Neutral hydrogen surveys in the last decade have revealed a new class of galaxies where the gas component can be dominant compared to the stellar one (Mhi/Lb < 1). These objects may be either young (< 1 Gyr) or in a way "retarded", having evolved at a very slow rate without efficiently converting their large amount of gas into stars within a Hubble time. Here we analyse three such galaxies in the Centaurus A group at a distance of about 4.5 Mpc (discovered with the HIPASS survey), which have no analogues in the Local Group (Banks et al. 1999). From their optical morphology they appear to be dwarf spheroidals and low surface brightness, yet they are gas-rich (Mhi/Lb > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. These systems should be favoured hosts for starburst, yet a faint star formation region has been detected in only one object. Are these galaxies truly young Or rather, what inhibits the conversion of gas into stars slowing down their evolution Is there a connection with the environment they are evolving in the attempt to answer some of the above questions we have analysed data in Hi, Ha, V and / bands (taken with the Hubble Space Telescope) and the optical spectrum of the Hll region of one dwarf (HIPASS J1337-39). In particular we have constructed /, (V - I) Colour Magnitude Diagrams (CMDs) and we have compared the data-sets with theoretical models, using isochrones of various metallicities (Bertelli et al. 1994) and simulated CMDs (Harris & Zaritsky 2001). All three galaxies have well determined Red Giant Branches (RGBs) which put them in the Centaurus A group at distances between 4.5 and 5 Mpc. The well populated RGBs suggest that these systems cannot . be younger than 2 Gyr. The evidence of asymptotic giant branch (AGB) stars in two out of three galaxies suggest that they contain populations with ages of up to 10 Gyr. Older Horizontal Branch (HB) and RR Lyrae stars would be two magnitudes fainter than the photometric limit reached with the HST at this distance, therefore we cannot infer the presence of an old (> 10 Gyr) stellar population. The age-metallicity degeneracy has been broken in one case (HIPASS J1337-39) where we measured a low metal abundance (Z 1/30 solar) from the analysis of the Hll region. The remaining two galaxies do also show properties which are at least consistent with similarly low metallicities. From the comparison of the observed and model CMD's we infer that all three galaxies are gas-rich because their Star Formation Rates have been very low during their long lives (< 10--3 A/ yr_1). We argue that in such systems, star formation (SF) may have been sporadic and local, although one object (HIPASS J1321-31) has a peculiar red plume in its CMD suggesting that most of its stars were formed in a "miniburst" 300--500 Myr ago. Neither their low Hi surface densities, nor their low metal content favour high star formation rates or starbursts, although we can not exclude connections with the local environment. Two out of three galaxies seem to be located in the outskirts of the group, being at about or less than 1 Mpc from the more massive galaxies of the group. The low metal abundances may be the consequence of the ejection of enriched material via supernovae-driven winds during the SF episodes, but it is likely they were not violent enough to blow away the majority of gas (Mac Low k Ferrara 1999, Ferrara & Tolstoy 2000). The question of why there are no similar dwarf galaxies in the Local Group remains open and the study of similar objects in the nearby groups is necessary to widen the sample of galaxies with such properties. In the Appendix A we present the first part of a still ongoing project with the aim of studying the stellar population of "retarded" galaxies at the other end of the mass scale---spirals with Mhi > 10io A/ and Mhi/Lb 1---The optical, near IR and 21cm observations are briefly presented and discussed.

