In this thesis I develop Bayesian approach to model galaxy surface brightness and apply it to a bulge-disc decomposition analysis of galaxies in near-infrared band, from Two Micron All Sky Survey (2MASS). The thesis has three main parts. First part is a technical development of Bayesian galaxy image decomposition package Galphat based on Markov chain Monte Carlo algorithm. I implement a fast and accurate galaxy model image generation algorithm to reduce computation time and make Bayesian approach feasible for real science analysis using large ensemble of galaxies. I perform a benchmark test of Galphat and demonstrate significant improvement in parameter estimation with a correct statistical confidence. Second part is a performance test for full Bayesian application to galaxy bulgedisc decomposition analysis including not only the parameter estimation but also the model comparison to classify different galaxy population. The test demonstrates that Galphat has enough statistical power to make a reliable model inference using galaxy photometric survey data. Bayesian prior update is also tested for parameter estimation and Bayes factor model comparison and it shows that informative prior significantly improves the model inference in every aspects. Last part is a Bayesian bulge-disc decomposition analysis using 2MASS Ks-band selected samples. I characterise the luminosity distributions in spheroids, bulges and discs separately in the local Universe and study the galaxy morphology correlation, by full utilising the ensemble parameter posterior of the entire galaxy samples. It shows that to avoid a biased inference, the parameter covariance and model degeneracy has to be carefully characterised by the full probability distribution.

The purpose of this dissertation is to locate evolved massive stars within the central 50 pc of the Galactic Center. These stars are considered to be the descendants of O stars and should be less than 10 Myr old. They trace young star clusters within the Galactic Center. Through these stars and young star clusters, we hope to understand the star formation mode and history within the Galactic Center, as well as the properties of evolved massive stars in the high metallicity environment. We first study the Chandra X-ray deep survey of the Arches and Quintuplet clusters, two of the three young massive star clusters within the Galactic Center. The diffuse X-ray emission is used to constrain their initial mass function and we find a deficiency of low-mass stars, which could be explained by an ongoing collision between the clusters and the adjacent molecular clouds. We then perform a systematic search of young massive stars on a large scale within the Galactic Center through our new HST/NICMOS Paschen-alpha survey. We produce mosaic maps of the Paschen-alpha line and continuum emission, giving an unprecedentedly high resolution and high sensitivity panoramic view of stars and photo-ionized gas in the nuclear environment of the Galaxy. Many new HII regions and extended emission regions have been found. Combined with the archived HST snapshot observations and spectroscopic observations, we construct a sample of 180 potentially evolved massive stars. A multi-wavelength study of these stars is conducted. We find that young massive stars have continued to form within the Galactic Center during the last 10 Myr and some of the evolved massive stars may represent star formation in small groups or even in isolation, compared to the three massive star clusters within the Galactic Center

One focus of research in cosmology regards the growth of structure in the universe: how we end up with stars, galaxies, galaxy clusters, and large scale structure in a universe that appears homogeneous and isotropic on large scales. Using cosmological perturbation theory, we investigate the evolution of density and velocity perturbations corresponding to a universe that is slowly contracting (Ijjas and Steinhardt), testing with and comparing different values for the equation-of-state parameter. This allows for the comparison of the growth of large scale structure in scenarios including a matter-dominated expanding universe, a dark energy-dominated expanding universe, and now, an ekpyrotic scalar field-dominated contracting universe. Further, we consider the timescales on which deviations from ΛCDM in favor of the model considered could become relevant.

Saturn's rings are a complex, dynamic system that can provide unique insight into the structure and features of the planet and surrounding system. We use stellar occultation data of Saturn's rings collected from the Cassini Ultraviolet Imaging Spectrograph to visualize and analyze bending waves present within the rings. Analysis of the propagation of these waves gives insight into the surface mass density of the local ring region and can be used to further our understanding of ring dynamics and ring formation. Our analysis of the Mimas 7:4 bending wave estimated a surface mass density between 30 g cm-2 and 43 g cm-2, corroborating the findings of Spilker et al. (2004) of 47 ± 6.2 g cm-2 and supporting our current understanding of linear wave theory. Our analysis of the Mimas 4:2 bending wave estimated the surface mass density to be between 33 g cm-2 and 47 g cm-2 and was of particular interest since this wave is found in the relatively uncharacterized B ring region.

