The European Space Agency’s cornerstone mission Rosetta is due for launch in January 2003. It will perform a rendezvous with comet 46P/Wirtanen beyond 3 AU and, following an initial mapping phase, deploy a lander to a selected site on the nucleus surface. The Rosetta Lander will provide unprecedented access to cometary material. Some of the most uncertain characteristics of the nucleus material are physical properties such as its density, the structure of the surface layers and its mechanical strength. MUPUS (Multi-Purpose Sensors for Surface and Sub-Surface Science) is one of the experiment packages selected for the Lander payload which will address certain physical properties and their evolution with time. This thesis focuses on the in situ measurement of the density of the surface layers by a radiation densitometer incorporated into the MUPUS thermal probe, and on the penetrometry measurements to be performed by an accelerometer mounted in the Lander’s anchoring harpoon. A concept for incorporation of a gamma ray attenuation densitometer into the thermal probe is presented and explored. A 137Cs radioisotope source will be mounted near the tip of the probe and semiconductor radiation detectors situated at the top of the probe will monitor the transmitted count rate during probe insertion, as the intervening material attenuates the radiation. Preliminary experiments to evaluate cadmium telluride (CdTe) detectors for this purpose are presented, as well as results from a specially-developed Monte Carlo computer code designed to model the absorption and scattering of photons in bulk material. Also presented is a control algorithm to dynamically re-budget the integration time and depth resolution of the instrument as it is inserted by the hammering mechanism. This is required due to: a) the wide range of possible densities the instrument may encounter, b) the variation vs. depth of required integration time, and c) the limited time in which the measurement must be performed. For lower than nominal densities, integration time may be wasted when it could be used to improve the accuracy and depth resolution. For higher densities the integration time at particular depths may not be sufficient to obtain acceptable accuracy; in this case some depth resolution could be sacrificed to improve the accuracy. The proposed algorithm uses the density measured at each point to update the time budget and depth resolution for the remaining stages of penetration. Although the use of the gamma ray backscatter type of densitometer was eventually rejected in favour of the aforementioned attenuation technique, investigation of the backscatter technique resulted in an extension to the Single Scattering Model– an analytic approximation of its operation. This extended model adds to our understanding of these devices' response to spatial inhomogeneity. Calculations show that anchoring of the Lander is necessary to avoid possible ejection from the nucleus by gas drag in the case of a landing in an active area. The use of the Lander’s anchoring harpoon to perform penetrometry measurements is reported, including the results of preliminary experiments and techniques for analysing the accelerometry data. It is shown that layers with distinctly different strengths may be identified, and that the mean deviatoric stress– a strength parameter– may be constrained to within a factor of about 2.2. This would be a significant improvement on current estimates, which vary by several orders of magnitude. Together with other investigations on the Rosetta mission the densitometry and penetrometry measurements will serve to constrain models of the physical state and evolution of the cometary material found at the landing site. In particular both instruments are sensitive to near-surface layering, which may be expected from theoretical models of cometary activity.

Most asteroids in the size range of approximately 100m to 100km are rubble piles, aggregates of rocky material held together mainly by gravitational forces, and only weak cohesion. They contain high macroporosities, indicating a large amount of void space in their interiors. How these voids are distributed is not yet known, as in-situ measurements are still outstanding. In this work, a model to create rubble pile asteroid simulants for use in SPH impact simulations is presented. Rubble pile asteroids are modelled as gravitational re-aggregating remnant fragments of a catastrophically disrupted parent body, which are represented by spherical pebbles. It is shown that this approach allows to explicitly follow the internal restructuring of rubble pile asteroids during impact events, while preserving the expected properties of the bulk asteroid as known from observations and experiments. The bulk behaviour of asteroid simulants, as characterized by the stability against disruption and fragment size distribution, follows the expected behaviour and is not sensitive to the exact distribution of voids in the interior structure, but rather to the void fraction as the amount of consolidated void space in between the constituent fragment pebbles. No exact a priory knowledge of the fragment size distribution inside the body is therefore needed to use this model in impact simulations. Modelling the behaviour of the large-scale rubble pile constituents during impact events is used as a tool to infer the internal structure of asteroids by linking surface features like hills or pits to the creation of sub-catastrophic craters. In this work, the small rubble pile asteroid (2867) Šteins is analysed. The flyby of the Rosetta spacecraft at Šteins has revealed several interesting features: the large crater Diamond close to the southern pole, a hill like feature almost opposite to the crater, and a catena of crater pits extending radially from the rim of the crater. A possible link between these two structures and the cratering event is investigated in a series of impact simulations varying the interior of a plausible shape of Šteins prior to the event that formed crater Diamond. A connection between the cratering event and the hill is shown to be highly unlikely. Therefore, the hill is most likely a remnant of the formation of Šteins. Its size therefore helps to infer the initial size distribution of fragments forming the asteroid. The formation of a fracture radially from the crater can be observed for rubble pile simulants with highly collimated voids. This fracture could plausibly form the catena of pits observed on Šteins. This can therefore serve as a link between observable surface features and Šteins internal structure. The interior of Šteins is most likely an aggregate of fragments that themselves are only lightly fractured, and large void spaces might be found inside the asteroid. As Šteins seems to be a good example of a YORPoid, an asteroid that has been evolved to a top-like shape by radiative forces due to the YORP effect, this gives first insights in the distribution of voids in the interior of this class of rubble pile asteroids.