The Equatorial Strip sample comprises 1077 galaxies selected purely by their Hi signature. This is a unique sample which covers a large area of sky, is free from optical selection effects and which has complete, very high quality, optical data (SDSS) for a subsample of 201 galaxies. The combination of the Hi data with optical data allows us to make a comprehensive analysis and study of the properties of Hi selected extragalactic sources and investigate vital global correlations which will help us to improve our understanding of gas-rich and Low Surface Brightness galaxies. The Equatorial Strip runs along the celestial equator from 6 < 8 < +10 and through all R.A.s. The Equatorial Strip sample represents 14% of the whole sky, covering an area of 5738 deg2 and a total volume of 2.76 x 106 Mpc-3. LSB galaxies make up 12% of the sample, however, no high luminosity or high Hi mass LSB galaxies have been found. Consequently, LSB galaxies make up no more than 6% of the high luminosity, gas-rich population, and massive LSB galaxies contribute no more than 13% of the population, at the 95% confidence level. The Bivariate Brightness Distribution and the Luminosity Function for the sample have been calculated and relationships found between surface brightness and optical luminosity, Hi, baryonic and dynamical mass. From these results I find that LSB galaxies contribute 3510 % t the number density of gas-rich galaxies in the Universe but only 72% to the luminosity density. They also contribute 21+6% and 12 3% respectively to the neutral hydrogen (Hi) and baryon density of gas-rich galaxies in the Universe. The Equatorial Strip sample has unveiled many objects not found in optical surveys, ranging from very low surface brightness, very blue galaxies to extremely gas-rich galaxies.

High redshift galaxy protoclusters are the precursors of today’s massive clusters; the sites of formation of the most massive galaxies in the present-day Universe. By studying these immature structures we can directly analyse the formation of galaxies in the densest environments without relying on extrapolations from low redshift. Finding protoclusters is challenging due to the need for very wide and deep surveys. Radio-loud active galactic nuclei (RLAGN) have been shown to preferentially reside in overdense environments at z > 1. By using these bright radio sources as beacons, protoclusters may be efficiently selected, without the need for large, blind surveys. In this thesis I study the properties of galaxies in high redshift (z > 1.3) clusters and protoclusters selected around RLAGN. Using a sample of 37 clusters and protoclusters from the Clusters Around Radio-Loud AGN (CARLA) survey, I show that the protocluster galaxies have an approximately unevolving, red observed-frame i'-[3.6] colour across 1.3 < z < 3.2. This is at odds with the simple models which are commonly used to explain the cluster red sequence in the local Universe, which predict cluster galaxy colours to become more blue at higher redshifts. Taking the full cluster population into account, I show that the formation of stars within the majority of massive cluster galaxies occurs over at least 2 Gyr, and peaks at z ~ 2–3. This is consistent with the cosmic star formation history, with star formation ending in clusters at 1 < z < 2. I further show that massive galaxies at z > 2 must have assembled within 0.5 Gyr of them forming a significant fraction of their stars. This means that few massive galaxies in z > 2 protoclusters could have formed via dry mergers. Some of the CARLA structures exhibit signs of being mature, collapsed clusters. In a pilot project, I report on the discovery of a z = 1.58 cluster with a strong red sequence around the RLAGN 7C 1753+6311. I demonstrate that the cluster has an enhanced quiescent galaxy fraction that is three times that of the control field. I also show that this enhancement is mass dependent: 91 +/- 9% of the M* > 10^10.5 Msun cluster galaxies are quiescent, compared to only 36 +/- 2% of field galaxies, whereas the fraction of quiescent galaxies with lower masses is the same in the cluster and field environments. This is in contrast to low redshift studies which have shown that mass and environmental effects on quenching star formation are separable. In the literature there is some debate as to whether RLAGN preferentially reside in clusters of a certain stage of collapse. The presence of a dense core and a well-formed, quiescent red sequence suggest that 7C1753+6311 resides within a mature cluster. This means that distant RLAGN do not solely reside in young, uncollapsed protoclusters, rather they can be found in clusters in a wide range of evolutionary states. Finally I present results from surveys of Halpha emitters in the fields around three high redshift RLAGN. I find that there is more dust-obscured star formation in protocluster galaxies than in similarly-selected control field galaxies at z ~ 2.5 and there is tentative evidence of a higher fraction of starbursting galaxies in the denser environment. However, on average I do not find a difference between the star formation rate (SFR)-mass relations of the protocluster and field galaxies and so conclude that the SFR of these galaxies at z ~ 2.5 is governed predominantly by galaxy mass and not the host environment. The stellar mass distribution of the protocluster galaxies is also skewed towards higher masses and there is a significant lack of low mass (M < 10^10 Msun) galaxies within the protocluster core. These results have implications for future protocluster surveys. The lack of low mass galaxies affects the level of overdensity which is detected. If only high mass galaxies are considered, the density of the protocluster field may be over-estimated. This means that it is important when quantifying protoclusters to compare their mass functions, rather than simply number overdensities. I also find that some radio galaxies do not reside in the centre, or densest region of the surrounding structure, meaning the overdensity measured in an aperture centred on the RLAGN will be underestimated. This means that future studies of (proto)clusters around RLAGN should use larger fields of view in order to establish the existence of a (proto)cluster.

