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We discuss the evolution of a relativistic outflow responsible for producing the emission associated with GRBs. We investigate how afterglows are produced in the inter- action between the outflow and the ambient medium. Understanding the properties of the outflow from afterglow emission can be coupled with information obtained from the prompt component to constrain the magnetisation of the outflow. We analytically and numerically evaluate the relative strength of the reverse shock emission as the out- flow propagates into either a wind or ISM -type environment. We find that previous estimates of magnetisation based on the relative strength of forward and reverse shock emission had been underestimated by up to a factor of 100. We then apply our revised magnetisation estimate to a sample of 10 GRBs and find that 5 of the 10 events can be described by the ISM model. As recent studies have indicated that the fraction of en- ergy stored in the magnetic fields are small, our findings would suggest that the ejecta is driven by thermal pressure. Finally we consider how inhomogeneities present in the outflow can lead to variations in the very early afterglow. Considering small gradi- ent in the ejecta density profile modifies the rising index of the afterglow and can be equivalent to changing the dimensionless parameter ξ by a factor of 2. Uncertainties in determining the width of the ejecta present difficulties in understanding the distribution of GRBs afterglow rising index.

A detailed understanding of the process of star formation is crucial for modern astrophysics. Stars form from the gravitational collapse of molecular gas clouds; it is the process by which cold molecular gas is transformed into the stars and planets that make up the many billions of galaxies in the observable Universe. However, there are a number of open questions that have yet to be answered and a comprehensive theory that explains and predicts how, where and why stars and their clusters form proves elusive. One such open question is how does the environment, on both local scales and galactic scales, influence star formation? The enormous radiative and mechanical outputs of high-mass stars (M > 8M_sol ) are known to have a strong impact on their surroundings and are able to erode their natal molecular clouds via their stellar winds, ionizing radiation and supernovae. It has been proposed that the shock fronts at the edges of expanding HII regions might trigger subsequent generations of star formation (e.g. Elmegreen & Lada, 1977; Bertoldi, 1989), and there are observational studies to support this (e.g. Thompson et al., 2012). It has also been proposed that large-scale effects such as the spiral structure of galaxies like the Milky Way might trigger the formation of stars in otherwise quiescent gas (e.g. Dobbs et al., 2008), though observations within the Galaxy appear to suggest that spiral arms are playing only a minor role, if any, in the triggering of star formation (e.g. Moore et al., 2012; Eden et al., 2015). To answer this question, and others concerning star formation, large samples of imminently and currently star-forming regions are required, and surveys of the plane of the Milky Way in various tracers are providing the data to acquire these. Molecular clouds are the initial conditions for star formation, and a complete theory of star formation must necessarily involve a detailed understanding of molecular clouds. In this thesis a survey of molecular gas in the Inner Galaxy known as CHIMPS is presented; these data provide measurements of denser and more optically thin molecular gas at a higher angular resolution than preceding surveys and over a significant area of the first quadrant of the Galactic plane. The combination of CHIMPS data with data from other surveys, such as Hi-GAL, allows the star-forming content of clumps of dense molecular gas to be studied. The clumps of molecular emission identified within CHIMPS appear to be highly turbulent in nature, and are over-pressurized with respect to the encompassing neutral gas. This would appear to suggest that they are transient features in a highly dynamic interstellar medium. The efficiency of star formation within the CHIMPS clumps is not found to vary significantly on kiloparsec scales between the spiral arms and their inter-arm regions, with the exception of the Scutum-Centaurus arm, within which the current level of star formation per unit gas mass appears to be somewhat suppressed. On a clump-to-clump basis, the distribution of star formation efficiency is log-normal, indicating that the efficiency is determined by many random processes, with no single dominant agent. The conclusion is that it is turbulence that controls the star formation efficiency, which is powered on a wide range of scales from the feedback of high-mass stars to the shear induced by the rotation of the entire Galaxy.

