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.

Setting constraints on the evolutionary history of a galaxy requires informations about the age and metallicity of its stellar content. However, outside the Local Group of galaxies representative surveys on the age and metallicity of stellar populations in galaxies of different types (e.g. luminosity, environment) can only be performed with photometry. The latter requires special care in order to overcome the age- metallicity degeneracy of broad band colors. Combining optical and near-infrared observations of globular clusters is a powerful tool in this respect, using the different sensitivity of broad band colors regarding both parameters. Based on Single Stellar Population (SSP) models we derive the cumulative age distribution for various globular cluster systems and compare it to simulated systems with a known age distribution. So far this method has been tested successfully on various early-type galaxies, revealing a second generation of globular clusters in two galaxies in a sample of eight with convincing reliability.

If the CDM paradigm is correct, then dwarf galaxies are far rarer than dark matter haloes of comparable circular velocity at the present day. Some process or processes must suppress galaxy formation almost completely on these small scales. Several plausible mechanisms may explain the scarcity of dwarf galaxies in the local universe, but it has been hard to distinguish between the different models observationally. The spatial distribution and kinematics of dwarf galaxies contain additional information about the age and formation history of these systems. Interpreted in the CDM framework, they provide powerful new constraints on the physics of dwarf suppression, ruling out at high significance the simplest models, in which the halo occupation probability is constant above some fixed mass or velocity threshold. We describe recent tests of small-scale galaxy formation models using the satellites of the local group and other nearby groups, and discuss the implications of these results for studies of the stellar halo, sub-galactic dark matter and the reionization of the universe at high redshift.

We report here and use the results of a set of three-dimensional SPH/Treecode simulations which model the formation and early evolution of disk galaxies, including the generation of heavy elements by star formation, to investigate the effects of dust absorption in quasar absorption line systems. Using a simple prescription for the production of dust, we have compared the column density, zinc abundance, and optical depth properties of our models to the known properties of damped Lyman alpha systems. We find that a significant fraction of our model galaxy disks have a higher column density than any observed DLA system. We are also able to show that such parts of the disk tend to be optically thick, implying that any background quasar would be obscured through much of the disk. This would produce the selection effect against the denser absorption systems thought to be present in observations.

The central problem of galaxy formation is understanding the cooling and condensation of gas in dark matter halos. It is now clear that to match observations this requires further physics than the simple assumptions of single phase gas cooling. A model of multi-phase cooling (Maller & Bullock 2004) can successfully account for the upper cutoff in the masses of galaxies and provides a natural explanation of many types of absorption systems (Mo & Miralda-Escude 1996). Absorption systems are our best probes of the gaseous content of galaxy halos and therefore provide important constraints on models for gas cooling into galaxies. All physical processes that effect gas cooling redistribute gas and therefore are detectable in absorption systems. Detailed studies of the nature of gas in galaxy halos using absorption systems are crucial for building a correct theory of galaxy formation.

We present evidence that FCC 046, a dE,N galaxy with an offset nucleus, suffered a counter-streaming instability. This instability may explain the presence of lopsided nuclei in other dE,Ns if these galaxies are weakly rotating. The counter-streaming needed for the instability may result from the destruction of box orbits during the assembly of the nucleus.

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 have studied the metallicity of 17 high column density forests at $N_{{\rm H}{\sc I}}=10^{15-17}$ cm$^{-2}$ and $2<z<3.5$ toward 7 high-$z$ QSOs taken with the VLT/UVES, in order to study the metallicity of absorption line systems as a function of H I column density and $z$. This relation provides an important constraint on metal enrichment mechanisms of the intergalactic medium as well as early star formation in galaxies. We used higher order Lyman series to measure a reliable H I column density for the saturated Ly$\alpha$ lines, and applied an ionisation correction assuming the Haardt-Madau UV background including both galaxies and QSOs. We have found 3, 6, and 8 systems with no metals, C IV-only, and additional ions other than C IV respectively. A photoionisation model with the Haardt-Madau background generally reproduces the observed metal line ratios. The median [C/H] is $-3.03$ and $-1.96$ for the 6 C IV-only systems and the 8 additional-ion systems respectively. The inferred H I number density is $n_{{\rm H}{\sc I}} \sim 10^{-3.5}$ cm$^{-3}$ and $\sim 10^{-2.5}$ cm$^{-3}$ for the C IV-only systems and the additional-ion systems respectively. When other ions are present, [C/H] of the high-$N_{{\rm H}{\sc I}}$ forest is similar to that of sub-damped Ly$\alpha$ systems.

The structural properties of a sample of 50 dEs in the Virgo cluster are here derived from Near InfraRed (NIR, H-band 1.65 $\mu$m) surface photometry and analysed. One-dimensional surface brightness (SB) profiles are extracted using elliptical isophote fitting. They are characterised by means of structural parameters, namely the half light radius $R_e$, the average surface brightness within $R_e$ ($\mu_e$), and a concentration index ($c_{31}$). We show that typical dEs have close-to-exponential NIR SB distributions.

The relations between dEs and giant ellipticals (Es) are investigated by comparing the NIR structural parameters of 273 Es in nearby clusters. Further analysis is conducted using the optical-NIR colour $B-H$ and by studying the relationships between structural and dynamical parameters (fundamental plane) for the two classes of galaxies. The transition between the two regimes is smooth and no dichotomy is seen.

Observations suggest systematic differences between chemical abundances of stars in satellite galaxies and those in the Milky Way halo. Specifically, for the same [Fe/H] values, stars in surviving satellite galaxies display significantly lower [α/Fe] ratios than stars in the stellar halo.

