Hubble Space Telescope (HST) observations are presented for a sample of 28 3CR radio galaxies in the redshift range 0.6 < <I>z</I> < 1.8, together with maps of their radio structure at comparable angular resolution obtained using the VLA interferometer, and infrared images of the field taken with UKIRT. The HST images display a wide range of bizarre structures which tend to be aligned along the radio axis. The morphology of the optical emission evolves strongly with the physical size of the radio source, suggesting that the predominant cause of the optical alignment is a massive burst of star formation induced by shocks associated with the passage of the radio hotspots through the host galaxy. Such a starburst would evolve and fade throughout the lifetime of the radio source. This model is supported by the HST observations of 3C34, which provide evidence for such a burst of star formation having been induced in a companion galaxy by the impact of the radio jet. In contrast, the infrared images show giant elliptical galaxies. The radial intensity profiles of the galaxies are well-matched at radii ≤ 30 kpc by a de Vaucouleur's law, whilst at greater radii they show an excess of emission similar to that of cD galaxy halos. Their infrared K-magnitudes are tightly correlated with redshift, consistent with them being a passively evolving population of galaxies which formed at high redshift. Passive evolution of their stellar populations is also required if their surface brightness <I>vs</I> characteristic size relation is to match the fundamental plane defined by low redshift giant elliptical galaxies. The implications of these results for the formation and evolution of massive galaxies and for the cosmic evolution of the radio source population are discussed.

Within the redshift range 2 ≤ <i>z</i> ≤ 4, a substantial contribution to the UV background at 1 Ryd from galaxies is required. Quasars dominate the ionizing photon emission at 4 Ryd. A significant contribution to the UV background at 4 Ryd by thermal emission from hot gas in galaxies and galaxy groups appears unlikely. The small spatial variance expected for a UV background dominated by ionizing emission from hot gas cannot be reconciled with the large fluctuations observed in the ratio of He II to H I column densities in the Lyα forest. it is shown that these large column density ratio fluctuations are expected if, at each point in space, only a small number of quasars contribute to the UV background at 4 Ryd. This is expected at the tail-end of the He II ionizing photons is similar to the average separation between quasars. At higher redshifts, <i>z</i> ≃ 6, the almost complete attenuation of quasar spectra blueward of the Lyα emission line indicates that the neural hydrogen fraction in the IGM is increasing with lookback time. Radiative transfer simulations are used to perform an in depth examination of the small regions of high transmissivity immediately blueward of the Lyα emission lines, produced by quasar emission. The sizes of these highly ionized near-zones cannot be used to reliably constrain the ionization state of the hydrogen in the surrounding IGM; the near-zone sizes provide no clear evidence for the tail-end of H I reionization occurring just above <i>z</i> = 6. The relative sizes of the Lyα and Ly<i>β</i> near-zones, as well as the widths of the absorption features within the Lyα near-zones, should provide more sensitive measurements of the IGM neutral hydrogen fraction at these redshifts.

As a result of this research, it is now accepted that quasars and galaxies are not unrelated phenomena. Instead, there is now substantial evidence that most galaxies have passed through an active phase at an early evolutionary stage. In spite of this observational progress, the modelling of quasars is still in a preliminary state. In most of the articles published before the last year the luminosity function of quasars is computed by convolving the halo mass function with prescriptions to associate a light curve to a dark matter halo. This procedure has been useful in demonstrating that the activity of quasars can be related to structure formation through hierarchical clustering, but is inadequate to study the link between quasars and galaxy formation, because it does not contain any (realistic) treatment of cooling and star formation. Attempts at more detailed modelling have fallen into two categories depending on the mechanism postulated for fuelling quasars: quasars may accrete most of their mass from cold gas in major mergers (Cattaneo, Haehnelt and Rees 1999; Kauffmann and Haehnelt 2000; Cattaneo 2001a,b; and Haehnelt and Kauffmann 2001), or they may be fuelled with hot gas in cooling flows (Nulsen and Fabian 2000). This thesis is one of the first attempts to model quasars in Monte Carlo simulations of galaxy formation. The simulations begin with the construction of merger trees for a set of dark matter haloes representative of the universe at <I>z</I> = 0. Subsequently these trees are populated with galaxies and quasars. It is assumed that, whenever two galaxies of comparable mass merge, an elliptical forms, while a fraction of the gas in the discs of the merging galaxies goes into the formation or refuelling of a supermassive black hole according to a specified accretion law. My research find results that are in substantial agreement with those of Kauffman and Haehnelt: the consumption of gas by stars is the main reason for the decline of quasars at low redshift but that decline is insufficient without a decrease in the fraction of accreted mass; not all the accretion is optically visible; the merger scenario underpredicts counts of very bright objects (chapter 3). Kauffmann and Haehnelt have used a much more sophisticated code for simulating galaxy formation, which includes a model for the synthesis of stellar population, while I have made a more systematic study of how the accretion law affects the quasar luminosity function. Moreover, I have considered that quasars can be an additional source of feedback besides supernova explosions. I have then repeated the same analysis with GalICS (Hatton <I>et al. </I>2002) a much more powerful software for simulating galaxy formation. GalICS uses merger trees extracted from N-body simulations and incorporates STARDUST (Devriendt, Guiderdoni and Sadat 1999), a routine for calculating stellar evolution, feedback and the reprocessing of light by dust. Calculations with GaIICS reconfirmed my previous results (with less need for radiatively inefficient accretion). Adding a quasar contribution to starbursts spectra can reproduce the broad variety in the observed spectral distribution of ULIRGs.

