abstract: Lyman-alpha (Lyα) galaxies (LAEs) and Lyα blobs (LABs) are objects identified and studied due to their bright Lyα emission lines. This bright emission allows LAEs and LABs to be studied in the distant universe, providing a glimpse into the physical processes occuring in the early universe. This dissertation presents three complementary studies of LAEs and LABs at z ~ 3.1. The two main foci of this work are (1) to understand the gas kinematics in both classes of objects and (2) to improve spectral energy distribution (SED) fitting processes to better determine the physical characteristics of LAEs. Gas kinematics in this dissertation means looking for signatures of large-scale winds. This is an exciting astrophysical endeavor, because the results can provide insight into how Lyα photons escape distant galaxies and traverse the IGM, and the results have implications for how the epoch of reionization can be studied with the Lyα line and because winds can be a signature of powerful star formation events. In the first two studies we find signatures of winds in three LAEs by measuring the velocity offset between the redshifts of [OIII] and Lyα in these galaxies. The first two LAEs presented here represent the first ever measurements of [OIII] in Lyα-selected field galaxies. The third study reports no velocity offset between [OIII] and Lyα when the methodology is transferred to a z ~ 3.1 LAB. This lack of velocity offset is an interesting result, however, as powerful outflows and star formation events, which should impart a velocity offset, have been hypothesized as power sources for LABs. In addition to understanding the kinematics of these objects, we introduce a new parameter into the SED fitting process typically used to characterize LAEs. This new parameter enables better determination of characteristics like the age, mass, metallicity, dust content and star formation history of the galaxies in our sample. These characteristics provide a snapshot of galaxies in the universe ~ 11 billion years ago and also provide insight into how these characteristics compare to galaxies at other epochs. / Dissertation/Thesis / Ph.D. Astrophysics 2012

abstract: Galaxies represent a fundamental catalyst in the ``lifecycle'' of matter in the Universe, and the study of galaxy assembly and evolution provides unique insight into the physical processes governing the transformation of matter from atoms to gas to stars. With the Hubble Space Telescope, the astrophysical community is able to study the formation and evolution of galaxies, at an unrivaled spatial resolution, over more than 90% of cosmic time. Here, I present results from two complementary studies of galaxy evolution in the local and intermediate redshift Universe which used new and archival HST images. First, I use archival broad-band HST WFPC2 optical images of local (d<63 Mpc) Seyfert-type galaxies to test the observed correlation between visually-classified host galaxy dust morphology and AGN class. Using quantitative parameters for classifying galaxy morphology, I do not measure a strong correlation between the galaxy morphology and AGN class. This result could imply that the Unified Model of AGN provides a sufficient model for the observed diversity of AGN, but this result could also indicate the quantitative techniques are insufficient for characterizing the dust morphology of local galaxies. To address the latter, I develop a new automated method using an inverse unsharp masking technique coupled to Source Extractor to detect and measure dust morphology. I measure no strong trends with dust-morphology and AGN class using this method, and conclude that the Unified Model remains sufficient to explain the diversity of AGN. Second, I use new UV-optical-near IR broad-band images obtained with the HST WFC3 in the Early Release Science (ERS) program to study the evolution of massive, early-type galaxies. These galaxies were once considered to be ``red and dead'', as a class uniformly devoid of recent star formation, but observations of these galaxies in the local Universe at UV wavelengths have revealed a significant fraction (30%) of ETGs to have recently formed a small fraction (5-10%) of their stellar mass in young stars. I extend the study of recent star formation in ETGs to intermediate-redshift 0.35<1.5 with the ERS data. Comparing the mass fraction and age of young stellar populations identified in these ETGs from two-component SED analysis with the morphology of the ETG and the frequency of companions, I find that at this redshift many ETGs are likely to have experienced a minor burst of recent star formation. The mechanisms driving this recent star formation are varied, and evidence for both minor merger driven recent star formation as well as the evolution of transitioning ETGs is identified. / Dissertation/Thesis / Ph.D. Astrophysics 2013

