Short hard gamma-ray bursts (GRB) are believed to be produced by compact binary coalescences (CBC) { either double neutron stars or neutron star{black hole binaries. The same source is expected to emit strong gravitational radiation, detectable with existing and planned gravitational wave observatories. The focus of this work is to describe a series of searches for gravitational waves (GW) from compact binary coalescence (CBC) events triggered by short gamma-ray burst detections. Specifically, we will present the motivation, frameworks, implementations and results of searches for GW associated with short gamma-ray bursts detected by Swift, Fermi{GBM and the InterPlanetary Network (IPN) gamma-ray detectors. We will begin by presenting the main concepts that lay the foundation of gravitational waves emission, as they are formulated in the theory of General Relativity; we will also brie y describe the operational principles of GW detectors, together with explaining the main challenges that the GW detection process is faced with. Further, we will motivate the use of observations in the electromagnetic (EM) band as triggers for GW searches, with an emphasis on possible EM signals from CBC events. We will briefly present the data analysis techniques including concepts as matched{filtering through a collection of theoretical GW waveforms, signal{to{ noise ratio, coincident and coherent analysis approaches, signal{based veto tests and detection candidates' ranking. We will use two different GW{GRB search examples to illustrate the use of the existing coincident and coherent analysis methods. We will also present a series of techniques meant to improve the sensitivity of existing GW triggered searches. These include shifting background data in time in order to obtain extended coincident data and setting a prior on the GRB inclination angle, in accordance with astrophysical observations, in order to restrict the searched parameter space. We will describe the GW data analysis and present results from a GW search around 12 short gamma-ray bursts detected by the InterPlanetary Network (IPN) between 2006 and 2007. The IPN{detected bursts usually have extended localization error boxes and a search for GW was performed at different sky locations across these error regions. Since no GW detection was made, we set upper limits on the distances to the GRB progenitors; we briefly discuss the implications that two IPN GRBs error regions overlap two nearby galaxies.

This thesis presents an investigation of local galaxies using new data from the Herschel Space Observatory. Herschel observes the entire far-infrared peak of galaxies, at higher sensitivities and angular resolution than previously possible, and can observe large samples of galaxies or areas of sky. I developed data reduction routines to optimise the data processing of SPIRE extragalactic fields, and found the best methods of flux extraction for galaxies and for fitting of spectral energy distributions. For all the objects I investigated, a single-temperature modified blackbody was a good fit to the global fluxes between 100–500 μm. Within an individual galaxy (i.e., M31, NGC4501 and NGC4567/8) the dust temperature varies between 15–30K. In M31 the dust emissivity index varies between 1.2–2.5 suggesting a change in the physical properties of the grains. The dust and gas are highly correlated in M31, with the gas-to-dust ratio varying from ∼20 in the centre to ∼200 at 18 kpc as expected from the metallicity gradient of the galaxy. By averaging the radial profiles of the late-type objects in the Herschel Reference Survey (HRS), I have shown that dust emission can be traced to at least twice the optical radius (R25) of the galaxy. Within the HRS, dust is detected in 24% of Ellipticals and 62% of S0s and has a mean temperature of 23.9 ± 0.8K for early-type galaxies, warmer than that found for other Herschel studies of late-type galaxies. The mean dust mass for the entire detected early-type sample is logMd = 6.1 ± 0.1M⊙ with a mean dust-to-stellar-mass ratio of log(Md/M∗) = −4.3 ± 0.1, a factor of ∼50 lower dust-to-stellar-mass ratio than for the spiral galaxies in the HRS. The wide range in the dust-to-stellar-mass ratio for ETGs and the lack of a correlation between dust mass and optical luminosity suggest that much of the dust in the ETGs detected by Herschel has been acquired as the result of interactions, although these are unlikely to have had a major effect on the stellar masses of the ETGs.

The main focus of the work described here within, is the development of an analysis pipeline that can detect gravitational waves transients that have a duration from a large fraction of a second up to 100 seconds. Firstly, we will present the basic theory of gravitational waves, their generation and their detection. We will then review the current state of the art of gravitational waves transient search algorithms and their limitations. We also introduce the coherent search pipeline that has been developed from the ground up to find such intermediate length gravitational waves, and discuss in detail how it differs from other pipelines, and what makes it so well suited to this task. Further, we will describe the powerful glitch rejection algorithm and sky localisation solution that using the spherical harmonic basis made possible. Finally, we demonstrate the use of this method on real data, and compare our results against a mature pipeline.