The evolution of galaxy cluster counts is a powerful probe of several fundamental cosmological parameters. A number of recent studies using this probe have claimed tension with the cosmology preferred by the analysis of the Planck primary CMB data, in the sense that there are fewer clusters observed than predicted based on the primary CMB cosmology. One possible resolution to this problem is systematic errors in the absolute halo mass calibration in cluster studies, which is required to convert the standard theoretical prediction (the halo mass function) into counts as a function of the observable (e.g., X-ray luminosity, Sunyaev-Zel'dovich flux, optical richness). Here I propose an alternative strategy, which is to directly compare predicted and observed cluster counts as a function of the one-dimensional velocity dispersion of the cluster galaxies. I show that the velocity dispersion of groups/clusters can be theoretically predicted as robustly as mass but, unlike mass, it can also be directly observed, thus circumventing the main systematic bias in traditional cluster counts studies. With the aid of the BAHAMAS suite of cosmological hydrodynamical simulations, I demonstrate the potential of the velocity dispersion counts for discriminating even similar ΛCDM models. Then, I compare the abundance of groups in the GAMA survey to the predictions from BAHAMAS to constrain the values of several cosmological parameters. Additionally, I investigate the role of active galactic nuclei (AGN) in galaxy evolution. The color bimodality of galaxy populations roughly divides galaxies into two groups: blue, star-forming galaxies, and red, quiescent galaxies. One theory that explains how high-mass, red, non-star-forming galaxies maintain this condition is the duty cycle hypothesis. This hypothesis invokes AGN feedback from low luminosity radio-loud AGN (LERGs) to deposit mechanical heating into the intergalactic medium, thus preventing star formation. I test this hypothesis by comparing the half-light radii of quiescent elliptical galaxies with LERG host galaxies using a large multi-wavelength sample from two surveys, UKIDSS/UDS, and ULTRAVISTA/COSMOS. The radius distribution of the two groups are similar, thus providing evidence for the duty cycle hypothesis. I also check the star formation activity of the LERGs. For the duty cycle to hold, LERGs should reside within non-star-forming galaxies. However, I find that a subset of LERGs appear to be dusty star forming galaxies.

I present an investigation into Type Ia Supernovae (SNe Ia). The aim of this investigation is to explain the physics and diversity of SNe Ia, motivated by the fact that, although SNe Ia are known to come from a thermonuclear explosion of a C+O Chandrasekhar mass (Ch-mass) White Dwarf (WD), their exact explosion scenario is one of debate, and their full diversity is not fully understood. As SNe Ia are used as cosmological distance probes, understanding their explosions and progenitors systems in more detail could have important consequences. To examine the diversity of SNe Ia, I first present a large sample analysis of their B and V - band light curves, separated by host galaxy type. A new method for calculating host galaxy extinction is implemented and the width luminosity relation (WLR) is examined. After correction for host galaxy extinction, ‘normal’ SNe Ia (∆m15(B) < 1.6 mag) fill a larger parameter space in the WLR than previously suggested. Even excluding fast declining SNe, ‘normal’ (MB <−18mag) SNeIa from star forming(S- F) and passive galaxies are distinct. This may indicate that various progenitor channels are prevalent in different galaxy types. Furthermore, it was also confirmed that sub- luminous SNe Ia tend to favour passive galaxies, which implies that this subset of SNe Ia come from an older progenitor system. There was a lack of transition SNe Ia in the dataset used in this project. These are SNe Ia with a luminosity between normal and sub-luminous SNe Ia. Understanding transitional SNe Ia is important in determining whether sub-luminous SNe Ia are a totally different population. With the aim of understanding how normal SNe explode, I first turn my attention to SN 2014J. SN 2014J was the closest type Ia in the last 410 years, and it was a once in a life time opportunity to study. Therefore, a detailed spectroscopic and photometric analysis and abundance stratification modelling of SN 2014J is presented. SN 2014J is a spectroscopically normal type Ia SN with a B band decline rate of 0.95 mag, before correction for extinction. It was located in the dusty starburst galaxy M82, and does not follow the average Galactic extinction law of Rv = 3.1. With the knowledge about the diversity of SNe Ia and the ability to carry out de- tailed modelling, SN 1986G was next chosen to be modelled. SN 1986G sits in an interesting area of parameter space in the WLR. It is located in the ‘gap’ between normal and sub-luminous SNe Ia. It has been theorised that sub-luminous SNe Ia come from a different progenitor system than standard SNe Ia. Therefore, understanding SN properties in this ‘gap’ is important for determining at which point SNe Ia properties begin to diverge from the normal scenario. A full abundance tomography modelling of SN 1986G was carried out. It was found that this SN is a low energy Chandrasekhar mass explosion. It had 70% of the energy of a standard W7 model. These findings raise the possibility that only SNe Ia with very large decline rates deviate from a Chandrasekhar mass.

