We present the results from a suite of N-body simulations of the tidal stripping of two-component dwarf galaxies comprising some stars and dark matter. We show that recent kinematic data from the local group dwarf spheroidal (dSph) galaxies suggests that dSph galaxies must be sufficiently massive ($10^9$–$10^{10}$M$_\odot$) that tidal stripping is of little importance for the stars. We discuss the implications of these massive dSph galaxies for cosmology and galaxy formation.

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.

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.

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).

We summarise recent results assessing the carbon and silicon abundances of the intergalactic medium (IGM) using the ‘pixel optical depth’ technique. We briefly discuss the implications of these results for models of intergalactic enrichment, focusing on distinguishing ‘early’ $z \gg 4$ enrichment by the first generations of stars and objects from ‘late’ enrichment by $2 \lesssim z\lesssim 5$ Ly-break galaxies. We then discuss the comparison of observed QSO spectra to simulated spectra generated from cosmological simulations that self-consistently include enrichment, and draw qualitative implications for the general picture of intergalactic enrichment at $z \gtrsim 2$.

One hundred early-type members of the Virgo Cluster have been observed with the Hubble Space Telescope (HST) as part of the ACS Virgo Cluster Survey. The survey and its extensions consists of F475W and F850LP imaging from the Advanced Camera for Surveys (ACS), supplemented by additional imaging and spectroscopy from WFPC2 HST, Chandra, Spitzer, Keck, KPNO, and CTIO. In this article, I report briefly on four scientific highlights from the program: (1) the discovery of a population of probable ultra-compact dwarf galaxies; (2) an improved measurement of the frequency of compact stellar nuclei in early-type galaxies; (3) a precise new measurement of Virgo's line-of-sight depth; and (4) new insights into the chemical evolution of galaxies from the color distributions of their globular cluster systems.

Absorbers seen in the spectrum of background quasars are a unique tool to select HI-rich galaxies at all redshifts. These allow us to determine the cosmological evolution of H I gas, $\Omega_{\rm HI+HeII}$, a possible indicator of gas consumption as star formation proceeds. The damped Lyman-$\alpha$ systems (DLAs with NHI $\ge$ 10$^{20.3}$ cm$^{-2}$) in particular are believed to contain a large fraction of the H I gas, but there are also indications that lower column density systems, namely “sub-damped Lyman-$\alpha$” systems play a role at high-redshift. Here we present the discovery of high-redshift sub-DLAs based on 17 $z>4$ quasar spectra observed with the Ultraviolet-Visual Echelle Spectrograph (UVES) on VLT. This sample is composed of 21 new sub-DLAs which, together with another 10 systems from previous ESO archive studies, make up a homogeneous sample. The redshift evolution of the number density of several classes of absorbers is derived and shows that all systems seem to be evolving in the redshift range from $z=5$ to $z\sim3$. The redshift evolution of the column density distribution, $f(N,z)$, down to NHI=10$^{19}$ cm$^{-2}$ is also presented. A departure from a power law due to a flattening of $f(N,z)$ in the sub-DLA regime is present in the data. $f(N,z)$ is further used to determine the H I gas mass contained in sub-DLAs at $z>2$. The complete sample shows that sub-DLAs are important at all redshifts from $z=5$ to $z=2$.

We use LUQAS, a sample of 27 high resolution high signal-to-noise UVES quasar (QSO) spectra (Kim et al. 2004), and the Croft et al. (2002) sample together with a set of high resolution large box-size hydrodynamical simulations run with the code GADGET-II to recover the linear dark matter power spectrum at $z>2$ and at scales of $1-40\,h^{-1}\,$ Mpc. These scales cannot be probed by any other observable. We address some of the uncertainties in the theoretical modelling of the Lyman-$\alpha$ forest structures such as the difference between full hydro-dynamical simulations and a more simplified scheme based on the HPM (Hydro-Particle-Mesh) technique. We combine these data sets with the Cosmic Microwave Background (CMB) data of WMAP in order to get tighter constraints on cosmological parameters. We focus on the recovered values of the power spectrum amplitude, the primordial spectral index and the running of the primordial spectral index. By considering models of slow-roll inflation we give constraints on inflationary parameters. We explore the implications for the mass of a warm dark matter particle, including light gravitinos and sterile neutrinos, and from the perspective of constraining the neutrino mass.

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.

We attempt to define the structural properties of Broad Absorption Line (BAL) QSOs and their relationship with the general quasar population using the Eigenvector 1 correlations. We identify 8 low-redshift quasars (z $<$ 0.5, 6 with a BALnicity index $>$ 0 km s$^{-1}$) where it was possible to combine optical and UV spectroscopic observations. The special utility of low-$z$ quasars involves our ability to discuss CIV$\lambda$1549 BAL QSOs in the context of the Eigenvector 1 optical parameter space and to have a reliable measure of the quasar rest frame. We find that the majority of the BAL sources are population A sources as defined in Sulentic et al. (2000). At least 2 sources that are hosted by ultra-luminous IR galaxies show intriguing effects in their nuclear spectra. A possible correlation between the terminal velocity and luminosity L also suggests that the luminosity to black hole mass ratio ($L/M$) is a governing factor with the largest terminal velocity BALs showing the highest $L/M$ values. The CIV$\lambda$1549 emission line profiles of classical BALs show the ubiquitous E1 population A blue-shift that supports a disk+wind scenario with an opening angle of $<$ 45$^\circ$. Observation of “secondary” BAL features roughly in correspondence with the mean radial velocity of the CIV$\lambda$1549 emission motivates us to model the BAL systems with an additional component that may share the BLR outflow and may be co-axial with the accretion disk, perhaps associated to a significant black hole spin.