Currently, solar observers are investigating spectroscopic images of the Sun's outermost atmosphere (the corona), which are challenging long-held views on the density and temperature structure of this environment. The corona is "filled" with magnetic strands but determining their precise nature is not straightforward. One way of revealing the nature of the coronal heating mechanism is by comparing simple theoretical one dimensional hydrostatic loop models with observations of the temperature and/or density structure along these features. The most wellknown method for dealing with comparisons like that is the x2 approach. In this research we consider the restrictions imposed by this approach and present an alternative way for making model comparisons using Bayesian statistics. In order to quantify our beliefs we use Bayes factors and information criteria such as AIC and BIC. Three simulated data-sets are analysed in order to validate the procedure and assess the effects of varying error bar size. Another three datasets (Ugarte-Urra ci at., 2005; Priest ci at., 2000; Young ci al., 2007) are analysed using the method described above. For the Ugarte-Urra ci at. and Young ci al. data-sets, we conclude apex dominant heating is the likely heating candidate, whereas the Priest ci al. data-set implies basal heating. Note that these new results (regarding the Ugarte-Urra ci at. and Priest ci at. data-sets) are different from those obtained using the chi-squared statistic. The second research project involves extensive model comparison against solar flare plasma observed cooling curves. After a solar flare erupts, flare-loops form which cool over thousands of seconds. How the plasma cools over time is investigated. In this case, we test the adequacy of the zero-dimensional EBTEL (Enthalpy-Based Thermal Evolution of Loops) model as introduced by Klimchuk, Patsourakos, and Cargill (2008). An interesting approach here is to define the form of the non-thermal heating input to the system and compare it with the thermal heating input. For the data-set under investigation (Raftery et al., 2009) a Full-Gaussian energy profile is proposed. Also, from the data it is not possible to distinguish which of the thermal or non-thermal heat flux is more dominant, so both can be equally considered for temperature, density and pressure evolution of the system. Finally, the last part of this research is dedicated to recurrent nova outbursts. RS Ophiuehi is a nova produced by a white dwarf star and a red giant. In this case the white dwarf will steadily acerete gases on its surface from the red giant's outer atmosphere. About every twenty years, enough material will be accreted on the white dwarf's surface in order to produce an eruption. Over the past one hundred years at least five such outbursts have been observed. As another application of Bayesian model comparison techniques, curve fitting models are tested against light curves of RS Ophiuchi outbursts in order to decide upon the one that best describes the data. Furthermore, the magnitude of the star is analysed using wavclet analysis techniques. Ways of deriving the Cone of Influence are presented. An outcome of this analysis is that we can quantitatively confirm that an outburst occurred around November 26, 1945, which was not recorded due to the observational seasonal gaps. This was originally proposed by Oppenheimer and Mattei (1993) but was never accepted as a confirmed outburst. Also, this method reveals a pre-outburst signal in the light curve. For this, the way in which the wavelet analysis can be beneficial for future outburst predictions is presented.

<p> Accretion on to black holes powers some of the most luminous objects in the universe. In this thesis I present a series of works aimed at constraining the properties of black hole accretion in a variety of astrophysical systems. Numerical methods are vital for studying the multi-scale and non-linear physics of these systems. First I introduce <tt>DiscoGR,</tt> the first implementation of numerical general relativistic magnetohydrodynamics on a moving mesh. <tt> DiscoGR</tt> is capable of efficiently and accurately simulating highly supersonic thin accretion disks, the objects responsible for many luminous accretion events. I apply <tt>DiscoGR</tt> to study minidisks: accretion disks around a single member of a binary black hole system. Spiral shock waves, excited by tidal forces from the binary companion, propagate throughout the disk, causing efficient accretion by purely hydrodynamical means. The shock-driven accretion has an effective alpha parameter of the order 0.01, comparable with accretion driven by the magnetorotational instability. Furthermore, shocks near the black hole contribute to a radiative signature brighter in the hard x-rays than the standard Novikov-Thorne model. Finally I present an analysis of gamma-ray burst (GRB) x-ray afterglow light curves. The analysis fits the data from the <i>Swift-XRT</i> directly to a suite of hydrodynamical simulations, constraining the jet opening angle and, for the first time, the viewing angle of these events. I find typically the viewing angle to be 0.57 of the jet opening angle. Observing a GRB off-axis can reduce the inferred energy of the central engine, thought to be a neutron star or accreting black hole, by up to a factor of four.</p><p>