<p>Galaxies live in extended, non-luminous haloes of dark matter. How dark matter haloes are affected by environment has been examined using cosmological simulations, and resulting predictions tested for isolated and cluster galaxies. However, predictions have have yet to be tested in the intermediate density environment of galaxy groups. We present a weak galaxy-galaxy lensing analysis of galaxies in groups, with the aim of examining how the group environment affects the dark matter haloes of member galaxies. In particular, we address three questions: 1) whether the dark matter haloes of galaxies in groups are truncated relative to galaxies in the field, 2) how dark matter is distributed within the group environment and 3) whether the halo-to-stellar mass ratio is different between field and group galaxies. We use a basic stacking method and a maximum likelihood technique to parameterize the dark matter haloes of group and field galaxies. Our samples of intermediate redshift group and field galaxies were identified by the Group Environment and Evolution Collaboration in the CNOC2 Redshift Survey. For these data, we measure the average radial extent of a group galaxy dark matter halo to be $s_* = 54^{+114}_{-39}$ kpc, which hints at the possible truncation of galaxy haloes in the group environment. We develop a method of examining the distribution of dark matter within the galaxy group itself, but obtain inconclusive results. Our preliminary analysis of star formation efficiency (halo-to-stellar mass ratio) indicates group galaxies may be less efficient at forming stars compared to galaxies in the field. Larger data samples are required in order to conduct a more rigorous analysis.</p> / Master of Science (MSc)

<p>The astrophysical ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate is a key quantity used in both classical nova and type I X-ray burst models that predict isotopic abundances produced during nucleosynthesis in the outburst. Currently, uncertainties in ³¹S structure parameters lead to a variation in the reaction rate by a factor of 20 at nova temperatures causing predicted isotopic abundance ratios in the Si-Ar mass region to vary by factors of up to 4. The ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate can be determined indirectly by measuring transfer reactions populating excited states in ³¹S. Nuclear structure information for ³¹S resonant states above the proton threshold of 6131 keV and within the Gamow window that contribute most significantly to the reaction rate can be used to re-evaluate the rate for nova and type I X-ray burst temperatures and reduce current uncertainties. We have performed an experiment in order to study the level structure of ³¹S via the ³²S(<em>d</em>,<em>t</em>)³¹S single-nucleon transfer reaction using the MP tandem accelerator and Q3D magnetic spectrograph at MLL in Munich, Germany. Excited states of ³¹S in the 6-7 MeV region were observed and spin-parity constraints have been suggested. In this work we describe the experimental setup, data analysis and results for both experiments and provide recommendations for further investigation of the ³⁰P(<em>p</em>,<em>γ</em>)³¹S astrophysical reaction rate.</p> / Master of Science (MSc)

<p>We use numerical simulations to examine the fragmentation of protostellar discs via gravitational instability (GI), a proposed formation mechanism for gas-giant planets and brown dwarfs. To accurately model heating and cooling, we have implemented radiative transfer (RT) in the TreeSPH code Gasoline, using the flux-limited diffusion approximation coupled to photosphere boundary cooling. We present 3D radiation hydrodynamics simulations of discs that are gravitationally unstable in the inner 40 AU; these discs do not fragment because the cooling times are too long. In prior work, one of these discs was found to fragment; however, we demonstrate that this resulted from an over-estimate of the photosphere cooling rate. Fragmentation via GI does not appear to be a viable formation mechanism in the inner 40 AU.</p> <p>We also present simulations of GI in the outer regions of discs, near 100 AU, where we find GI to be a viable formation mechanism. We give a detailed framework that explains the link between cooling and fragmentation: spiral arms grow on a scale determined by the linear gravitational instability, have a characteristic width determined by the balance of heating and cooling, and fragment if this width is less than twice their Hill radius. This framework is consistent with the fragmentation and initial fragment masses observed in our simulations. We apply the framework to discs modelled with the commonly-used beta-prescription cooling and calculate the critical cooling rate for the first time, with results that are consistent with previous estimates measured from numerical experiments.</p> <p>RT is fundamentally important in the star formation process. Non-ionizing radiation heats the gas and prevents small-scale fragmentation. Ionizing radiation from massive stars is an important feedback mechanism and may disrupt giant molecular clouds. We present methods and tests for our implementation of ionizing radiation, using the Optically-Thin Variable Eddington Tensor method.</p> / Doctor of Philosophy (PhD)

