We used our database of ESO VLT-UVES spectra of quasars to build up a sample of 67 damped Lyman-$\alpha$ (DLA) systems with redshifts $1.7<z_{\rm abs}<3.7$. For each system, we measured average metallicities relative to Solar, [X/H] (with either ${\rm X}={\rm Zn}$, S, or Si), and the velocity widths of low-ionisation line profiles, $W_{1}$. We find that there is a tight correlation between the two quantities, detected at the $5\sigma$ significance level. The existence of such a correlation, over more than two orders of magnitude spread in metallicity, is likely to be the consequence of an underlying mass-metallicity relation for the galaxies responsible for DLA absorption lines. The best-fit linear relation is $[{\rm X}/{\rm H}]=(91.35\pm 0.11)\log W_{1} -(3.69\pm 0.18)$ with $W_{1}$ expressed in km s$^{-1}$. While the slope of this velocity-metallicity relation is the same within uncertainties between the higher and the lower redshift bins of our sample, there is a hint of an increase in the intercept point of the relation with decreasing redshift. This suggests that galaxy halos of a given mass tend to become more metal-rich with time. Moreover, the slope of this relation is consistent with that of the luminosity-metallicity relation for local galaxies. The DLA systems having the lowest metallicities among the DLA population would therefore, on average, correspond to the galaxies having the lowest masses. In turn, these galaxies should have the lowest luminosities among the DLA galaxy population. This may explain the recent result that the few DLA systems with detected Ly$\alpha$ emission have higher than average metallicities.

We report the current results of our on-going search for molecular hydrogen (H$_2$) in damped Lyman-$\alpha$ (DLA) and sub-DLA systems at $1.8<z_{\rm abs}<3.4$ using the Ultraviolet and Visible Echelle Spectrograph (UVES) installed at the ESO's Very Large Telescope (VLT), Unit Kueyen.

We report the results obtained from optical and near-infrared spectroscopy and also broad-band photometry of lensed low-luminosity galaxies ($1.3\leq z\leq 2.3$) located in the core of lensing clusters. The amplification factor allowed us to obtain the physical properties (SFR, abundance ratios, mass, age of the burst, dust contents, etc.) of star-forming galaxies, 1 to 2 magnitudes fainter than galaxies in a blank field, using the same indicators as in studies of low-redshift galaxies. A part of this study was concentrated on the dynamics of distant lensed galaxies using Integral Field Spectroscopy (SINFONI/VLT science verification programme).

A sample of distant ($z>0.4$) luminous infrared galaxies (LIRGs) selected from ISOCAM deep survey fields (CFRS, UDSR, UDSF) have been studied on the basis of their high-quality optical spectra from VLT/FORS2 (R=5Å). Robust estimates of dust extinction can be considered via the energy balance between the infrared and H$\beta$ luminosities, after correcting the underlying Balmer absorption properly. Oxygen abundances [12+log(O/H)] in the interstellar medium of the sample galaxies estimated from the “strong-line” method show a range from 8.36 to 8.93, with a median value of 8.67, which is 0.5 lower than that of local bright disks (i.e. $L^*$) at the given magnitude. The timescale to double the stellar masses of such LIRGs can be very short, 0.1-1 Gyr. A significant fraction of distant large disks are indeed LIRGs. Such massive disks could have formed $\sim50$% of their metals and stellar masses since $z\sim1$.

HE0512–3329 is a gravitationally-lensed double QSO with damped Ly$\alpha$ systems observed at $z=0.931$ in front of both QSO images (DLA A and B). We have obtained spatially resolved HST STIS and optical VLT UVES spectra of both QSOs in order to study differences in the metal abundances across the lines of sight. We detect substantial differences, of roughly 0.5 dex, in both [Mn/H] and [Fe/H], on a transverse scale of 5 $h^{-1}$ kpc. Differential dust depletion appears as the most plausible explanation.

We present initial results from an high-resolution imaging search for damped Ly$\alpha$ absorbers toward the high-redshift quasar APM 08279+5255 by using the Adaptive Optics system (AO) attached to the Subaru telescope. We detected in total 11 objects within a $23''\times23''$ field around the quasar. Among these detected objects, we identified a possible candidate for the galaxy giving rise to the damped Ly$\alpha$ absorption at z$_{abs}$=2.974 with a criteria of distance from the sight-line toward the quasar and luminosity.