In this thesis I discuss two projects that have been a major part of my postgraduate studies. The aim of these projects is to study the dynamics and chemodynamics of simulated Milky Way analogues. Specifically, I investigate chemical abundances in the solar-neighbourhood and of the outflow rates of gas of Milky Way analogue discs. In the first project, I describe a galaxy simulated with the code RAMSES-CH and compare this with chemical abundance data from the Gaia-ESO survey and the RAVE survey. The aim of this work is to improve matching the chemical abundances in stars within the galaxy to those in observational surveys. This is done by sampling our simulation to best match what an observer does in observational surveys. In addition to carrying out an observationally-motivated spatial selection within the simulation and thus comparing like for like, we take into account observational uncertainty and the selection effects (photometric, effective temperature and surface gravity). Incorporating these factors within the simulation data, we find that simply taking a spatial cut alone within a simulation model is not sufficient to match simulated abundances like for like with observational surveys. For complete observational selection functions, like that in the Gaia-ESO survey, the selection function has a minimal impact on the ages and metal abundances. However for a narrower selection function like in the RAVE survey, the impact becomes more noticeable. The method that improves simulation abundance patterns with observations however is the inclusion of an observationally motivated scatter based on the uncertainties of the observational survey you are studying. In my second project I study the outflow abundances of gas from the disc of Milky Way-like galaxy in isolation along with inflow from a hot gas halo. I generate a galaxy model from the initial conditions generator code GalactICS and run the simulation with the meshless Lagrangian Godunov-type code GIZMO. The simulation's aim is to investigate gravity driven turbulence of a gas disc in the absence of more commonly considered sources of feedback such as supernovae. Our goal is to place a lower limit on this effect in Milky-Way analogues, which from initial investigations conducted without full self-gravity is admittedly anticipated to be small. We present a study the outflow of gas and its relation to the surface density and radius of the disc. In comparison to more idealised parameter studies, we find that the outflows of gas from high surface density regions are suppressed by the cooling flow of the gas halo. This contradicts results from small box simulations, but is reflective of the physics in a full disc model. Nonetheless, outflows up to 1.5 kpc in height are found, and vertical velocity dispersions are in broad agreement with other work that includes additional sources of feedback.

High time resolution observations of two Be stars ( K, Dra and ɣCas) have been made with an Image Isocon television camera in an attempt to detect rapid radial velocity and/or intensity variations of the Hydrogen emission lines. Analysis of variance tests have been used to determine the significance of possible variations. Fluctuations of the intensity of the H alpha line of ɣ Cas were seen on a time scale of a few minutes. Upper limits to the radial velocity variations were imposed by an apparent non-linear expansion of the camera reading beam scanning raster. The variations of H alpha for K Dra were marginally significant, showing a slow change in the position of the line with no significant change in intensity over the 30 minutes of observation. No significant variations were present in observations of H beta and H gamma of ɣCas. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

It is believed that a wide range of physical processes conspire to shape the observed galaxy population but it remains unsure of their detailed interactions. The semi-analytic model (SAM) of galaxy formation uses multi-dimensional parameterizations of the physical processes of galaxy formation and provides a tool to constrain these underlying physical interactions. Because of the high dimensionality and large uncertainties in the model, the parametric problem of galaxy formation can be profitably tackled with a Bayesian-inference based approach, which allows one to constrain theory with data in a statistically rigorous way. In this thesis, I present a newly developed method to build SAM upon the framework of Bayesian inference. I show that, aided by advanced Markov-Chain Monte-Carlo algorithms, the method has the power to efficiently combine information from diverse data sources, rigorously establish confidence bounds on model parameters, and provide powerful probability-based methods for hypothesis test. Using various data sets (stellar mass function, conditional stellar mass function, K-band luminosity function, and cold gas mass functions) of galaxies in the local Universe, I carry out a series of Bayesian model inferences. The results show that SAM contains huge degeneracies among its parameters, indicating that some of the conclusions drawn previously with the conventional approach may not be truly valid but need to be revisited by the Bayesian approach. Second, some of the degeneracy of the model can be broken by adopting multiple data sets that constrain different aspects of the galaxy population. Third, the inferences reveal that model has challenge to simultaneously explain some important observational results, suggesting that some key physics governing the evolution of star formation and feedback may still be missing from the model. These analyses show clearly that the Bayesian inference based SAM can be used to perform systematic and statistically rigorous investigation of galaxy formation based on various observations and help to design new observations that can effectively discriminate theoretical models.

