptions that could be tracers of the warm photoionised IGM ($T < 10^5$ K) and warm-hot ionised medium (WHIM, $T \sim 10^5-10^6$ K). For a reliable assessment of the baryonic content of the Ly$\alpha$ forest at low $z$ it is crucial to determine the relative numbers of photoionised absorbers vs. hot collisionally ionised absorbers. Preliminary results indicate that 30–40% of the total baryons are in the photoionised IGM and at least 20% in the WHIM. We also discuss the observed evolution of the Doppler parameter with redshift.

We study the physical conditions in damped Lyman-$\alpha$ systems (DLAs), using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H$_2$ by Ledoux et al. (2003). H$_2$ is detected in 13-20% of the DLAs in our sample. Using the rotation level populations of H$_2$ and fine-structure excitations of C I we show the mean kinetic temperature of H$_2$ components is 153$\pm$78 K, $n_{\rm H}$ = 10$-$250 cm$^{-3}$, and the ambient radiation field is similar to or slightly higher than the mean diffuse UV field of the Galaxy. Combining this with the success rate of detecting H$_2$ in DLAs we conclude that at least 13$-$20% of DLAs at $z_{\rm abs}\ge1.9$ show the presence of CNM and substantial star-formation activity. C II$^*$ absorption is detected in all the components where H$_2$ absorption is seen. The level populations of C II in these systems is consistent with the physical parameters derived from the excitation of H$_2$ and C I. We detect C II$^*$ in about 50% of the DLAs and therefore in a considerable fraction of DLAs that do not show H$_2$. The absence of C I absorption, the measured N(C II$^*$)/N(C II) and N(Al III)/N(Al II) ratios in these systems are consistent with the gas having lower density (n$_{\rm H} = 0.3-6 cm^{-3}$) than that seen in the H$_2$ components. 50% of the DLAs that do not show C II$^*$ are consistent with them originating from a low density warm neutral medium.

To date, half a dozen DLAs have been detected in GRB afterglows, but most have modest signal-to-noise ratios. Our detection of a DLA in the afterglow of GRB 030323 is unambiguous: its neutral hydrogen column density is log N(HI)=21.90$\pm$0.07, higher than any (GRB- or QSO-) DLA HI column density inferred directly from Ly$\alpha$ in absorption. We discuss several other properties such as the metallicity, the detection of fine-structure lines of SiII, and our strong upper limit on the H$_2$ content. The GRB afterglow allows us to infer all these quantities, even with the host being as faint as V=28. The successful launch of the Swift satellite (November 2004) has made high-resolution spectroscopy of a large sample of GRB afterglows possible. This will offer a unique way to study in detail the interstellar medium inside GRB host galaxies as a function of redshift.