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.

Until recently, dwarf elliptical galaxies (dEs) were mostly believed to be gas-less systems, having lost their interstellar medium (ISM) through a galactic wind or through ram pressure stripping in the dense cluster environment. However, recent H$\alpha$ and HI surveys indicate that some dEs in cluster environments are able to retain part of their gas (Drinkwater et al. (2001), Conselice et al. (2001)). In a detailed observational study of the ISM of a sample of Fornax dEs, we detected genuine dEs containing warm and cold gas (De Rijcke et al. (2003), Michielsen et al. (2004), Buyle et al. (2005)).

Using an N-body/SPH code, we explore the two gas-removing scenarios in order to assess their efficiency in removing the ISM from dEs. We simulated dEs subjected to ram pressure stripping by the intracluster medium (ICM) and found that, while small dEs are instantaneously stripped, more massive dEs are able to retain part of their ISM over long time-scales. Isolated dEs on the other hand are expected to lose their gas through a galactic wind. Our simulations show that supernova explosions can blow (super)bubbles in the ISM, thereby ejecting part, though not all, of the ISM.

We have entered a new era in the observational study of the kinematics of dwarf spheroidal and dwarf elliptical galaxies. Large telescopes with multi-object spectrographs can now obtain radial velocities of hundreds if not thousands of individual stars in the nearer dSph systems, while new sensitive longslit spectrometers can measure the kinematics of dE galaxies from their integrated light to unprecedented faint surface brightness levels. I review where we have been, where we are, and where we are going in the kinematic studies of these dwarfs systems.

We present first studies of dE galaxies by means of 3D spectroscopy, and address questions concerning data processing and analysis techniques used to extract internal kinematics and stellar population properties from integrated light spectra.

A colour analysis of a carefully selected sample of over 200 Virgo cluster dEs and 8 giant ellipticals (Es) in the SDSS yields the following results: a) the nucleated dEs (dENs) follow a tight colour-magnitude relation (CMR) which does not broaden significantly at fainter ($M_r \ge -15$) magnitudes, b) the CMR of dENs smoothly transitions to the CMR of Es, but the latter changes its slope at about $M_r \approx -20$, c) the brighter dENs are consistent with ages of $t \ge 5\rm{Gyr}$ and a decrease of metallicity towards fainter magnitudes, but a possible additional luminosity-age relation is not ruled out, d) it is crucial to treat dS0s separately from dEs since their different colour properties would otherwise bias the comparison of nucleated and non-nucleated dEs (dEnoNs), e) the dEnoNs show a weak trend towards bluer colours than dENs.

The Tully-Fisher (TF) relation is an empirically establish correlation between the luminosity of a spiral galaxy and its rotational velocity (Tully-Fisher, 1977). We used the Tully-Fisher relation to probe the dark matter (DM) distribution in the optical regions of spiral galaxies. We investigated a sample of rotation curves that includes 957 galaxies. We applied a new technique that takes advantage of the full knowledge of the galaxy rotation curves.

We present results from photometric and spectroscopic survey of giant stars in the Leo I dwarf spheroidal galaxy. We find compelling evidence that this galaxy has been tidally disrupted.

We have studied the distribution of dwarf-galaxy satellite systems around our Milky Way (MW) and the Andromeda (M31) galaxy. The anisotropy is quantified and the form of their distribution is found to be incompatible with that expected if they were cold-dark-matter sub-structures. The origin of these satellites therefore can not be cosmological. Rather the Milky Way and Andromeda satellites probably stem from a local evolutionary mechanism.

We study the faint end of the HI mass function (HIMF) in order to test the predictions of the CDM theory on the number density of objects with small (dark) masses. The neutral hydrogen is much better tracer of the underlying mass distribution compared to the luminous matter and can be used to test the existence of a population of small galaxies in which the star formation has been partially or completely suppressed during cosmic evolution. Due to technical limitations, the existing HI surveys are not very sensitive on masses below 10$^8$ HI $M_{\odot}$. We designed a blind HI survey to be sensitive for objects with small HI masses. The surveyed area is in the Canis Venatici groups of galaxies and covers in total $\approx$86 deg$^2$ of sky, with the observed velocities in the range $-350 < cz < 1400$ km s$^{-1}$. We detected 69 objects, 22 of them for the first time in HI. All new HI detections fall in the lower part of the mass-histogram, confirming our ability to detect galaxies with small HI masses. The calculated HIMF is flat in the faint end regime ($\alpha {\sim}{-}$1), different from the steep rise predicted by CDM models. Possible effects of the environment on the estimated HIMF parameters are discussed.

An all-sky list of 88 nearby dwarf spheroidal (dSph) galaxies with distances $D < 10$ Mpc is considered. Most of the objects have recently been found by Karachentseva & Karachentsev based on POSSII/ESO-SERC survey. A hundred more dSph galaxies are expected in this volume, being missed so far because of their low luminosity and low surface brightness. Apart from 22 dSph members of the Local Group, there are 33 dSphs in other nearby groups that have been resolved recently into stars with HST. Only 5% of the local dSphs are situated outside the known groups.

We discuss observational correlation between basic parameters of the dwarf spheroidal galaxies,in particular, absolute magnitude, surface brightness, metallicity, and so-called “tidal index”. The observed number of dSphs in group increases with luminosity of its brightest galaxy. In a “synthetic” nearby group, dwarf spheroidal galaxies are distributed in depth quite symmetrically about the principal galaxy, having an rms distance scatter of 200 kpc. Projected radial distribution of dSphs in the synthetic group follows the profile $N(R) ~ exp(-R/200$ kpc).