I present new optical and infrared photometry for a statistically complete sample of seven sources selected at 1.1 mm. Comparing their photometric redshifts with redshifts from previous 850 μm selected surveys indicates that 1.1 mm selected surveys may be better at finding high redshift sources. I also perform a banded Ve/Va analysis on the sample, and find no evidence for a redshift cutoff in the space density of sources. However, the sample size is very small. I perform the same analysis on a statistically complete sample of 38 galaxies selected at 850 μm. I find a evidence for a drop off in space density of sources beyond between z∼1-2 as well as for the existence of two differently evolving sub-populations separated in luminosity. I present a sample of SPIRE sources, selected at 250 μm, with a set of previously collected ancillary photometry and either spectroscopic or photometric redshifts. I find that only ∼1/4 of the sources at redshifts z∼1 show evidence of undergoing a major merger. I find evidence to support a downsizing model of galaxy evolution, where the most massive galaxies form first. I find some correlation with star formation rate and the gas mass of a galaxy, therefore the drop in cosmic star formation rate since z = 1 may be caused by a coinciding drop in the average gas mass of galaxies over this era. Finally, I discuss the possibility of using imaging spectrometers to break through the confusion limit. Taking the SAFARI instrument as a test case I find that I can uniquely identify galaxies by their redshift, determined via an automated method. I find that this method works for galaxies with fluxes as much as ten times below the traditional continuum confusion limit. I also find that I can uniquely identify spectrally confused sources

The thesis concerns the statistical characterisation of large scale properties of the Universe. Two complementary data sets are considered: all-sky maps of the cosmic microwave background (CMB) temperature fluctuations from the Wilkinson Microwave Anisotropy Probe (WMAP); and large area maps of galaxies detected through the sub-millimetre electromagnetic emission using the Herschel Space Observatory. The standard model predicts the distribution of temperature fluctuations in the CMB to be Gaussian, homogeneous and isotropic. Since they could deviate from the standard model in many different ways, a number of complementary descriptors are required. All-sky maps of the CMB are often decomposed into spherical harmonic modes. Any modes aligned with the Galactic plane are particularly interesting because anomalous behaviour in them could indicate errors in the subtraction of Galactic foreground. Here a simple statistical analysis of these modes is tested and shown to be a useful diagnostic of possible foreground subtraction systematics. In addition, two methods for characterizing large-scale anisotropy in all sky CMB maps are discussed. They are tested against simulated anisotropic cosmologies and both show promise as effective diagnostic tools. The second part concerns analytical models of the correlation function for the distribution of galaxies. The 'Halo' model is comprehensive, but it is also rather complex. We promote a simpler alternative based on fitting functions found from numerical simulations. Both models compare well to the observational data, showing that the fitting function method can be a quick and easy option. Also, we show that a 'key' Halo model assumption, intra-halo correlations, is not required to produce a good fit. We summarise by discussing the different approximations used in the current galaxy clustering models, the limits of the currently available data and future areas of development.

The study of ancient stellar systems in the vicinity of the Milky Way provides a wealth of information on the conditions, in the early Universe, that led to the properties we observe today in galaxies and in their constituent components. Resolved stellar populations enable us to gain detailed insights on the age and chemical composition of such stellar systems, tracing their properties on a fine spatial scale. The deep investigation of Local Group objects revealed that even very old, low mass, stellar systems host unexpected complexities in their stellar populations. Such complexities remain largely unexplained, our understanding limited by observational and theoretical limitations. Here I present work aimed at a deeper characterisation of the complex stellar popula- tions in dwarf spheroidal galaxies and Galactic globular clusters. I use a combination of observational and modelling techniques to shed light on the detailed stellar properties of these objects. Part of my investigation focuses on the horizontal branch of dwarf spheroidal galaxies. By careful modelling of the horizontal branch in the galaxy Carina, which has well known star formation history, I demonstrate that the horizontal branch contains precious information, that can be used to refine age measurements in nearby galaxies. To this aim, I develop a new modelling method that, for the first time, combines constraints from the main sequence turn-off and the horizontal branch to provide very precise measurements of the star formation history in resolved galaxies. The combined information from different regions of the colour-magnitude diagram permits to recover the value of mass loss experienced by red giant branch stars with very high precision. I test this technique on a range of synthetic populations and on the well studied galaxy Sculptor, demonstrating the increased age resolution that this approach provides. I apply my modelling tool to the distant galaxy Tucana, determining a very detailed star formation history, where multiple events of star formation can be clearly distinguished. The identification of the different stellar populations on the horizontal branch permits us to characterise the spatial distribution of the star formation events in this galaxy. I also perform a photometric study of the massive globular cluster M13, focussing on the multiple stellar populations present in this object. I identify and trace the different stellar populations out to most external regions of this cluster. The spatial distribution of these populations, which shows no sign of radial segregation, reveal the very advanced dynamical evolutionary stage of the cluster. The work presented in this manuscript constitutes a step forward to understand the formation of low mass ancient stellar systems and paves the way for deeper studies of large samples of stellar systems in the Milky Way vicinity.