Galaxy clusters are the most recent of cosmological structures to have formed by the present time in the currently favoured hierarchical scenario of structure formation and are widely regarded as powerful probes of cosmology and galaxy formation physics alike. Over the past few years, it became increasingly clear that precision cluster cosmology requires the development of detailed, realistic theoretical models of galaxy clusters and the confrontation of synthetic surveys generated using these models with observations. This motivates a campaign of large cosmological hydrodynamical simulations, with plausible 'sub-grid' prescriptions for the relevant galaxy formation physics. This thesis presents a new suite of large-volume cosmological hydrodynamical simulations called cosmo-OWLS. They form an extension to the Overwhelmingly Large Simulations (OWLS) project, and have been designed to help improve our understanding of cluster astrophysics and the non-linear structure formation, which are now the limiting systematic errors when using clusters as cosmological probes. Starting from identical initial conditions in either the Planck or WMAP7 cosmologies, the most important 'sub-grid' physics, including feedback from supernovae and active galactic nuclei (AGN) has been systematically varied. Via the production of synthetic surveys of the simulations and comparisons with observations, the realism of these state-of-the-art models was explored. At the same time, the simulations were shown to providea valuable tool for interpreting the observational data, as well as powerful means for testing commonly-employed methods for estimating, for example, cluster masses and determining survey selection functions, which are crucial for cluster cosmology. The properties of the simulated galaxy groups and clusters were first compared to a wide range of observational data,such as x-ray luminosity and temperature, gas mass fractions, entropy and density profiles, Sunyaev-Zel'dovich flux, I-band mass-to-light ratio, dominance of the brightest cluster galaxy, and central massive black hole (BH) masses, by producing synthetic observations and mimicking observational analysis techniques. These comparisons demonstrated that some AGN feedback models can produce a realistic population of galaxy groups and clusters, broadly reproducing both the median trend and, for the first time, the scatter in physical properties over approximately two decades in mass (¹³M⊙≲500≲10¹⁵M⊙) and 1.5 decades in radius (0.05≲500≲1.5). However, in other models, the AGN feedback is too violent (even though they reproduce the observed BH scaling relations), implying calibration of the models is required. The production of realistic populations of simulated groups and clusters, as well as models that bracket the observations, opens the door to the creation of synthetic surveys for assisting the astrophysical and cosmological interpretation of cluster surveys, as well as quantifying the impact of selection effects. A study of the scatter and evolution of the hot gas properties of the populations of galaxy groups and clusters, such as X-ray luminosity and temperature, gas mass and Sunyaev-Zel'dovich flux, as a function of the important non-gravitational physics of galaxy formation was then conducted. The median relations and the scatter about them are reasonably well-modelled by evolving broken power-laws. The non-radiative model and the model that neglects AGN feedback are consistent with having selfsimilar mass slopes, whereas the mass slopes of the AGN feedback models deviate significantly from the self-similar expectation. Self-similar evolution, which is widely adopted in current cosmological studies, was also found to break down when efficient feedback is included. The log-normal scatter varies mildly with mass, is relatively insensitive to non-gravitational physics, but shows a moderately strong decreasing trend with increasing redshift. The X-ray luminosity has a significantly larger scatter than all the other hot gas proxies examined. It is thus the poorest one, while the 'best' one is the mean X-ray temperature. Synthetic Sunyaev-Zel'dovich observations, generated using a 'multi-purpose' light cone software package developed during the thesis, were used to check the veracity of some of the results reported by the Planck collaboration at the end of 2012. Taken at face value, their results seem to favour a close to self-similar scaling relation between the Sunyaev-Zel'dovich flux and total mass all the way down to individual galaxy haloes, which is in contradiction with X-ray and absorption lines observation. The matched filter used by the Planck collaboration recovers fluxes which are biased increasingly high as feedback intensity increases. Two likely causes for the bias, i.e. confusion and deviations from the universal pressure profiles were investigated. Confusion was found to have a negligible effect when the signal is averaged over a large number of systems. Instead a shape mismatch (in terms of pressure profiles) was identified as being mostly responsible for the bias. Finally, synthetic X-ray observations, generated using a combination of the developed light cone software and of the XMM-Newton simulator and processed with the detection pipeline of the XXL survey, were used to start quantifying the selection function of the XXL survey. Preliminary results suggest that: (i) XXL is only able to find a very small fraction of the galaxy group population, (ii) the survey is best at finding lowmass clusters (14.0≲log₁₀[M₅₀₀(M⊙)]≲14.5) at z ≲ 0:75, and (iii) the detection pipeline misses a few very massive, very extended, nearby systems.