We present preliminary results of two surveys of weak Mg II (rest frame equivalent width $W_r$(2796) $<$ 0.3Å) absorbers at $\langle z \rangle$=0.15 and at $\langle z \rangle$=1.75. For the low redshift survey, we used 25 HST/STIS echelle quasar spectra ($R$=45,000) that covered Si II$\lambda$1260 and C II$\lambda$1335 at 0 $<z<0.3$. These transitions were used as a tracer of Mg II$\lambda$2796, allowing a survey of redshift path-length $g(z)$=5.3. Our high redshift survey covers a redshift path-length of $g(z)$=6.7 in 15 high resolution QSO spectra obtained with UVES on VLT. We find that $dN/dz=0.96\pm 0.19$ for $0<z<0.3$ and $0.02<W_r$(2796) $<$ 0.3Å, and $dN/dz=1.05\pm 0.16$ for $1.4<z<2.2$ over the same equivalent width range. These numbers are to be compared to the results of a previous survey of weak Mg II absorbers: $dN/dz=1.74\pm 0.10$ for $0.4<z<1.4$ (Churchill et al. 1999). There appears to be a peak epoch for weak Mg II absorbers at $z\sim$0.9. At higher redshifts, the observed value is significantly less than expected for cosmological evolution, and somewhat smaller than expected considering the larger extragalactic background radiation at $\langle z\rangle$=1.75. We conclude from this that the processes that produce weak Mg II absorbing structures are equally, or somewhat less, active at $\langle z\rangle$=1.75 than at $\langle z \rangle$=0.9. At lower redshifts, $\langle z\rangle$=0.15, we would expect a larger number of weak Mg II absorbers than observed (comparing to intermediate redshifts). A significant fraction of these expected objects would result from the evolution of low density structures in which only C IV absorption was observable at higher redshifts. The overall evolution of weak Mg II absorbers is consistent with them being related to some type of global star formation activity, perhaps restricted to certain environments. More detailed examination of the evolution of the properties of weak Mg II absorbers is needed.

Using H I absorption alone, we attempt to separate H I absorption lines in quasar spectra into two categories; HDLs (Higher Density Lines) and LDLs (Lower Density Lines), and we discuss the difference in their physical properties. We deblend and fit all H I lines with Voigt profiles, and make an unbiased sample of H I lines covering a wide column density range ($12 < \log N_{HI} < 19$ cm$^{-2}$). To reduce the influence of line blending, we simultaneously fit several Lyman series lines. As a result of a two-point correlation analysis, we found that higher column density H I lines are clustering at $\Delta v < 200$ km s$^{-1}$, while lower ones at $\Delta v < 100$ km s$^{-1}$. We define HDLs as H I lines with $15<\log N_{HI} < 19$ cm$^{-2}$ and all H I lines within $\pm$200 km s$^{-1}$ of a line with $\log N_{HI} > 15$ cm$^{-2}$, and LDLs as others with $12 < \log N_{HI} < 15$ cm$^{-2}$. We found that the HDLs have smaller minimum $b$-values for a given column density than the LDLs. This difference is successfully reproduced by our hydrodynamic simulation. The LDLs seem to be cool or shock-heated diffuse IGM gas, while the HDLs are likely to be cooler dense gas near to galaxies.

The cosmological variability of $\alpha$ is probed from individual observations of pairs of Fe II lines. This procedure allows a better control of the systematics and avoids the influence of the spectral shifts due to ionisation inhomogeneities in the absorbers and/or non-zero offsets between different exposures. Applied to the Fe II lines of the metal absorption systems at $z_{\rm abs}=1.839$ in Q1101–264 and at $z_{\rm abs}=1.15$ in HE0515–4414 observed by means of UVES at the ESO-VLT, it provides $\Delta\alpha/\alpha = (0.4\pm1.5_{\rm stat})\times10^{-6}$. The result is shifted with respect to the Keck/HIRES mean $\Delta\alpha/\alpha = (-5.7\pm1.1_{\rm stat})\times10^{-6}$ (Murphy et al. 2004) at a high confidence level (95%). Full details of this work are given in Levshakov et al. (2005).

We investigate the evolution of damped Ly$\alpha$ absorption systems (DLA) from hierarchical structure formation models by exploring the basic properties of DLA host-galaxies (DLA galaxies) to clarify the connection between DLAs and galaxies. We find that DLA galaxies primarily consist of low surface brightness dwarf galaxies. For example, at redshift $z \le 1$, DLA galaxies have typical radius 3 kpc, surface brightness 22–27 mag arc-sec$^{-2}$, and star formation rate $10^{-2}$ M$_{\odot}$/yr. We discuss selection effects in the detection in emission caused by compactness and brightness. Finally, we explore the radio properties at $z=0$ by comparing the observational results from blind radio surveys.

We present a measurement of $\left[\rm{O}/\rm{Fe}\right]$ versus $\left[\rm{Fe}/\rm{H}\right]$ in the $z>2$ IGM derived from a sample of 23 Lyman Limit systems whose column density lies in the range $19.0\le\log N_{HI}\le 20.2$ cm$^{-2}$. The systems span a metallicity range of $-2.70 \le \left[\rm{Fe}/\rm{H}\right]\le -0.50$, allowing for a direct comparison with similar studies of stars within the Milky Way. Although the Lyman Limit systems can be highly ionised, the effects of ionisation on the determination of $\left[\rm{O}/\rm{Fe}\right]$ is minor for most of the systems in the sample. The data appear to show a decrease in $\left[\rm{O}/\rm{Fe}\right]$ with increasing metallicity with an approximate slope of $m\simeq-0.55$. We also determine a best fit power law slope of $\alpha=-1.86 \pm 0.2$ to the column density distribution $f_{HI}(N)$ for $19.0 \le\log N_{HI}\le 20.2$ cm$^{-2}$.