Spiral galaxies are the most numerous type of massive galaxy in the low-redshift Universe. There have been a number of mechanisms proposed for their origin, including global density waves and swing amplified arms growing around local density perturbations. In order to understand the origin of arms fully, one requires a detailed study of all types of spiral structure. This thesis examines the global properties of the spiral galaxy population in large, complete samples of low-redshift galaxies. We utilise data from Galaxy Zoo, which provides visual classifications for a flux-limited sample of Sloan Digital Sky Survey (SDSS) galaxies, complete for m_r <17.0. In order to study galaxy morphology, we developed techniques to measure the two fundamental properties to fully describe spiral arms: the arm number and pitch angle (how tightly wound the arms are). Accurate arm number measurements were obtained from the Galaxy Zoo 2 dataset. This required the development of a new method to remove the effect of redshift-dependent classification bias, an effect that galaxies viewed from further away have more difficult to distinguish features. To measure pitch angles, we used a tool called SPARCFIRE. This automatically detects spiral arms in input images, and measures a number of properties of the detected arms, including pitch angles. Accurately measuring pitch angles required the use of a machine learning algorithm trained on a subset of images checked by human inspection. Our method achieves a completeness of 75 per cent and contamination of 19 per cent in detecting real spiral arms in images. Using these data, the demographics of spiral galaxies with different spiral arm numbers are compared. It is found that whilst all spiral galaxies occupy similar ranges of stellar mass and environment, many-armed galaxies display much bluer colours than their two-armed counterparts. I also combined UV and mid-IR photometry from GALEX and WISE to measure the rates and relative fractions of obscured and unobscured star formation in a sample of low-redshift SDSS spirals. Total star formation rate has little dependence on spiral arm multiplicity, but two-armed spirals convert their gas to stars more efficiently. I find significant differences in the fraction of obscured star formation: an additional ~ 10 per cent of star formation in two-armed galaxies is identified via mid-IR dust emission, compared to that in many-armed galaxies. The latter are also significantly offset below the IRX-beta relation for low-redshift star forming galaxies. I present several explanations for these differences versus arm number: variations in the spatial distribution, sizes or clearing timescales of star forming regions (i.e., molecular clouds), or contrasting recent star formation histories. I also compare overall demographics with respect to arm pitch angle. A stellar mass-complete sample of ~ 6,000 SDSS spiral galaxies was selected. The star formation properties of galaxies vary significantly with arm number, but not pitch angle. We find that galaxies hosting strong bars have spiral arms substantially (4 − 6 degrees) looser than unbarred galaxies. Accounting for this, spiral arms associated with many-arm structures are looser (by 2 degrees) than those in two-arm galaxies. In contrast to this average trend, galaxies with greater bulge-to-total stellar mass ratios display both fewer and looser spiral arms. This effect is primarily driven by the galaxy disc, such that galaxies with more massive discs contain more spiral arms with tighter pitch angles. This implies that galaxy central mass concentration is not the dominant cause of pitch angle and arm number variations between galaxies, which in turn suggests that not all spiral arms are governed by classical density waves or modal theories. Finally, I confront analytical predictions for swing amplified arms. By using a number of measured properties of galaxies, and scaling relations where there are no direct measurements, I model samples of SDSS and S4G spiral galaxies in terms of their relative halo, bulge and disc mass and size. Using these models, I test predictions of swing amplification theory with respect to directly measured spiral arm numbers from Galaxy Zoo 2. We find that neither a universal cored or cuspy inner dark matter profile can correctly predict observed number of arms in galaxies. However, by invoking a halo contraction/expansion model, a clear bimodality in the spiral galaxy population emerges. Approximately 40 per cent of unbarred spiral galaxies at z < 0.1 and M > 10 log(M_sun) have spiral arms that can be modelled by swing amplification. This population displays a significant correlation between predicted and observed spiral arm numbers, evidence that they are swing amplified modes. The remainder are dominated by two-arm systems for which the model predicts significantly higher arm numbers. These are likely driven by tidal interactions or other mechanisms.