No description available.

This research investigates the connection between the submillimetre (sub-mm), millimetre (mm) and gamma-ray emission in a sample eight blazars (consisting of high- and intermediate-frequency peaked BL Lacertae Objects (HBLs and IBLs respectively) as well as Flat Spectrum Radio Quasars (FSRQs)) and one radio galaxy. Light curves using both short-term (~6 days to ~3 months) and long-term (5.5 - 12 years) observations at gamma-ray and (sub-)mm wavebands are presented. Long-term light curves at (sub-)mm wavelengths show evidence that emission at these wavelengths is source class dependent. Similarities in emission patterns and flux variability were observed between FSRQs and IBLs. The correlation between emission at 1.35 mm and the 100 MeV to 100 GeV gamma-ray wavebands was studied both qualitatively and statistically using the discrete correlation function (DCF) method. Results from the DCF analysis showed that while all sources exhibit different behavioural patterns at different epochs, some general trends can be drawn based on the source type. It was found that IBLs behaved more like FSRQs than HBLs, although IBLs are classified as BL Lacertae objects, with HBLs showing the weakest correlation and variability at both emission bands and FSRQs showing the strongest evidence for correlation and variability within this sample set. This provides further evidence for the hypothesis of the unified Active Galactic Nuclei model, in which these objects evolve from FSRQ type blazars into IBLs and then into HBLs. This is because IBLs exhibit behavioural patterns of both FSRQs and BL Lacertae objects. In addition to this, time delay analysis of 3C 454.3, BL Lacertae and 3C 273 in the correlation study yielded separation distances between the emission regions ranging between ~0.1 pc to ~19 pc in the rest frame of the observer. These are in agreement with other studies in the literature (e.g. (214;42)). However, much greater separation distances were observed for 3C 279 and OJ 287 (> 70 pc). These findings suggest that the nature and geometry of blazar jets are highly complex and vary from epoch to epoch. It was concluded that extensive consistent long-term multiwavelength studies of a larger sample of sources from all blazar sub-classes would help further constrain the location of the wavelength-dependent emission regions in the jet.

Disformally coupled fields are predicted to occur in nature, and cosmology in particular, by fundamental theories of strings and branes. They also arise independently from considerations of the most general relation permissible between two metric tensors of a given theory of gravitation. This work explores the cosmological consequences that arise when such couplings are added to the standard model of cosmology and the disformally coupled field is asked to play the role of dark energy. Among other things, it is shown that disformal interactions modify the angles of light cones and can induce motion damping of the field, similar to the well known Hubble friction, in the cosmological background. In addition, an extension is considered to the theoretical framework whereby the disformal interaction strengths can vary from species to species. Some models based on this generalisation are found to be well constrained by both astronomical and ground based particle experiments (discussed in chapter 3), whereas others (discussed in chapter 4) are actually able to avoid these constraints, while simultaneously offering insight into potential dark energy-dark matter interactions in the cosmos. Finally, a particularly well behaved form of disformal coupling is invoked to address to the cosmological constant problem (chapter 5).

No description available.