Here we investigate whether the observed differences can be explained in the framework of hierarchical structure formation. We model the chemical enrichment of a typical Milky Way galaxy in a ΛCDM Universe using, in combination, i) a semi-analytical code and numerical simulations that model the accretion and disruption of halo substructure and ii) a chemical evolution model that takes into account each satellite's star formation, metal enrichment and stellar feedback. Our results suggest that the observed chemical abundance patterns are a natural outcome in the process of hierarchical assembly of the Galaxy. We find that the stellar halo is built up from satellite galaxies accreted early on (more than 8–9 Gyr ago) and enriched in α-elements produced in Type II supernovae (average [α/Fe] values between 0.2–0.5). In contrast, satellites which survive today were typically accreted late (within the last 4–5 Gyr) and had at the time of accretion nearly solar [α/Fe] values as a result of the longer contribution of Type Ia supernovae.

Molecular absorption lines in the line of sight to distant quasars are an extremely powerful tool to probe the dense interstellar medium and its chemical composition in intervening galaxies from low to high redshifts. The absorption line measurements of different isotopomers even allow us to study isotopic ratios, which can be interpreted as the signature of past nucleosynthesis activity, and put some constraints on the chemical evolution models.

In this paper, we present the study of molecular absorption lines in front of the quasar PKS 1830-211. The absorption is due to an intervening galaxy at $z=0.89$ which is identified as a nearly face-on spiral galaxy. We have carried out a survey of absorption lines of various HCO$^+$, HCN, HNC, and CS isotopomers with the Plateau de Bure Interferometer and derived for the first time the C, N, O, and S isotopic ratios in such a distant object.

This $z=0.89$ absorption system offers an unique opportunity to study the chemical composition in the disk of a spiral galaxy only a few Gyr old. Our results show significantly different isotopic ratios as compared to those measured in the Solar System or in the local ISM, indicating a poorly enhanced abundance of material processed by intermediate and low mass stars.

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.

By investigating properties from the infrared (IR) to the optical of IR-selected QSOs (IR QSOs), optically selected QSOs (PG QSOs), and narrow-line Seyfert 1 galaxies (NLS1s) in the local Universe, we find that the IR excess in IR QSOs is mostly in the far-IR, and their IR spectral indices suggest that the excess emission is caused by starbursts rather than AGNs. The ratio of the star formation rate and the accretion rate is about several hundred for IR QSOs, but decreases with the central black hole mass. We also study an optically selected QSO sample at high redshift with hyper-luminous far-IR luminosities. We find that similar to IR QSOs at low redshift, these high redshift QSOs have a far-IR excess also, which should be due to the contribution of starbursts heating the dust. But the ratio of star formation rate to accretion rate for QSOs at high redshift is typically smaller than that for IR QSOs at low redshift, which hints at the relatively faster growth of black holes at early epochs.

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$^{-2}$.

A cold dark matter halo big enough to host the Milky Way contains hundreds of subhalos massive enough to host dwarf galaxies. The difference to the much smaller observed number of satellite galaxies seems to be a problem for CDM. The galaxy number density profile and disk like configuration are also different form the total subhalo populations in CDM simulations. A number of different models of dwarf galaxy formation which are able to reproduce the right number of luminous subhalos have been proposed. Some of them also give the right radial distributions and make disk like configurations more probable. Additional information about the typical formation times and sites of dwarf galaxies can be found in the stellar halo of the Milky Way, i.e. from the stellar debris of tidally disrupted dwarfs. A stellar halo with a realistic concentration is obtained when most dwarfs form early (before redshift 10) in small halos (virial mass above $10^8$ solar masses). This mass scale found from simulations of dark matter structure formation coincides with the virial temperature of $10^4 K$ which is needed for efficient atomic cooling.

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.

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.

We present new Keck/HIRES observations of seven of the highest-redshift known quasars. These include four with $z_{\rm qso}\ge 5.8$ and two with $z_{\rm qso}\ge 6.3$. The data will be used to produced a complete statistical description of the Ly$\alpha$ forest at $z > 5$, allowing us to better assess the significance of the strong absorption seen at $z>6.2$. We introduce the statistics of transmission gaps as a means of characterising the high-redshift evolution of the forest. In addition, we identify several clear absorption lines of low optical depth in the quasar proximity regions and briefly discuss using these to place constraints on the thermal history of the IGM at $z>5$.

We are conducting a survey of 44 early-type galaxies in the Fornax Cluster using the Advanced Camera for Surveys on the Hubble Space Telecope: the ACS Fornax Cluster Survey. We briefly describe the survey, the selection of the target galaxies and some of the scientific objectives.

The recent development of large, complete samples which identify high-redshift galaxies at $z\sim5.7$ and $z\sim 6.5$ from deep, wide-field surveys provides detailed information on the earliest galaxies, their numbers, spatial and kinematic distributions, and implications for early reionisation of the IGM. In this contribution we present results of spectroscopic studies of $z\sim 5.7$ and $z\sim6.5$ galaxies identified from our deep, Lyman alpha narrow-band and multicolour surveys conducted with the SuprimeCam mosaic CCD camera on the 8.3-m Subaru telescope and observed with the DEIMOS multi-object spectrograph on Keck. The luminosity function of the $z\sim6.5$ galaxies is shown to be similar to the luminosity function of the $z\sim 5.7$ galaxy samples, suggesting that a substantial star-forming population is already in place at $z\sim 6.5$. Comparisons of both individual and stacked spectra of galaxies in these two samples show that the Lyman alpha emission profiles, equivalent widths, and continuum break strengths do not substantially change over this redshift interval. The wide-field nature of the surveys also permits mapping of the large-scale distribution of high-redshift galaxies in spatial structures extending across individual SuprimeCam fields ($\sim 60$ Mpc). Field-to-field variations in the number of objects at $z\sim 6.5$ may shortly be able to place constraints on the porosity of the reionisation boundary.