We present preliminary results of a detailed analysis of the $z=3.55$ Lyman limit system observed in the optical spectrum of Q2000-330. We have fitted Voigt profiles to the system to determine the column densities and Doppler parameters of its many components. For its major components we have also determined the ionisation fraction of the gas and obtained crude estimates of metallicities. We find order of magnitude variations in ionisation fraction and metallicity in components separated by as little as 150 km s$^{-1}$ in velocity space. This work is presented as an example of a campaign we have begun to collect such information on a sample of $\sim 100$ such systems in an effort to accurately characterise this enigmatic and important class of quasar absorption line systems.

We discuss the physical properties and the baryonic content of the Warm-Hot Intergalactic Medium (WHIM) at low redshifts. Cosmological simulations predict that the WHIM contains a large fraction of the baryons at $z=0$ in the form of highly-ionised gas at temperatures between $10^5$ and $10^7$ K. Using high-resolution ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) we have studied the WHIM at low redshifts by searching for intervening O VI and thermally broadened Lyman $\alpha$ (BLA) absorption toward a number of quasars and active galactic nuclei (AGNs). Our measurements imply cosmological mass densities of $\Omega_b$(O VI)$\geq0.0022 h_{75}^{-1}$ and $\Omega_{b}$(BLA)$\geq0.0035\,h_{75}^{-1}$. Our results suggest that the WHIM at low $z$ contains more baryonic mass than stars and gas in galaxies.

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 H I Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density ($N_{\rm HI}<10^{15}$ cm$^{-2}$) Lyman-$\alpha$ absorbers from the literature. The absorber-galaxy cross-correlation function shows that on scales from 1–10 h$^{-1}$ Mpc, absorbers are embedded in halos with masses similar to that of galaxy groups.

We present preliminary results from the Hamburg/ESO survey for damped Ly-$\alpha$ (hereafter, DLA) systems. This survey is characterised by (i) good knowledge of the biases affecting the parent QSO survey, and (ii) the brightness and (iii) relatively wide magnitude distribution of the background QSOs. Therefore, it is well–suited to study possible magnitude–dependent biases in DLA surveys, such as the one expected from dust obscuration.

We have systematically searched for damped Lyman-$\alpha$ line candidates in 5 Å, resolution spectra of the 188 QSOs that constitute our statistical sample. These candidates have later been re-observed with UVES at the ESO–Very Large Telescope (VLT) for confirmation and accurate $N(HI)$ measurements. In the redshift range covered by the survey, 19 DLA systems have been discovered. Over the whole survey, we find that the number density $n(z)$ and cosmological density of gas $\Omega_\mathrm{gas}$ have comparable values to the ones obtained by CORALS (Ellison et al. 2001).

However, the number densities of DLA systems $n(z)$ in two sub–samples of equal absorption distance path defined by the magnitude of the background QSOs differ by a factor of $\approx 5$. We estimate that the probability that $n(z)$ is equal in the two sub–samples is $<0.003$. A similar, only slightly less significant difference is found for $\Omega_\mathrm{gas}$.

We investigate the variations in the broadband luminosities of low and intermediate redshift galaxies due to the stochastic nature of the neutral hydrogen distribution present in the form of Ly$\alpha$ absorbers in the intergalactic medium. This effect is caused by variations in the distribution and properties (redshift, column density, Doppler parameter) of the absorbers along different lines-of-sight out to a given redshift. Using a set of observationally constrained redshift-, column density-, and Doppler parameter distributions we perform Monte Carlo simulations for a large number of lines-of-sight towards galaxies at a given redshift $z_{em}$ and calculate attenuated FUV/NUV magnitudes and corresponding 1-, 2-, and 3$\sigma$ variations. We predict significant variations in luminosity ranging from -0.22 to +0.48 magnitudes at the 1$\sigma$level for galaxies at$z=1.5$inGALEXdata.

With the information era in astronomy coming, this “data avalanche” may provide many answers to important problems in contemporary astrophysics. The most important problem is sifting through massive amounts of data to mine knowledge. In this paper, we positionally cross-identify multi-wavelength data from optical, near-infrared, and X-ray bands, and then employ Alternating Decision Trees (ADTree) to quickly and robustly separate AGN candidates to a high degree of accuracy. We emphasise the application of the method due to the development of large survey projects and the establishment of the virtual observatory, and conclude that the application of data mining algorithms in astronomy is of great importance to discover new knowledge impossible to obtain before, and promote the development of astronomy.

Significant progress has been made in the last decade toward an understanding of the intergalactic He II absorption. At redshifts $z<2.8$, individual absorbers are found to be ionised mainly by a hard quasar continuum, but their ionisation states vary considerably and the ionising radiation field may have contributions from a soft stellar continuum. The He II opacity increases significantly at $z\sim 2.8$, suggesting a possible end of the general reionisation of intergalactic He II. Using the massive SDSS database and UV selection with HST and GALEX, we have found new quasars at $z>3.5$, with detected He II absorption. Future observations will pinpoint the epoch of reionisation of the intergalactic He II.