Massive stars emit strong fluxes of ionising radiation and their dynamical impact on their natal clusters is expected to be severe. The outflows generated expel residual gas from the cluster and can potentially gravitationally disrupt it. The loss of its reserves of molecular gas also prevents the cluster forming more stars. Star-formation and star cluster evolution cannot be fully understood without a proper treatment of feedback. I present a novel technique I have developed to allow the inclusion of the effects of ionising radiation in smoothed particle hydrodynamics (SPH) simulations of star clusters. The new algorithm is able to reproduce the results of simple analytical models and also gives results in good agreement with a more sophisticated Monte Carlo radiative transfer code when tested under highly anisotropic conditions. I simulate the effects of ionising radiation in globular clusters and compare my results with one-dimensional calculations with which I find good agreement. I investigate three cases in which different quantities of gas are distributed in my model cluster such that the as becomes fully ionised either during the HII region’s formation phase, or during its expansion phase, or such that the HII region is trapped inside the cluster core. I find gas expulsion to be quite efficient in the calculations in which the HII region escapes the core. I observe an instability in the second calculation which causes the shocked shell driven by the ionisation front to fragment as the HII region exits the core. The instability produces new structure from the smooth gas in the system, but this structure is rapidly destroyed by the radiation field and the effect of the instability on the evolution of the system is minimal. I also simulate feedback in the context of young embedded clusters, a highly inhomogeneous and anisotropic environment. I find that, again, photoionisation is able to produce novel structure in the ambient gas, causing it to fragment into filaments and beads. This fragmentation of the neutral gas, together with compression by hot ionised gas, which decreases the Jeans mass, lead me to conclude that feedback promotes star formation.

No description available.

A new mechanism for the evolution of primordial magnetic fields is described and analysed. The field evolution is followed from the time of its creation until the epoch of structure and galaxy formation. The mechanism takes into account the turbulent behaviour of the early universe plasma. Of crucial importance are the plasma opacity and conductivity which inhibit the field evolution through Alfven wave propagation and Thomson scattering effects. A number of other related issues such as the case of an electro-weak plasma are also considered. For demonstrational reasons the mechanism is applied to two specific toy models of primordial field generation. An attempt to create a primordial magnetic field using a realistic model the above mechanism is employed in the context of False Vacuum Inflation. Considering the inflation dominated regime, the field is produced by the Higgs-field gradients, resulting from a grand unified phase transition, which occurs during the inflationary period. The evolution of the field is followed from its creation through to the epoch of structure formation, subject to the relevant constraints. Particular attention is paid to the reheating period and also to the inflation's quantum fluctuations at the time of phase transition, since both the above could jeopardise the stability of the magnetic field. It is found that it is possible to create a magnetic field of sufficient magnitude. Another approach to the problem of seed field generation involves superconducting cosmic strings. It is a growing belief that, for a wide range of theoretical models, cosmic strings may be superconducting and carry substantial currents. A network of charged-current carrying cosmic strings may generate a primordial magnetic field through dynamical friction between the strings and the plasma. The field is created by vorticity, generated in the primordial plasma due to the strings' motion and gravitational pull.

Observational constraints on evolutionary models of low-mass stars (≲ 1.0 M_⊙) are presently extremely scarce on the pre-main sequence. Recent observational evidence has indicated substantial discrepancies between the predictions of these models and observations made in binary star systems, where the masses can be measured dynamically. It is clear that in order to resolve these issues, a larger sample of precise measurements will be needed to anchor the theory. This work pursues two avenues to do this, using time-series photometric measurements in young open clusters (ages 1 – 200 Myr) obtained as part of the Monitor project, a large-scale survey using 2 – 4 m class telescopes. Stellar rotation periods are readily measured using photometry alone, and yet probe directly a fundamental stellar property: the angular velocity. These measurements place direct constraints on models of rotational evolution, had hence on the stellar models themselves. By examining rotation periods in six of the Monitor open clusters, I show that simple rotational evolution models can partially describe the data, with rapid rotators better-described by a model assuming rotation as a solid body, and slower rotators by a model including differential rotation between the radiative core and convective envelope in stars with masses ≳ 0.4 M_⊙, but that more theoretical work is clearly needed to resolve discrepancies between the models and the data. Eclipsing binary systems provide some of the most precise and accurate determination of stellar masses and radii available, from combined analysis of radial velocity and light curves. Searching for these systems is one of the primary science goals of Monitor. Dynamical solutions are presented for four of these systems, and two found to be on the pre-main sequence are compared to the predictions of the models, finding reasonable agreement in the mass-radius plane, but that there may be significant discrepancies in the effective temperatures, as found by several other authors.