The majority of the matter in the known universe is believed to be in the form of Dark Matter, and its widely accepted description is done by Cold Dark Matter (CDM). Nevertheless, its exact properties and composition are still unknown, and it is one of the most active areas of research in Cosmology. The use of Cold Dark Matter has been successful to describe the general behaviour of Dark Matter at large scales. However, it has encountered problems explaining phenomena at other regimes as on the scale of galaxy halos. Therefore, other models have been proposed over time which are able to retain the reasonable success of CDM on large scales and extent it to other regimes where CDM has problems to explain the observed data. One of such models is Scalar field Dark Matter (SFDM). Its properties allow it to produce similar results at large scales and solve the problems encountered at galactic scales. Nevertheless, the difficulty to obtain direct observations of Dark Matter makes it difficult to give a definitive comparison between the models. Therefore, it is important to study dark matter through different methods of analysis that would allow to increase the validity of its scope, and these methods are constantly being researched. In this work, a particular density profile known as Wave Dark Matter is implemented as a gravitational lens to study its behaviour in the cases where it produces strong lensing of light and of gravitational waves. Analytical functions for the description of a soliton core and a soliton core + NFW tail are applied to a sub-sample of 6 galaxies from The Sloan Lens ACS Survey to constrain the lensing parameters and their relation with the profile. Furthermore, by considering the soliton core to be the main contributor to the mass profile, this is implemented as a lens for the case of the wave approximation and further to describe the major effects of the lens on gravitational waves. It was found that the soliton core is too compact and dense in order to reproduce the observed values of the data for the lensed galaxies. However, adding a NFW tail alleviates the problem and reaches radii and masses within the range reported in the literature, although the size of the NFW tail cannot be properly constrained. Meanwhile for gravitational waves, it was found that the lensing parameters of the soliton core, if they are expected to describe a galaxy, will be such that they are more likely to be observed spaceborne gravitational wave detectors. In summary, therefore, a wave dark matter soliton in combination with a NFW tail is able to represent a galaxy, and the combination of ligh and gravitational waves should give new insight on the validity of the profile as a description of Dark Matter galactic haloes.

Minute-long gravitational-wave (GW) transients are currently a little-explored regime, mainly due to a lack of robust models. As searches for long-duration GW transients must rely on minimal assumptions about the signal properties, they are also sensitive to GWs emitted from unpredicted sources. The detection of such sources offers exciting and strong potential for new science. Because of the large parameter space covered, all-sky long-duration transient searches require model-independant processing and fast analysis techniques. For my PhD thesis, I integrated a set of fast cross-correlation routines in the spherical harmonic domain (SphRad) [50] into X-pipeline [95], a targeted GW search pipeline commonly used to search for GW counterparts of short and long duration GRBs & core-collapse supernovae. Spherical harmonic decomposition allows for the sky position dependancy of the coherent analysis to be isolated from the data [40] and cached for re-use, saving both time and processing units. Moreover, the spherical harmonic approach offers a fundamentally different view of the data, allowing for new possibilities for rejecting non-Gaussian background noise that could be mistaken for a GW signal. The combined search pipeline, X-SphRad, underwent a thorough internal review within the LIGO collaboration, which I led. The pipeline good functioning was assessed by rigorous tests including comparing a test data set with a standard sky grid-based analysis. I have developed a novel pixel clustering method that does not depend on the amplitude of potential signals. By using an edge detection algorithm, I quantify each pixel in the spectrogram by its similarity with its neighbours then extract features of sharply changing intensity (or ‘edge’). The method has shown promising results in preliminary tests. A simplified version of the algorithm was implemented in X-SphRad and large-scale testings are currently being processed.