The standard model of cosmology, namely the CDM model, is based on Einstein's theory of General Relativity (GR) with a Cold Dark Matter (CDM) content and a positive cosmological constant , in addition to ordinary matter and radiation components. While it provides a paradigm in very good agreement with many observations, from Big Bang Nucleosynthesis (BBN) to Cosmic Microwave Background (CMB), several questions remain open and various theoretical extensions seem necessary in order to address them. An extensively studied ingredient of the CDM model is the inflationary scenario, which solves some of the issues associated with the initial conditions that the original hot Big Bang model cannot address, such as the homogeneity and flatness problems. Furthermore, it ts very well with current data, in particular, the spectrum of temperature anisotropies in the CMB. As we recall in Chapter 2, in some scenarii, the end of inflation may lead to the formation of Cosmic Strings (CS) or Cosmic SuperStrings (CSS), which can have a significant impact on some observables, even though they have been proven not to be the main source of CMB anisotropies. We focus on a particular phenomenological consequence of C(S)S, Gravitational Waves (GWs), which are becoming an important tool to gather new information on our universe. More specifically, energetic high frequency GW Bursts (GWB) are thought to be emitted by cusps, which are points on C(S)S temporarily reaching the speed of light. We investigate the occurrence of such phenomena in a particular setup where a light string is stretched between two heavy, almost fixed strings, as could appear in a C(S)S network. First, an analytical study allows us to draw simplifying hypotheses, such as the periodicity of the non-interacting movement of the string, and yields an effective rule to identify cuspy strings. In addition, we implement these assumptions in a numerical simulation, which settles the free parameter of this criterion. Also, the string and the network parameters are found to influence strongly the average number of cusps and thus the amount of energy released in the form of GWB. In particular, both the analytical and numerical studies demonstrate that the smaller the correlation length is (that is, the wavier the string is), the more cusps the string holds. String/M-theory yields a large variety of scenarii and thus a large phenomenological diversity, from inflation to Dark Matter (DM) candidates. It generally implies additional dimensions and additional ingredients, such as scalar fields (often involved with inflation) or extended objects (such as Cosmic SuperStrings). It can also provide a description of our universe, on which we focus in Chapter 3, in which all fields but the graviton live on a (3+1) brane, itself embedded in a larger-dimensional bulk. We consider a model where the bulk is populated with a gas of punctual, effectively 0-dimensional defects, which interact with our brane universe. Their collisions with open strings attached to the brane generate a recoil velocity of such D0-branes, later called D-particles. This additional vector eld acts as a new content of the universe, which from the low energy point of view behaves as a Dark Matter/Dark Energy (DE) mixture. The modifications of the graviton equations of motion are related to its squared field strength, which under certain circumstances condensate and plays the r^ole of an extra scalar field. This model, called the D-material universe, can not only give a mechanism for the growth of large scale structure but, as we show here, can also lead to a successful inflationary scenario, the condensate appearing as the slowly rolling inflaton. Moreover, it provides an effective DM fluid which fits restricted - by our model's hypotheses - lensing data, thus diminishing the need of conventional DM without overclosing the universe. Finally, this supplementary ingredient alters the graviton propagation as it brings in an effective mass term and affects the refractive index experienced by radiations. This study, which spans several cosmological eras and covers several length scales, leads to constraints on the free parameters of the model including the number density of D-particles and the string scale. Such analyses of models beyond the CDM model may provide important information | alternative exploration routes as well as additional possible bounds on the parameters | that would help us understand the dynamics of our universe.

