We present a new semi–analytic treatment of the evolution of galactic winds within high resolution, large scale cosmological N–body simulations of a $\Lambda$CDM Universe. We focus our analysis on the impact of winds on the diffuse intergalactic medium at $z=3$ and we make predictions for the volume filling factor of winds as a function of our model parameters. We then verify this prediction by extracting a set of synthetic spectra along random lines of sight through our simulated box and by calculating the probability distribution function (PDF) of the spectral flux. We find that galactic winds do not significantly modify the PDF. We finally argue that the increased flux transmissivity found by Adelberger et al. (2003) around a small sample of Lyman break galaxies may be explained by the presence of hot ionised bubbles due to pressure–driven winds outflowing from the galaxies. However, this effect cannot be explained by cooled, momentum–driven winds. We conclude that the result of Adelberger et al. (2003) may be the outcome of a selection effect.

Understanding the sources responsible for reionizing the Universe is a key goal of observational cosmology. A discrepancy has existed between the meta-galactic hydrogen ionisation rate, $\Gamma_{\rm HI}$, predicted by early hydrodynamical simulations of the Lyman-$\alpha$ forest if scaled to appropriate assumptions for the IGM temperature, when compared to values predicted from the proximity effect. We present new estimates for $\Gamma_{\rm HI}$ in the redshift range $2<z<4$ based on hydrodynamical simulations of the Lyman-$\alpha$ forest opacity. Within the current concordance cosmology, and assuming updated QSO emissivity rates, a substantial contribution to the UV background from young star-forming galaxies appears to be required over the entire redshift range. Our results are consistent with lower-end estimates from the proximity effect. It is also found that the errors on the ionisation rate are dominated by uncertainties in the thermal state of the intergalactic medium and the r.m.s fluctuation amplitude at the Jeans scale.

We summarise our detections of the 2175 Å feature due to dust extinction in intervening galaxies toward quasars with redshifts $z>0.9$ from the Sloan Digital Sky Survey first and second data release. For the 15 Mg II absorption systems selected from $\sim$22,000 quasar spectra, the observed extinction 2175 Å “bump” with various strengths can be well fitted using template quasar spectra reddened with dust located in the Mg II absorber redshift. The dust extinctions are characterised by either a parameterised Galactic extinction law, or empirical extinction curves for the Large Magellanic Cloud and the Small Magellanic Cloud. We observe that our sample of intervening galaxies exhibiting dust features are all strong Mg II absorbers ($EW_{\lambda 2796}>1.7$Å). The various strengths of the absorption features imply a diversity of dust properties at high redshift. The Mg II systems showing the 2175 Å absorption are characterised by their large Mg II EW, high H I column density, and heavy reddening.

There has been a renewed interest in searching for diffuse interstellar bands (DIBs) due to their probable connection to organic molecules and, thus, their possible link to life in the Universe. Our group is undertaking an extensive search for DIBs in DLAs via QSO absorption-line systems. Six of our DLA targets are presented here. Our equivalent width (EW) limits for the $\lambda$5780 DIB line strongly suggest that DIB abundance is below the Milky Way expected value or that metallicity plays a large role in DIB strengths.

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.

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 confront predictions of the currently popular theories for dE formation and evolution with the observed position of dEs in $\log L_B$ vs. $\log \sigma$, $\log L_B$ vs. $\log R_{\rm e}$, $\log L_B$ vs. $\log I_{\rm e}$, and $\log R_{\rm e}$ vs. $\log I_{\rm e}$ diagrams and in the ($\log \sigma,\log R_{\rm e},\log I_{\rm e}$) parameter space in which bright and intermediate-luminosity elliptical galaxies and bulges of spirals define a Fundamental Plane (FP). We show that the dE sequences in the various univariate diagrams are disjunct from those traced by bright and intermediate-luminosity elliptical galaxies and bulges of spirals. It appears that semi-analytical models (SAMs) that incorporate quiescent star formation with an essentially $z$-independent star-formation efficiency, combined with post-merger starbursts and the dynamical response after supernova-driven gas-loss, are able to reproduce the position of the dEs in the various univariate and bivariate diagrams.

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.