The advent of gravitational wave (GW) astronomy has provided a new window through which to view and understand the Universe. To fully exploit the potential of GW astronomy, an understanding of all the potential electromagnetic counterparts to a gravitational wave detected source will help maximise the science returns. Here I present a study of the electromagnetic emission from relativistic jets that accompany the merger of binary neutron stars or black hole-neutron star systems. These counterparts provide a probe for the structure and dynamics of these relativistic outflows. Binary neutron star, or neutron star-black hole, mergers are thought to be the dominant progenitor of short gamma-ray bursts (GRBs). Here we investigate the possibility that there is a hidden population of low-Lorentz factor jets resulting in failed GRBs, on-axis orphan afterglows, and what kind of counterparts can be expected given a merger-jet population dominated by these failed-GRB jets. I find that for GW detected mergers, ∼ 80% of the population of on-axis events may result in a failed GRB afterglow. The afterglow of a failed GRB is characterised by the lack of any prompt emission; where the γ-rays are emitted within an optically thick region of the low-Lorentz factor (Γ) outflow and significant suppression via pair production and a high opacity results in the photons coupled to the pair plasma. This plasma will undergo adiabatic expansion, and the photons will decouple at the photospheric radius. The energy in the prompt photons, for a sufficiently low-Γ outflow, will have been significantly suppressed. GW detected mergers have a Malmquist bias towards on-axis events (i.e. the rotational axis of the system), where the peak of the probability distribution is an inclination ∼ 300. If the jets from these mergers have an intrinsic structure out to wider angles, then the majority of mergers will be accompanied by electromagnetic counterparts from these various jet structures. By making some simple assumptions about the energetic structure of a jet outside of a bright core region, the various temporal features that result from a given jet structure can be predicted. Where the population of merger jets is dominated by a single structure model, I show the expected fractions of optical counterparts brighter than m_AB = 21. On 17 August 2017, the Light Interferometer Gravitational Wave Observatory (LIGO) in collaboration with Virgo detected the merger of a binary neutron star system. Various electromagnetic counterparts were detected: the GRB 170817A by Fermi/GBM and INTEGRAL; an optical, blue to red, macro/kilo-nova from ∼ 1/2 day post merger to ∼ 5 − 10 days; and a brightening radio, and X-ray counterpart from ∼ 10 days. Optical detection of this counterpart at a magnitude ∼ 26 was made at ∼ 100 days post-merger. Analysis of this counterpart is consistent with the afterglow of a Gaussian structured jet viewed at the system inclination, ∼ 18 ± 80. If all short GRB jets have a similar jet structure, then the rates of orphan afterglows in deep drilling blind surveys e.g. the Large Synoptic Survey Telescope (LSST), will be higher than those expected from a homogeneous, or ‘top-hat’ jet, population. The rates for the various jet structures for orphan afterglows from mergers are discussed, showing that for a population of failed GRBs, or an intrinsic Gaussian structure, an excess in the orphan rate may be apparent. Understanding the dynamics and structure for the jets from black-hole systems born at the merger of a compact binary can help give clues as to the nature of jets from black holes on all scales. As an aside, I show empirically that regardless of black hole mass or system phenomenology, the relativistic jets from such systems share a universal scaling for the jet power and emitted γ-ray luminosity. This scaling could be due to the similar efficiencies of various processes, or alternatively, the scaling may be able to give insights into the emission and physical processes that are responsible for high-energy photons from these outflows. GW astronomy offers a probe of the most extreme relativistic outflows in the Universe, GRBs. The predicted electromagnetic counterparts from these outflows, in association with GW detections, provides a way to probe the Lorentz-factor distribution for merger-jets. Additionally, the phenomenological shape of the afterglows, at various inclinations, gives an indication of the intrinsic structure of these jets. An understanding of these dynamical and structural qualities can be used to constrain the parent population, merger rates, and binary evolution models for compact binary systems.