In this thesis we discuss two key problems: the cosmological constant problem (CCP), an issue that primarily manifests itself in late universe cosmology; and the process of thermalisation during the post-inflationary reheating phase of the early universe. We start by giving a brief review of general relativity, discussing both its successes and failures, in particular, why one might consider modifications of it. We then delve into the aspects of early and late universe cosmology that we aim to address in the research discussed in this thesis. Starting with an overview of the inflationary paradigm, and the need to reheat the universe post-inflation, we give a review of previous research that has been conducted in this area. We then move on to discuss the CCP in detail, in particular, why it is such an issue. After setting the scene for this problem, we proceed to discuss how to approach finding a resolution to it, highlighting certain stumbling blocks that one needs to be mindful of. Having set the scene, we then present a potential solution to the CCP, involving a scalar-tensor modifed theory of gravity, so-called Horndeski theory. Building upon a class of Horndeski theories providing self-tuning solutions to the CCP, we provide a generalisation in which matter interacts with gravity via a disformal coupling to the spacetime metric. We establish the form of the disformally self-tuning Lagrangian on a cosmological Friedmann-Robertson-Walker background, and show that there exist non-trivial self-tuning solutions. In the latter half of this thesis, we move on to review the literature on the non-perturbative description of the early stages of reheating, so-called preheating. With the motivation to study the less well understood thermalisation process that must necessarily take place in this phase, we then present a toy model preheating theory, in which we account for the effects of thermalisation from its onset. Within the density matrix formalism, we derive a (self-consistent) set of quantum Boltzmann equations, which are able to describe the evolution of an ensemble of self-interacting scalar particles that are subject to an oscillating mass term. In particular, we apply this to the preheating scenario in order to study the evolution of scalar particle number densities throughout this process. We then conclude by discussing our numerical analysis of the Boltzmann equations, drawing attention to some important results and features that manifest using this approach, in particular, how the process differs from the standard analysis through the inclusion of thermalisation.

Many gravitational wave sources will produce electromagnetic signals as they emit gravitational waves. An important example is binary neutron star mergers. The joint observations and discoveries of the electromagnetic signatures of these gravitational wave sources can produce substantial scientific benefits in physics, astrophysics and cosmology. To maximize the scientific outcomes of such gravitational events as much as possible, the detections of their electromagnetic signatures are necessary. The first detection of the inspiral signals from binary neutron stars by LIGO and VIRGO, and the observations of the associated electromagnetic counterparts throughout the electromagnetic spectrum have served an excellent example. These detections and discoveries have also ushered in a new era of both gravitational wave astronomy and multi-messenger astronomy. However, using gravitational wave interferometric detectors, the sky location estimates of the gravitational wave signals from binary neutron star can span a few hundreds square degrees, unless there are three or more detectors observing the event simultaneously. The large sky localization error poses a challenge for astronomers scanning the localization error to look for the electromagnetic signals of these gravitational wave events. The electromagnetic counterparts may also not be readily detectable depending on the distance and orientation of the sources, which presents further difficulties in detecting their signals. To alleviate the situation, we develop an algorithm to maximize the detection probability of the electromagnetic counterparts of gravitational wave events. The algorithm we develop is able to generate an observing strategy that optimizes the probability of successful electromagnetic follow-up observations given limited observational resources. This is achieved by using a greedy algorithm for tiling the sky location error and Lagrange multiplier for assigning observation times to observation fields. The analysis with the algorithm also allows an estimate of the detection probability. In Chapter 3, we present a proof-of-concept demonstration of this algorithm to four telescopes Subaru-HyperSuprimeCam, CTIO-Dark Energy Camera, Palomar Transient Factory and Pan-Starrs, for three different simulated binary neutron star events, assuming kilonova to be the target electromagnetic counterpart. By applying the algorithm to telescopes with arbitrary field of view and sensitivity within a range, we provide an insight into the potential of future telescopes and other telescopes not directly included in our analysis. Moreover, the algorithm is applied to the design of a space based mission, the Einstein Probe, to find the optimal combination of the size of field of view and the sensitivity. The localization of gravitational wave sources, which is determined both by the gravitational wave signals and the detectors, is an important factor to the success of electromagnetic follow-up observations. We investigate the localization of binary neutron star mergers detected with the Einstein Telescope and Cosmic Explorer. Compared to the existing detectors, the improvement in the sensitivity of the Einstein Telescope and Cosmic Explorer in the low frequency band has many important implications. One of them is the considerable increase in the length of the in-band of the signals from binary neutron stars, which is useful in localizing the sources. In Chapter 4, using a Fisher matrix approach, we estimate the sky localization error of binary neutron stars as a population and distributed at various distances. As the extended in-band duration of signals also increases the possibility of identifying and releasing the presence of a signal prior to merger, known as early warning, we investigate the prospect for early warning of binary neutron star merger events with these detectors. While the Einstein Telescope and Cosmic Explorer hold promising future for gravitational wave astronomy, they are not likely to be operative until the 2030s. In the literature, detectors designed with more advanced technologies than LIGO and VIRGO are proposed to fill the gap in time. We estimate the localization of binary black holes with two such detectors in Australia and China and seconds generation detectors such as LIGO, LIGO India, VIRGO and KAGRA. In chapter 5, we study electromagnetic observations of binary neutron star mergers with the Large Synoptic Survey Telescope. The Large Synoptic Survey Telescope is a telescope designed with large size of field of view and excellent sensitivity in its observing bands. Such a telescope provides a promising prospect for multimessenger astronomy with gravitational waves. With its sensitivity and field of view, the telescope is expected to enable electromagnetic follow-up observations with shorter exposure time and fewer observation fields than many existing telescopes. We define a simple procedure for electromagnetic follow-up observations triggered by gravitational waves using the telescope. Taking advantages of the Fisher matrix approach in Chapter 4 for the sky location estimates, we quantify the observation time necessary for the telescope to perform electromagnetic follow-up observation of binary neutron star mergers detected with different networks of gravitational wave detectors.