The motivation of this thesis was the study of radio-loud, active galaxies. These galaxies house relativistic jets at their centres, powered by accretion onto a super massive black hole. The focus was on the optical flux and polarised emission produced by these powerful jets. An automated pipeline was developed to reduce data from the Liverpool Telescope Ringo2 and Ringo3 polarimeters. As part of this work, the Ringo3 instrumental polarisation and depolarisation were characterised by repeated observations of standard stars. The Ringo2 and Ringo3 optical polarimetry and photometry of a sample of 20 gamma-ray bright blazars were combined with Fermi gamma-ray space telescope data and were used to explore possible correlations and thus probe the emission sites in the jet. We found that optical and gamma-ray fluxes had strong, positive correlations. This suggests that the dominant source of optical and gamma-ray emission is from shared emission regions. If the Inverse Compton model is adopted to explain the gamma-ray emission (i.e. upscattering of photons by relativistic electrons), this correlation suggests that synchrotron self-Compton emission processes are occurring in the jet, along with inverse Compton upscattering from nearby electrons (rather than those outside the jet). The gamma-ray flux and optical degree of polarisation were not significantly correlated. The optical flux and degree of polarisation were weakly positively correlated (with correlations that did not improve with an introduced lag). Both of these results imply that there is no large scale highly ordered magnetic field in the region where the gamma-ray emission originates. We found that the maximum degree of polarisation differs depending on the location of the source's synchrotron-peak. This may be a result of the viewing angle of the observer with respect to the jet. This suggests that the majority of optical polarisation is produced in shocked regions within the jet, downstream of the main emission region. We found that the degree of polarisation was lower during a period of polarisation angle rotation compared with a period of non-rotation. This implies that the downstream magnetic field structure is either helical or compressed in a direction transverse to that of the jet. Consistent with other work, our Ringo3 colour analysis showed that, with the exception of one source, flat spectrum radio quasars had a `redder' when brighter property. This suggests that when the source is more luminous, the jet (i.e. non-thermal) emission dominates over the thermal emission from the accretion disk (which is powerful in FSRQs). We found that BL~Lacs had a `bluer' when brighter behaviour, suggesting that the brighter emission may come from more energetic photons within the jet. We presented data from our long-term, multi-colour, blazar monitoring campaign. We found that all but one source had a `redder' polarisation when the polarisation was higher. This implies that the highest polarisation is associated with higher densities of lower energy particles in the jet. Well-sampled, regular cadence data is very important for the effective study and interpretation of blazars. This is particularly crucial for the interpretation of the position angle rotations, which can afford information about the electric vector angle (and hence the magnetic field angle). In this work, we presented the design of a new multicolour polarimeter, MOPTOP. The optical components in MOPTOP allow as much of the light from the source to be exploited as possible by replacing the rotating Polaroid (from the Ringo polarimeter design) with a rotating half-wave plate and beam splitter. MOPTOP's design minimises exposure times, allowing more frequent observations and a better sampling of data. A densely sampled monitoring program that is not interrupted by periods of sunlight would be highly desirable for the study of blazar jets.