<p>One of the most fundamental problems in theories of planet formation in protoplanetary disks is planetary migration that arises from resonant, tidal interactions of forming planets with the natal disks. This rapid inward migration, also known as type I migration, leads to the well-known problem that its timescale is about two orders of magnitude shorter than the typical disk lifetime, so that (proto)planets plunge into the host stars within the disk lifetime. This provides a huge hurdle for understanding the statistical properties of observed extra solar planets that now amount to more than 700.</p> <p>In this thesis, we focus on one of the most general properties of protoplanetary disks - inhomogeneities. A large amount of theoretical and observational work currently suggests that protoplanetary disks are most likely to possess several kinds of inhomogeneities. Planetary migration is highly sensitive to the disk properties such as the surface density and temperature of disks, and the sensitivity leads to the formation of trapping sites for rapid type I migration at disk inhomogeneities. These local sites capturing planets undergoing migration are referred to as planet traps. We perform both analytical and numerical studies for exploring formation mechanisms of planet traps at disk inhomogeneities and their consequences for the formation and evolution of planetary systems. We focus on three kinds of the disk inhomogeneities: dead zones, ice lines, and transitions of heat sources in protoplanetary disks we refer to as heat transitions. Dead zones are an inevitable consequence of disk turbulence originating from magnetorotational instabilities (MRIs) that take place in (partially) ionized disks threaded by weak magnetic fields. One of the fundamental properties of the dead zone is a low level of turbulence there, which is the outcome of the high density, preventing the region from being ionized due to X-rays from the central stars and cosmic rays. Ice lines are formed due to low disk temperatures which lead to condensation of specific molecules there. Heat transitions arise as a consequence of the switching of the dominant heating process from viscous heating to stellar irradiation as the distance to the host stars increases.</p> <p>We summarize our major findings. 1) rapid dust settling arising in dead zones leaves a dusty wall at the outer edge of the dead zones beyond which the disks are quite turbulent, so that dust is fully mixed with the gas. Efficient heating of the wall by stellar irradiation and the subsequent backward heating of the dead zones by the wall result in a positive temperature gradient in the dead zones. This inversion in the temperature profiles leads to outward migration there. 2) Any protoplanetary disk is likely to possess up to three types of planet traps that are specified by characteristic disk radii (dead zone, ice line and heat transition traps). Disk evolution, driven by disk viscosity, lowers both the accretion rate and surface density of gas and moves traps inward at different rates. This suggests that the interactions of (proto)planets captured at different traps play the dominant role in constructing planetary system architectures. Furthermore, the distribution of planet traps depends largely on stellar masses and accretion rates, so that they are one of the principle parameters for regulating the (initial) scale of planetary systems. 3) Both multiplicity and mobility of planet traps are crucial for understanding the statistical properties of observed extra solar planets. For instance, the mass-period relation - observational manifestation that planetary mass is an increasing function of orbital periods - can be understood by constructing and following evolutionary tracks of accreting planets in planet traps. These three contribution are new results in the field.</p> / Doctor of Philosophy (PhD)