Line emission is strongly dependent on the local environmental conditions in which the emitting tracers reside. In this work, we focus on modelling the CO emission from simulated giant molecular clouds (GMCs), and study the variations in the resulting line ratios arising from the emission from the J = 1 − 0, J = 2 − 1 and J = 3 − 2 transitions. We first study the ratio (R2−1/1−0) between CO’s first two emission lines and examine what information it provides about the physical properties of the cloud. To study R2−1/1−0 we perform smooth particle hydrodynamic simulations with time dependent chemistry (using GADGET-2), along with post-process radiative transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic emission maps of a MC. R2−1/1−0 has a bimodal distribution that is a consequence of the excitation properties of each line, given that J = 1 reaches local thermal equilibrium (LTE) while J = 2 is still sub-thermally excited in the considered clouds. The bimodality of R2−1/1−0 serves as a tracer of the physical properties of different regions of the cloud and it helps constrain local temperatures, densities and opacities. Then to study the dependence line emission has on environment we perform a set of smoothed particle hydrodynamics (SPH) simulations with time-dependent chemistry, in which environmental conditions – including total cloud mass, density, size, velocity dispersion, metallicity, interstellar radiation field (ISRF) and the cosmic ray ionisation rate (CRIR) – were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the 3 transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to ±0.3 dex, triggered by changes in the environmental conditions. Changes in the ISRF and/or in the CRIR have the largest impact on line ratios since they directly affect the abundance, temperature and distribution of CO-rich gas within the clouds. We show that the standard methods used to convert CO emission to H2 column density can underestimate the total H2 molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds. One of the underlying assumptions in star formation is that stars are formed in long lived, bound molecular clouds. This paradigm comes from examining the virial parameter of molecular clouds. To calculate the virial parameter we rely on three quantities: velocity dispersion, size and mass, each of which have their own underlying assumptions, uncertainties and biases. It should come as no surprise that variations in these quantities can have a significant impact on our assessment of cloud dynamics and hence our overall understanding of star formation. We therefore use CO line emission from synthetic observation to study how the dynamical state of clouds changes as a function of metallicity and to test how accurately the virial parameter traces these changes. First we show how the ”observed” velocity dispersion significantly decreases with lower metallicities and how this is reflected on the virial parameter. Second we highlight the importance of understanding the intrinsic assumptions that go into calculating the virial parameter, such as how the mass and radius are derived. Finally, we show how the virial parameter of a cloud changes with metallicity and how the ’observed’ virial parameter compares to the ’true’ value in the simulation.