Filamentary structures are observed to be common over a wide range of spatial scales and are strongly linked to star formation. In this thesis I present the results of a range of numerical simulations which investigate the stability, collapse and fragmentation of filaments. The global longitudinal collapse timescale for filaments is found to be considerably longer than for equally dense spheres, allowing sufficient time for local collapse to occur, and to solely occur via the distinctive end-dominated mode. A new freefall timescale equation for filaments is presented, as well as a semi-analytic model of longitudinal collapse. The fragmentation of accreting filaments is found to be more complicated than that of equilibrium filaments, and is dominated by the behaviour of longitudinal gravo-acoustic oscillations. This results in the fastest growing perturbation mode being independent of filament width. The non-equilibrium model presented here allows observers to estimate the age of a fragmenting filament and the mass accretion rate. Simulations of filaments accreting from a inhomogenous, turbulent medium show that turbulence has a large impact on the fragmentation of a filament. When the turbulence is sub-sonic, a filament fragments in a two-tiered hierarchical manner. As the energy in the turbulent field increases, the filament fragments into elongated fibre-like sub-structures. The formation of these fibre-like structures is intimately linked to the vorticity of the velocity field in the filament and the accretion onto the filament. In addition, I present synthetic C18O observations and show that the fibrelike sub-structures appear as velocity-coherent structures, well separated in velocity space, similar to the fibres observed by Hacar & Tafalla (2011).

Since the early 1990s, several groups of comoving, coeval stars younger than 100Myr and within 100 pc have been revealed. Studying and identifying members in these ‘Nearby Young Moving Groups’ (MGs) is vital because they provide: well characterised samples to test pre-main sequence evolution; ideal targets for direct imaging of exoplanets, discs and brown dwarfs; observational evidence for the birthsites of stars in the Solar neighbourhood. Spectroscopy is used to perform tests of membership for 24 M-dwarf candidates of both the Beta Pictoris MG (BPMG) and AB Doradus MG, confirming 8 and 6 new members, respectively. Measurements of lithium provide a precise age for BPMG, using the ‘Lithium Depletion Boundary’ (LDB) technique. This represents the most accurate age yet determined for this important MG and is about double what has been commonly assumed in the literature from other methods. A kinematically unbiased sample of 146 X-ray emitting FGK stars in the Northern hemisphere with short rotation periods chosen from the SuperWASP All-Sky Survey were spectroscopically investigated to assess their ages and kinematics. The search identified 26 stars younger than 200Myr based on their photospheric lithium. Whilst most of these were not associated with any MG, seven are comoving with the sparse (mostly Southern) Octans-Near MG. Infrared photometry is used to identify debris discs amongst M-dwarfs in MGs and their debris disc fractions are compared as a function of mass and age. Eight percent of the sample younger than 40Myr were identified as debris disc objects, although some may have remained undetected because the sensitivity limits for detecting debris discs around M-dwarfs is lower than for higher-mass stars. No debris discs were observed in MGs older than 40Myr, suggesting the timescale for disc removal is more rapid than for higher-mass stars.

Currently the Spitzer Space Telescope is the most reliable telescope for conducting secondary eclipse observations of exoplanets. The depth and the time of mid-eclipse are two important parameters that come from a secondary eclipse analysis. The eclipse depth gives information on the temperature of the atmosphere, and can provide evidence for the presence of molecules in the atmosphere of the planet. If multiple wavelengths have secondary eclipse depths measured then it is possible to constrain the spectral energy distribution (SED) of the atmosphere given some assumptions on, for example, the metallicity of the planet's atmosphere. The time of mid-eclipse gives e cos which, with an analysis including transit, radial velocity and secondary eclipse data, can strongly constrain the eccentricity of the planet's orbit. To fully understand the conclusions drawn from these two parameters realistic error bars must be quoted on the measurement of these parameters. It is generally understood that error bars that come from MCMC analyses of secondary eclipse observations are underestimated because the correlated noise in the data is not accounted for in the analysis. The goal of this thesis was to find a method to improve the estimates of the uncertainties on these two parameters as derived from Spitzer secondary eclipse lightcurves at 3.6 um and 4.5 um. This work was conducted through the generation and fitting of semi-synthetic Spitzer secondary eclipse light curves. I estimate the amount the uncertainties on these parameters need to be inflated by and show how my results compare with other similar work in the field. I show that the amount of inflation does affect the conclusions drawn when fitting these data with model atmospheres. This could also mean that for systems where complex chemistry is invoked to explain the observed data, simpler model can now fit the data due to the increase in the error bars. I also find that when multiple realisations of the same, simulated, secondary eclipse lightcurve are fit with the standard MCMC code, the amplitude and time of mid-eclipse can be recovered and found to be more than 3 away from the true value of the injected signal. This can mean that, because usually only 1 lightcurve is obtained per observation of the secondary eclipse, some detections of eccentricity and molecules may not be real detections but simply a result of noise in the data.