<p>Protostellar discs are generally thought to drive molecular outflows and jets observed in star forming regions, but there has been some debate as to how they form. The details of the driving and collimation of outflows help determine how much mass is cleared out and how much energy is fed back into the surroundings. Recently it has been argued that the magnetic brake is so strong that early protostellar disks cannot form.</p> <p>We have performed 3D ideal magnetohydrodynamic (MHD) simulations of collapsing Bonnor–Ebert spheres, employing sink particles within an AMR grid and using a cooling function to model radiative cooling of the gas. This allows us to follow the formation and early evolution of the accretion disc (2−8)×10⁴ years further into the Class 0 phase of its evolution. We form a rotationally dominated disc with a radius of 100 AU embedded inside a transient, unstable, flattened, rotating structure extending out to 2000 AU. The inner disc becomes unstable to a warping instability due to the magnetic structure of the outflow, warping 30 deg with respect to the rotation–axis by the end of the simulation. The disc is unstable to a Parker instability and sheds magnetic loops, degrading the orientation of the mean threading field. This reduces and locally reverses the magnetic braking torque of the large scale field back upon the disc. The reduction of magnetic braking allows a nearly Keplerian disc to form and may be the key way in which low mass stellar systems produce rotationally dominated discs. We discuss the relevance of our disc misalignment concerning the formation of mis–aligned hot Jupiters.</p> <p>Protostellar outflows are implicated in clearing mass from collapsing cores, and limiting the final mass of newly formed stars. The details of the driving and collimation of outflows help determine how much mass is cleared out and how much energy is fed back into the surroundings. The simulations generate outflows which are precessing, kinked, contain internal shocks and extend to a scale of 0.1 pc end–to–end. Our disc–wind theory describes magneto–centrifugal driving throughout the outflow bubble. The bulk properties of the outflow agree well with observations. The outflow has two components, a larger low speed wind (v_r < 1.5 km/s) dominated by a toroidal magnetic field Bφ, and an inner centrifugally driven jet dominated by Bp with speeds up to 20 km/s. The ratio of mass flux from the disk surface com- pared to accretion in the disk is measured to be M_out/M_in ∼ 0.1 from the inner component, whereas in the outer component M_out/M_in ∼ 1.0. The jet is misaligned and precesses as the disc warps by 30 deg with respect to the z–axis. We measure star formation efficiencies of ε_core = 0.63 (and growing), higher than theoretical predictions of ε_core = 0.29−0.39 and observations ε_core = 0.33.</p> <p>These new results reported in this thesis, show that disks can form in strongly magnetized media, in agreement with the observations - and that outflows are not as efficient in clearing away collapsing gas as has been assumed in various theoretical models. Both of these results have important implications for disk formation, and the origin of the IMF, as described in this work.</p> / Doctor of Philosophy (PhD)

In the 4d transition metal oxides, the extension of the 4d orbitals leads to comparable and thus competitive kinetic and coulomb energies. As a result, small perturbations can induce significant changes in their physical properties, giving rise to a class of exotic phenomena that are rarely found in other materials. The ruthenates materials with readily tunable parameters open an avenue to study the strong electronic correlation in the rarely explored territory: the 4d transition metal oxides. The bilayered system, Ca3Ru2O7, belongs to the Ruddlesden-Popper series in which the physical properties are intimately linked to the lattice degrees of freedom. Ca3Ru2O7, with its quasi-2D and severe structure distortion, is believed to be placed in a unique position at which the role of orbital degrees of freedom is highlighted. The system displays strikingly different behaviors when the field is applied along different crystalline axes. A ferromagnetic (FM) state with full spin polarization is achieved for B||a-axis, but colossal magnetoresistance is realized only for B||b-axis by avoiding the ferromagnetic state. In addition, for B rotating within the ac-plane, slow and strong SdH oscillations periodic in 1/B are observed for T.1.5 K in the presence of metamagnetism. For B|| [110], oscillations are also observed but periodic in B (rather than 1/B) and persist up to 15 K. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are driven by the orbital degrees of freedom. Complex thermodynamic properties are also observed in the other ruthenates system such as Sr4Ru3O10 and Pr3RuO7. The Sr4Ru3O10 is a triple-layered system that shows a dedicate balance between fluctuations and order. Besides the anomaly at TC=102K, anomalous behavior at low temperatures are also observed in the thermal study, indicative of an unusual magnetic order in this material. The Pr3RuO7 shows one-dimensional structure with zig-zag chain of corner sharing RuO6 octahedra running in parallel with the rows of edge-shared PrO8 pseudo-cubes. Magnetic and thermal properties studies on its single crystals indicate that the exchange interaction is strongly anisotropic. A Schottky-type anomaly at low temperature suggests that the gorderedh chain Pr ions are still sensitive to a crystal field.