In recent years neutrino experiments have begun to challenge the Standard Model assumption that neutrinos are massless. There is now firm evidence that neutrinos undergo quantum mechanical oscillations between flavours. This would imply that neutrinos possess mass and that neutrino flavours are mixed by the weak interaction. Atmospheric neutrinos, produced by the interactions of cosmic rays in the earth’s atmosphere, can be used to study these oscillations. The MINOS Far Detector has been collecting atmospheric neutrino data since 1^st August 2003 using a 5.4 kT steel-scintillator sampling calorimeter located 700 m underground (2100 m water-equivalent) at the Soudan Underground Laboratory, Minnesota. The Far Detector is the first massive underground detector to possess a magnetic field. This makes the separation of atmospheric <i>v_μ</i>and <i>v_μ</i> charged current interactions possible for the first time. This thesis presents a study of atmospheric neutrino oscillations in the Far Detector, based on a total detector exposure of 316 days (3.3 kT-Yrs fiducial exposure). The separation of atmospheric neutrinos from the high background of cosmic muons is outlined. A total of 82 candidate events are observed, with an expectation of 109.9± 21.4 events in the absence of oscillations. Of the selected events, 40 events have a clearly identified charge, with 27 events tagged as neutrinos and 13 events tagged as anti-neutrinos. This represents the first direct observation of atmospheric <i>v</i>_μ and v̄_μcharged current interactions. A maximum likelihood analysis is used to determine the allowed region for the oscillation parameters Δ<i>m²₂₃</i> and sin²2θ₂₃. This disfavours the null oscillation hypothesis at the 79% confidence level. With current low statistics, the sensitivity of the analysis is limited. The expected future sensitivity of the atmospheric neutrino analysis is discussed.

In this thesis we investigate the microphysics of cosmic strings in non-minimal quantum field theories. In particular we consider theories in which fermion fields couple to the strings, and those with larger symmetry groups, such as grand unified and supersymmetric theories. By considering these extensions to the minimal model, we obtain a more realistic picture of the properties of cosmic strings. In considering grand unified theories, which have multiple phase transitions, we show that a cosmic string formed at one phase transition can cause the creation of another string-line solution at a later transition. This string-like solution will have many of the properties and implications of a normal cosmic string. We consider this effect for a general string solution, and illustrate it with a realistic <I>SO</I>(10) unified theory. As well as the usual abelian strings, this theory also contains more exotic string solutions. We consider both types of cosmic string. Separately, we examine the form of cosmic string solutions in supersymmetric theories, and the effect of soft supersymmetry breaking on them. We investigate the existence of conserved fermion currents in a variety of cosmic string models. We show that supersymmetry may be used to find the form of some solutions analytically. We also derive an expression for the number and type of massless fermion currents in a general model. The existence of conserved current can conflict with observations, so these results may be used to constrain models. We find the number of massless currents in the <I>SO</I>(10) and supersymmetric theories mentioned above. We show that currents present on a string can be destabilised by later phase transitions or supersymmetry breaking. This may allow any conflict that the current's existence has with observations to be avoided. We also examine massive fermion currents in a simple model, and determine the spectrum of such states.

The Cambridge Infrared Panoramic Survey Spectrograph, CIRPASS, is an infrared spectrograph that operates in the 0.9 to 1.8 <i>m</i>m wavelength region. CIRPASS was developed entirely at the Institute of Astronomy, Cambridge, and the design, construction and performance of CIRPASS are presented in this thesis. CIRPASS provides integral field spectroscopy (IFS). Spectra are simultaneously obtained from 499 contiguous spatial elements in a two-dimensional area of sky using an integral field unit (IFU). The construction of the IFU, which used a lenslet array and fibre optic bundle, is presented, as well as a review of IFS techniques. The sensitivity of CIRPASS has been maximised by ensuring the background signal detected by the instrument is reduced to an absolute minimum. CIRPASS has the capability to remove, both in hardware and software, the dominant infrared sky background from OH emission in the upper atmosphere. The advantages that this offers, and the benefits of both suppression techniques are presented. The thermal background of CIRPASS was reduced by cooling the entire spectrograph to -40°C. The instrument was cooled using a conventional industrial refrigeration system and resulted in significant improvements to the instrument's performance. The characteristics of the HAWAII infrared detector array used were measured, to determine their impact on the instrument's sensitivity, and found to be within specification. Software was written to reduce CIRPASS data that is applicable to any IFS instrument producing data in the Euro 3D format. A data reduction pipeline, that can be used for rapid data reduction at the telescope and for more detailed analysis afterwards, was implemented. The pipeline implementation and the techniques adopted for reducing closely packed spectral data on an infrared detector are discussed. The performance of CIRPASS was quantified by observing the Cambridge night sky with CIRPASS attached to a small telescope. The results provide an accurate measure of the instrument's performance on a larger telescope and are detailed. The expected limiting magnitudes for 3 hour observations of a point source, on an 8 m telescope, resulting in a signal-to-noise ratio of 5 are J=21.3 and H=19.3. The equivalent limiting line fluxes are J=4.4x10^-18 ergs/s/cm² and H=9.0x10^-8 ergs/s/cm².