We present a discovery of a giant stellar halo in NGC 6822, a dwarf irregular galaxy in the Local Group. This halo is mostly made of old red giants, showing striking features: 1) it is several times larger than the main body of the galaxy seen in the optical images, and 2) it is elongated in the direction almost perpendicular to the HI disk of NGC 6822. The structure of this stellar halo looks similar to the shape of dwarf elliptical galaxies, indicating that the halos of dwarf irregular galaxies share the same origin with those of the dwarf elliptical galaxies.

We use recent observations of the HI mass function to constrain the amount of cold gas in dark matter halos. It is found that the cold gas mass in a halo decreases rapidly with decreasing halo mass for low-mass halos with $M<10^{12} h^{-1}{\rm M_\odot}$. This result is in conflict with the standard model, in which most of the gas in a low-mass halo is assumed to settle into a gaseous disk, and the cold gas is depleted by star formation and supernova-driven outflow until the disk becomes gravitationally stable. Heating by the UV background can reduce the amount of cold gas in halos with masses $<10^{10} h^{-1}{\rm M}_\odot$, but is insufficient to explain the observational result. A consistent model can be found if low-mass halos are embedded in a preheated medium, with a specific gas entropy $\sim 10\,{\rm keV\,cm^{2}}$. Such a model can also explain why the faint-end slope of the galaxy luminosity function is flat. We propose a preheating model in which the medium around low-mass halos is preheated by gravitational pancaking. Since large-scale tidal fields tend to suppress the formation of low-mass virialised halos while promoting the formation of pancakes, the formation of massive pancakes may precede that of low-mass dark halos at the present time. ‘Previrialisation’ of such pancakes can heat the intergalactic medium. The preheated gas has a cooling time longer than the age of the Universe at $z\lesssim 2$, and has a specific entropy comparable to that required to reduce the amount of cold gas and star formation activity in galaxy halos.

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.

The Surface Brightness Fluctuation method has been shown to be a powerful distance indicator for dwarf elliptical galaxies to very low surface brightness levels. It is applicable to stellar systems that are out of reach for classical indicators requiring resolved stellar populations such as the tip magnitude of the red giant branch. I briefly discuss a few results from recent SBF studies of dEs to demonstrate the significance of the SBF method to address longstanding issues related to cosmography, dark matter in galaxy groups, substructures in clusters, and the discrepancy between the mass function of collapsed objects and the faint end of the galaxy luminosity function. For the analysis of the large number of galaxy images that need to be processed as part of such SBF studies we are currently developing a fast, semi-automatic reduction pipeline that will be made readily available to the astronomical community.

We study properties of globular cluster candidates (GCCs) and the diffuse stellar populations in nearby low surface brightness dwarf galaxies, using HST WFPC2 photometry in the V and I bands. Our sample consists of 18 dwarf spheroidal (dSph), 36 dwarf irregular (dIrr), and 3 transition-type galaxies, with projected linear diameters less than 3.5 kpc and mean blue surface brightness $>23$ mag/arcsec$^2$ situated at the distance 2–6 Mpc in the field and in nearby groups. Our sample dwarf spheroidal galaxies were not detected in HI and are located at the distances of up to $\sim$1 Mpc away from a nearby bright galaxy. Transition-type galaxies, which are distributed like dSph galaxies, form a very rare class of galaxies. dIrr galaxies show a weaker concentration to the nearest massive neighbors than dSph and transition-type galaxies. At a given surface brightness and luminosity, they exhibit lower mean metallicities than dSphs. In contrast to dIrr galaxies, the majority of dSph galaxies at a similar mean surface brightness contains GCCs. The percentage of GCCs located near the centers of dSph galaxies is much higher than that for dIrr galaxies. The composite population of GCCs in dSphs is spatially more concentrated than in dIrr galaxies. The color distributions of GCCs in dSph and dIrr galaxies show major differences. While the latter shows obvious bimodality with the peaks near $(V-I)_0 \sim 0.5$ and $\sim$1.0 mag, the GCC color distribution in dSph galaxies shows only one peak with a mean color $(V-I)_0 \sim 1.0$ mag. There is a tendency of increasing half-light radii with increasing projected galactocentric distances for a large number of GCCs in dSph galaxies, which is also observed in the Galactic globular cluster system. We embarked on a spectroscopic survey of GCCs in low-mass galaxies to obtain their chemical compositions and relative ages.