Stars are of fundamental importance to the entire field of astronomy. The conversion of elements and the distribution of energy throughout the lifetime of stars drives the evolution of the Universe. Despite this, we do not have a unified understanding of the formation process for all stars. This thesis attempts to move forward this understanding, by focussing on the question: How do the initial conditions of star-forming regions vary across environments, and do these influence the process of star formation? To investigate the initial conditions of star formation, regions on the verge of forming stars have to be first identified and analysed. These regions have to be untouched by the disruptive effects of stellar feedback, such that the natal conditions of the gas – e.g. kinematics and chemistry – are not destroyed. Quiescent regions that are expected to form low-mass stars have been well studied over the past few decades, and the general process of low-mass star formation is well understood. Only relatively recently, however, has a group of objects being identified as being potential hosts of these initial stages of high-mass star formation: Infrared Dark Clouds (IRDCs). The study of these objects is difficult, due to both their rarity and complexity. An end-to-end understanding of high-mass star formation is, therefore, much less developed compared to their lower mass counterparts. This thesis presents the study of a sample of IRDCs within the Disc and Centre of the Milky Way; two very different environments. Several key aspects of the star formation process within IRDCs from these environments are investigated. Firstly, a chemical signpost – the deuterium fraction of N2H+ – is used to identify the regions of dense and cold gas on the verge of forming high-mass stars within a quiescent Disc IRDC, which can be used to study the initial conditions for star formation. Omitting potential beam dilution effects, chemical modelling suggests that the cloud could have reached a global chemical equilibrium, and, if so, would also be dynamically old (survived for several free-fall times). This timescale, with estimates of the embedded stellar mass, is used to determine star formation rates and efficiencies. Secondly, the kinematic structures within two apparently similar Disc IRDCs are identified using dense gas tracers – C18O and N2H+. The properties of these structures appear to be very similar, hinting at a similar formation scenario for both clouds, or, potentially, that these may be inherent to the larger Disc IRDC population. The dynamics of these filaments also show that they may be merging, which would suggest a compressive mode of turbulence driving. These structures are then linked to the larger kinematic structures – identified using a lower density tracing molecule, 13CO – and found to show good coherence with the brightest, most extended structures. These are then placed in the context of the previously identified Galactic scale structures, and in doing so show that IRDCs could be the densest parts of the much larger arm or inter-arm filamentary structures. Thirdly, the level of star formation within the Galactic Centre is investigated on both global (∼ 100 pc) and local (∼ 1 pc) scales. On a global scale, the star formation rate has been determined from all the available observational star formation diagnostics – i.e. direct counting of young stellar objects and integrated light measurements – and found to be in agreement with previous studies; i.e. around one-to-two orders of magnitude lower than predicted by the star formation models. On individual cloud scales, the star formation efficiency per free-fall time is in better agreement with the model predictions. However, uncertainties on the properties of these regions, such as the mode of turbulence driving, limit the further verification or falsification of the star formation theories. Lastly, the investigation of the local scale star formation within the Galactic Centre highlighted a particular part of the parameter space as the most promising to further test the star formation theories. In light of this, high-spatial resolution ALMA observations have been taken of two Galactic Centre clouds within this regime. Early results show that they have a complex structure, similar to that seen within Disc IRDCs, containing both filamentary and core-like features. Investigation of the brightest, most compact core region shows that it contains a very rich chemistry, and, of particular interest, is the rigorous detection of the pre-biotic molecule formamide (NH2CHO). When placing the results of this thesis in the bigger context of star formation theory, they appear to show interesting implications for the initially posed question – what is the influence of environment on the process of star formation? It is found here that despite the very different cloud scale properties of these regions, the star formation efficiency per free-fall time is surprisingly similar. To investigate this, the properties of the individual sites of high-mass star formation, the high-mass star-forming cores, are compared. Interestingly, despite the different environmental conditions, several key properties of the cores, such as their size and mass distribution, are also found to be very similar. The similarity of high-mass core properties and star formation rate per free fall time implies that once a region has produced high-mass cores, the evolution of these cores towards star formation must be similar. The difference in the global/environmental properties of the gas must then be setting the total star formation rate within these regions, by limiting the number of cores that can form. In particular, the mode of turbulence driving may play a major role in governing the fraction of gas that can be converted into stars per free-fall time within these two environments.