The magnitude of the rate at which cosmic dust enters the Earth’s atmosphere has been highly uncertain, with a daily mass influx ranging between 5 t d-1 and 270 t d-1. In fact, this issue has an important implication because if the upper limit of the estimates is correct, then vertical transport in the middle atmosphere must be faster than is generally assumed. On the other hand, if the lower limit is correct, then our understanding of the cosmic dust evolution in the solar system, and the transport mechanisms from the middle atmosphere to the Earth’s surface will need to be revised. The aim of the work described in this thesis is to estimate a Meteor Input Function (MIF) that allows to understand different atmospheric phenomena in the Mesosphere lower Thermosphere (MLT). For this purpose, the Zodiacal Cloud Model (ZCM) which is contrained by mid-Infrared observations of the zodiacal dust is evaluated. The ZCM is a detailed dynamical model that attempts to explain the origin of cosmic dust and accounts for the directionality, and mass and velocity distribution of Interplanetary Dust Particles (IDPs) in the inner solar system. The thesis is divided into four parts. First, the ZCM – constrained by measurements of the Infrared Astronomical Satellite (IRAS) – is evaluated using the Chemical ABlation MODel developed at the University of Leeds. These results are compared with those obtained from two quite different distributions: the Long Duration Exposure Facility (LDEF) and the incoming flux measured by meteor head echo detections with High- Power and Large-Aperture (HPLA) radars. Second, a newly-developed laboratory Meteor Ablation Simulator (MASI) is used to test the thermodynamic model within CABMOD as well as the use of the Hertz-Knudsen relation to describe the kinetics of evaporation; the Na, Fe, and Ca ablation rate profiles modelled by CABMOD are then refined. Third, the absolute contribution of each cosmic dust population –Jupiter-Family Comets (JFCs), Asteroids (ASTs), and Long-Period Comets (LPCs)– to the global input is estimated accounting for the most recent version of the ZCM constrained by Planck satellite observations, the cosmic flux accretion rate at the bottom of an ice chamber at the Amudsen-Scott base at South Pole, and recent measurements of the vertical fluxes of Na and Fe atoms above 87.5 km in the atmosphere. Finally, the impact of cosmic dust on the atmospheres of Venus, Mars and Titan is examined.

The properties of dark matter - its microphysical form, and its cosmological origin and history - are one of the most important mysteries in fundamental physics. So far, evidence for matter beyond the Standard Model comes entirely from gravitational effects. However, other experiments are reaching the point where the 'simplest' models of dark matter are coming into tension with data, and may be strongly constrained by medium-term observations. This, along with theoretical considerations, motivates the exploration of other possibilities for the history and composition of dark matter, especially those with the possibility of new, generic observational signatures. In this thesis, we explore some different classes of new dark matter models, focussing on regimes in which they may display approximately model-independent phenomenology. Firstly, we look at a class of dark matter models featuring large-number, stable composite states, and investigate how these may be synthesised in the early universe. As the example of Standard Model nuclear physics and Big Bang Nucleosynthesis demonstrates, the properties of small-number composite states in strongly-coupled theories may be complicated, and sensitive to the precise details of the theory. However, it may reasonably be expected that the properties of large enough composite states will obey simple geometrical scaling laws. In this case, if large enough states are synthesised in the early universe, the overall results of the synthesis process may become broadly independent of the detailed parameters of the model, and of initial conditions. We model 'dark nucleosynthesis' in such a regime, and find that the late-time number distribution takes on one of two characteristic forms, in both cases with weak dependence on small-number initial conditions and behaviour. Following on from this, we consider the scattering phenomenology that would result from dark matter being made up of such large composite states. This includes the coherent enhancement of scattering rates - for example, at direct detection experiments - compared to e.g. collider production processes. The spatially extended nature of composite dark matter states could also lead to characteristic momentum-dependent form factors in scattering processes, which may be identifiable in direct detection experiments. In addition, inelastic interactions between dark matter states may be important in astrophysical settings. Illustrating the effects of dark-sector energy injections, we present calculations for dark matter halo modifications through velocity kicks. As an example application, we discuss a different class of asymmetric dark matter models, in which late-time decays of part of the dark matter can re-populate a symmetric component, giving annihilation signals in galactic halos. The velocity kicks arising from the decay process may modify the spatial profile of such signals, to the extent to eliminating them almost completely from low-escape-velocity systems such as dwarf galaxies.