The propagation of cosmological ionisation fronts (I-fronts) during reionisation is strongly influenced by small-scale structure. Here we summarise our recent attempts to understand the effect of this small-scale structure. We present high resolution cosmological N-body simulations at high-z ($z>6$) which resolve a wide range of halo mass, from mini-halos to clusters of large, rare halos. We also study how mini-halos affect I-fronts, through simulations of mini-halo photo-evaporation including numerical gas dynamics with radiative transfer. Furthermore, we modify the I-front propagation equations to account for evolving small-scale structure, and incorporate these results into a semi-analytical reionisation model. When intergalactic medium clumping and mini-halo clustering around sources are included, small-scale structure affects reionisation by slowing it down and extending it in time. This helps to explain the observations of the Wilkinson Microwave Anisotropy Probe, which imply an early and extended reionisation epoch. We also study how source clustering affects the evolution and size of H II regions, finding, in agreement with simulations, that H II regions usually expand, and rarely shrink. Hence, “relic H II regions” are an exception, rather than the rule. When the suppression of small-mass sources in already-ionised regions by Jeans-mass filtering is accounted for, H II regions are smaller, delaying overlap. We also present a new numerical method for radiative transfer which is fast, efficient and easily coupled to hydrodynamics and N-body codes, along with sample tests and applications.

We present preliminary results of 3-D hydro simulations of the interstellar medium evolution in dwarf spheroidal galaxies undergoing star formation for the first time. The star formation is assumed to occur in a sequence of instantaneus bursts separated by quiescent periods. Different models are made changing the number and the intensity of the bursts in such a way that the final mass of the formed stars remains the same. We followed the enrichment of the ISM taking into account the contribution of both type Ia and II supernovae. The aim of our models is to find a star formation history compatible with the observed spread of stellar age and metallicity in such galaxies and to reproduce the observed mass-metallicity relation.

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 consider the feedback effects of the early reionization on the formation of small galaxies. For this purpose, we perform 3D radiation hydrodynamic simulations with incorporating the radiative transfer for ionizing photons. As a result, it is found that the early reionization is so devastating for low mass systems with $M_{\rm vir} \lesssim 10^8 M_\odot$ or $v_{\rm circ} \lesssim 20 {\rm km/s}$, and almost all gas is photo-evaporated in more than 95% of low mass systems. These results indicate that the low mass dwarf galaxies are not formed directly from isolated CDM density perturbations.

Radio-selected samples of quasars with complete optical identifications offer an ideal dataset with which to investigate dust bias associated with intervening absorption systems. Here, we review our work on the Complete Optical and Radio Absorption Line System (CORALS) survey whose aim is to quantify this bias and assess the impact of dust on absorber statistics. First, we review previously published results on the number density and gas content of high column density absorbers over the redshift range $0.6<z<3.5$. We then present the latest results from CORALS which focus on measuring the metal content of our unbiased absorber sample and an investigation of their optical–IR colours. Overall we find that although dust is unarguably present in absorption galaxies, the level appears to be low enough that the statistics of previous magnitude limited samples have not been severely affected and that the subsequent reddening of background QSOs is small.

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 $\Omega_\mathrm{gas}$.

This article reviews the popular reasons for the belief that dwarf elliptical galaxies and (ordinary) elliptical galaxies are distinct and separate species. They include: light–profile shape (or similarly image concentration); the magnitude–central surface brightness diagram; the magnitude–effective surface brightness diagram (or similarly the magnitude–effective radius diagram); and the Fundamental Plane. It is shown how a continuous trend between luminosity and a) light–profile shape, and b) central surface brightness (until the onest of core formation at $M_B \sim -20.5$ mag), results in a unification of the dwarf elliptical and (ordinary) elliptical galaxies. Neither the above four reasons, nor the luminosity function (at least in the Virgo cluster) provide evidence for a division at $M_B=-18$ mag between the dwarf elliptical and (ordinary) elliptical galaxies. Instead, they appear to be continuous extensions of each other.