Understanding the formation and evolution of our galaxy, the Milky Way, has been an ongoing process, which with the development of large-scale surveys has picked up considerable pace. Together with these new surveys, pipelines have been constructed which allow for the rapid and automatic processing of this wealth of new data. These codes are able to turn raw data files into tables of stellar parameters and chemical abundances in far less time than if they were analysed by hand. The results from these surveys open new windows on to the history of our galaxy and other disk galaxies. In this thesis, we present the development of a new pipeline, the STellAR Parameter AND Abundances pipeline (STARPANDA), which is able to rapidly derive stellar parameters, CNO abundances and other elemental abundances by utilising measurements of spectral features in both observed and synthetic spectra. We take the observed spectra, synthetic spectra and line lists employed by the APOGEE survey and produce new values for the stellar parameters, CNO abundances and Al abundances of the APOGEE stars. We then compare our results with those achieved by the APOGEE pipeline.

Stripped-envelope supernovae (SE-SNe) are a subset of core-collapse supernovae; the explosive death of a massive star. Their defining characteristic is the lack of promi- nent He and/or H envelope suggesting significant mass loss prior to explosion. Their progenitors may be high mass single stars (> 30 M⊙) or lower mass stars that are stripped via binary interaction. Since their discovery as a separate population in 1983, and until recently, the data on these objects steadily increased. SN discoveries have increased year on year since the early 2000s with the advent of targeted and untargeted surveys looking at the skies for transient objects. As a result, some of these surveys have amassed photometric and spectroscopic data on a large number of SE-SNe. The last few years has seen this data made available, dramatically increasing the number of objects with data. I present an investigation into the bulk properties of SE-SNe, using a large database accumulated from public sources, the Palomar Transient Factory, the Public ESO Spectroscopic Survey of Transient Objects, and my own observations. I begin the investigation by constructing and analysing the largest set of bolometric light curves of SE-SNe to date – 85 objects. The light curves are analysed to derive temporal characteristics and peak luminosity Lp, enabling the construction of a lumi- nosity function. Subsequently, the mass of 56Ni synthesized in the explosion, along with the ratio of ejecta mass to ejecta kinetic energy, are calculated. It is found that broad-lined SNe Ic (SNe Ic-BL) and gamma-ray burst SNe are the most luminous sub- types with a combined median Lp, in erg/s, of log10 (Lp)= 43.00 compared to 42.51 for SNe Ic, 42.50 for SNe Ib, and 42.36 for SNe IIb. It is also found that SNe Ic-BL synthesize approximately twice the amount of 56Ni compared with SNe Ic, Ib, and IIb, with median MNi = 0.34, 0.16, 0.14, and 0.11 M⊙, respectively. SNe Ic-BL, and to a lesser extent SNe Ic, typically rise quicker than SNe Ib/IIb; consequently, their light curves are not as broad. Next I examine the spectroscopic properties of these SNe using analytical methods. For He-rich SNe, the presence of H becomes the focus. The strength, velocity, and ratio between absorption and emission of H are measured, along with additional analysis of He I lines, in order to categorize the SNe. The He-poor SNe are ordered according to the number of absorption features N present in the spectra, which is a measure of the degree of line blending. The kinetic energy per unit mass Ek/Mej is strongly affected by mass at high velocity, and such situations principally occur when the outer density profile of the ejecta is shallow, leading to the blending of lines. Using the results, the existing SE-SN taxonomic scheme is adapted I then present the data and analysis of 19 SE-SNe observed since 2012. These SNe are analysed within the context of the earlier findings in this work, as well as exam- ining the ejecta mass distributions as derived from an analytical light curve model. The results support the assertion that SE-SNe reside in a parameter space which is still under-sampled as approximately 20 – 25 percent of these objects have properties that deviate significantly from that of the bulk population. The statistics of the ejecta mass distributions also provide evidence that these SNe arise from relatively low mass progenitors (< 25 M⊙) as the mean ejecta mass for all SN types is 2 – 4 M⊙. Furthermore, distribution of ejecta mass appears unimodal, which suggests that SE-SNe are preferentially arising from one channel; stars that undergo binary interaction. Understanding SE-SNe is important as their stripped pre-explosion progenitor stars are hot, making them sources of ionizing radiation. Their explosions influence their local environment by injecting energy, both radiative and kinetic, and seeding the ISM with the ashes of nucleosynthesis. Finally, they are a source of neutron stars and stellar mass black holes in the universe, which gives rise to other astrophysical events such as X-ray binaries, pulsars, and strong gravitational wave events.

In this thesis I took a combined sample of dust-selected galaxies (from Clark et al. 2015) and HI-selected galaxies (from De Vis et al. 2016). The dust selected sample contains a large fraction of intriguing galaxies dubbed the ‘Blue And Dusty Gas Rich Sources’ (BADGRS), while the HI selected sources revealed another population of blue and gas rich systems which are instead dust-poor. I investigated whether the unique properties of these galaxies could be explained by variations in their recent star formation activity. I showed that the BADGRS are younger, and have typically experienced more recent bursts of star formation compared to the non-BADGRS. Splitting the sample into dust-rich and dust-poor sources showed that both subpopulations are of similar age, although the dust-rich sources have experienced a burst of star formation more recently. I took the chemical evolution model of Morgan & Edmunds (2003), used more recently used in Rowlands et al. (2014), and updated many of the functions and libraries in line with recent literature. I then produced a suite of models to investigate the dust and metal properties of 425 Herschel sources. These models showed (i) a delayed star formation history is required to match the observed star formation rates; (ii) inflows and outflows are required to explain the observed metallicities at low gas fractions; (iii) a significantly reduced contribution of dust from supernovae is necessary to explain the dust poor sources with high gas fractions. We also showed the dust-to-metal ratio is not definitively constant in all galaxies, and that there is evidence for a decrease in the dust-to-metals ratio towards lower metallicity. This thesis proposes a model in which the dust, gas, metals and stars can be modelled in a consistent and coherent manner, and gains insight into the dust-to-gas evolution at early epochs.

This thesis is an investigation into the emission from an individual Narrow Line Seyfert 1 (NLS1) Active Galactic Nuclei (AGN) NGC 4051. NLS1s have all the properties of Seyfert galaxies but show peculiar characteristics, including the narrowest Balmer lines, strongest Fe II emission and extreme properties in the X-rays. NGC 4051, is one of the most X-ray bright Seyferts and it has been studied extensively by a number of X-ray observatories. Recent studies have also revealed that Seyfert cores are variable at radio wavelengths, however, there are very few Seyfert radio variability investigations, and this is one of the first that also investigates the radio/X-ray (jet/disc) coupling. It has been known for some time that both the X-ray and continuum radio observations provide an optimal tool to access the innermost regions of the AGN. A combination of X-ray, radio and optical data is used here to give an in depth analysis of both the core and extended emission regions of NLS1 NGC 4051. This work reveals that there is no clear evidence for radio variability in the core emission of NGC 4051 at 8.4 GHz with the possible exception of very low amplitude » 0.12 mJy variations detected in VLA A configuration. Deep VLA observations reveal a mean spectral index value of a » −0.3 for the core, suggesting a self absorbed jet. The surrounding radio emission has steeper spectral index values in the range of −0.5 < a < −1.6 which suggests the extended radio emission is optically thin synchrotron emission. During the A configuration observations both VLA radio data sets (2000-2001 and 2008-2009) reveal a very weak positive correlation between the radio and much larger amplitude X-ray variations but there is no evidence for a b value much greater than » 0.1 for the LR µ LbX relationship, which is consistent with a constant radio luminosity for the core. Collimated VLBI structure is detected which hints at the presence of an unseen jet. The proposed jet is of non-negligible power and estimates of the buoyancy speed of the lobes, and the break timescales from the radio spectral index provide evidence for radio activity in NGC 4051 occurring on timescales greater than > 106 years. Also, a change in the relative distance of the SW hotspot with respect to the core (seen in VLBI images) gives an apparent jet velocity 0.012 pc yr−1, equivalent to a speed of 11,700 kms−1 (» 0.04 c). Deep VLA radio imaging of NGC 4051 shows double lobed radio emission, which lies along the same PA as optical [OIII] emission. The nucleus of the [OIII] emission is coincident with the core radio emission. This structure suggests the presence of a double sided ionisation cone, where both radio and optical emission are collimated by the same disc or tori.