If dwarf-elliptical galaxies formed their stars very rapdily (on timescales of less than 1 Gyr), they may in principle be detectable out to high redshift. Prior to the discovery of cosmic acceleration, it appeared that rapid and late formation dwarf elliptical galaxies might be required to explain the number counts of faint galaxies. A plausible hypothesis emerged: that photoionization by the UV background prevents gas cooling in low-mass halos until $z \lesssim 1.5$. The discovery of cosmic acceleration eased the tension between predicted galaxy number counts and galaxy-evolution models. Nevertheless, there is some evidence for relatively late star formation in nearby dE's, and the photoionization delay mechanism still appears to have some merit. It is thus of interest to look back in time to see if we can find starbursting dwarf galaxies at moderate redshift. We review the connection between faint-blue galaxies and bursting-dwarf galaxies and discuss some attempts to identify progenitors to dE galaxies in the Hubble Ultra Deep Field (HUDF) observations. We find roughly 85 galaxies in the HUDF with redshifts $0.6<z<1.1$ that appear to have formed most of their stars at $z<1.5$. Of these, 70% have half-light radii less than 1.5 kpc. These are thus “smoking gun” candidates for dwarf galaxies that are either collapsing for the first time at moderate redshifts or have otherwise been unable to form stars for more than 1/3$ of a Hubble time.

We present the observed metallicity distribution for 437 red giants in the Carina dwarf spheroidal (dSph) galaxy that was derived from FLAMES medium-resolution spectroscopy of the near-infrared Ca triplet (CaT). We find a mean [Fe/H] of −1.7, dex and a full spread in metallicities of at least one dex. Most of this width is due to the occurrence of three populations at different [Fe/H], which are likely to be related to the three star forming (SF) episodes that governed Carina's evolution and also correlate with peaks in the age-distribution. Both the lack of any correlation between stellar colour and metallicity and a comparison of the metallicities with chemical evolution models, which suggest a major role of infalling gas, contribute to the detailed understanding of Carina's evolutionary history. Moreover, we find a mild dependence of metallicity on radial location, reflected in a shift of the distribution's peak towards the metal poor end in the outer regions.

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.

The evolution of dwarf galaxies, particularly any potential morphological evolution among the different types of dwarf galaxies, remains poorly understood. The interstellar medium (ISM) in dwarf galaxies is thought to be more strongly disturbed by star formation episodes in these low mass galaxies, including supernovae-driven winds during starbursts and star formation induced by galaxy-galaxy interactions. Loss of the ISM has been postulated to result in the evolution of dIrrs to dE/dSphs. However, disentangling the relative importance of environmental (galaxy harassment) versus local stocastic processes on star formation, and their effects on subsequent morphological evolution is still a very active area of research. So-called “transition dwarf” galaxies are galaxies that have properties that fall somewhere between gas-rich dIrrs and gas-poor dE/dSphs, and may provide clues to the overall evolutionary history of dwarf galaxies. We report on some recent work on a sample of vigorously star-forming dwarf “transition” galaxies, and suggest that they may be examples of the bridge between dIrrs and dEs/dSphs.

We investigate the chemical abundances of local star-forming galaxies which cause damped Lyman alpha lines. A metallicity versus redshift diagram was constructed, on which the chemical abundances of low-redshift star-forming galaxy populations are compared with those of high-redshift damped Lyman alpha systems. We discuss two types of experiments on individual star-forming galaxies. In the first, the damped Lyman alpha line is created against an internal ultraviolet light source generated by a star-forming cluster or a supernova explosion. In the second, the damped Lyman alpha line is seen against a background quasar. The metallicities measured from ionised gas in the star-forming regions, and neutral gas in the damped Lyman alpha systems, are compared with one another on a case-by-case basis. We highlight the occurrence of the star-forming galaxy/quasar pair SBS 1543+593/HS 1543+5921, where the emission- and absorption-line derived abundances give the same result. We argue that we therefore can in principle interpret damped Lyman alpha system metallicities as an extension of star-forming galaxy metallicities to higher redshifts, supporting the idea that gas-rich galaxies had lower chemical abundances when they were younger.

Recent progress in our knowledge of stellar populations in local dwarf spheroidal galaxies is briefly discussed. A few results are summarized including wide field observations of stellar populations and their spatial variations, studies of AGB and variable stars, extension to near-infrared wavelengths, and the interpretation effort based on synthetic color-magnitude diagrams and chemical evolution models.

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.