Ultraluminous infrared galaxies (ULIRGs) are among the most powerful galaxies and play an important role in galaxy formation and evolution. In this dissertation, I present by far the most detailed and latest studies of ULIRGs both in the nearby universe and at epochs only half of today’s universe’s age, through their color-magnitude relation (CMR) and morphology. I studied the most luminous local ULIRGs in the SDSS, and found that local ULIRGs are optically luminous, on average 1 magnitude brighter than field galaxies, and they are as blue as the blue cloud in the color-magnitude diagram of galaxies. Only few ULIRGs appear in the green valley, the low density region between the red sequence and blue cloud, and none of which harbors an AGN. Therefore, we are not witnessing the AGN quenching of star formation, and the blue colors of ULIRGs is consistent with a working hypothesis in which the dust geometry is patchy and blue lights originate from unshielded stellar components, many of which are at large distances. We also imaged nine most luminous ULIRGs at z ∼ 1 using HST in ACS/F814W and NICMOS2/F160W filters. The z ∼ 1 ULIRGs, unlike the z ∼ 0 ULIRGs, appear to be more concentrated in the green valley. The concentration of z ∼ 1 ULIRGs in the green valley is possible due to the color selection in studying the CMR. Quantitative morphology measurements have been widely used in describing galaxy morphology and classifying galaxies, but their limitations have not been systematically studied. In this dissertation I present detailed studies of the limitations of non-parametric quantitative morphology measurements, the Gini coefficient (G) and M 20. Both z ∼ 0 and z ∼ 1 ULIRGs distribute heterogeneously in G – M20 space, which is unexpected from the proposed effectiveness of G –M 20 in selecting merging galaxies. I performed a series simulations, which shows that seen against higher background noise, G decreases and M20 increases systematically. Observationally G and M20 are also more sensitive to certain merging stages, consistent with the findings from other numerical simulations. As a conclusion, cautions must be made when applying the quantitative morphology measurements. There are strong color gradients across almost all z ∼ 0 and z ∼ 1 ULIRGs. Benefited from high spatial resolutions of HST, I developed a new technique to investigate the pixel-based color distribution and produced pixel-based color-magnitude diagrams (pCMDs) for the z ∼ 1 ULIRGs. A unique feature in the pCMD, "horizontal blue shelf", has been discovered among all z ∼ 1 ULIRGs, which is consistent with typical colors of today's disk, and can be modeled by young stellar populations with an age between 10 Myr and 500 Myr with moderate extinction.

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 lowmass 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-α survey. We produce mosaic maps of the Paschen-α 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.

We study the distribution and nature of neutral hydrogen in various environments. We start by exploring the disk and halos of galaxies in Chapter 1. Our sample consists of 23 sightlines through 15 low-redshift foreground galaxy - background quasar pairs with impact parameters ranging from 1.7 kpc up to 86.7 kpc. We detected one absorber in the GBT survey from the foreground dwarf galaxy, GQ1042+0747, at an impact parameter of 1.7 kpc and another possible absorber in the VLA imaging of the nearby dwarf galaxy, UGC 7408. In Chapter 2, we present a detailed study of the absorber associated with the galaxy, GQ1042+0747, at an impact parameter of 1.7 kpc. The width of the main absorption line indicates that the gas is cold with kinetic temperature < 283 K, and the HI column is surprisingly low. The lower limit on the size of the absorber is 27.1 x 13.9 pc. In turn, this indicates a low density for a cold cloud, n(HI)= 3.5 cm-3. In Chapter 3, we explore the distribution and evolution of tidally stripped HI in a complete distance-limited sample of 22 Hickson Compact Groups (HCGs) with at least four true members. We detected an average HI mass of 8 x 10 9 cm-2 M o, which is significantly larger than previous single-dish measurements. Spectral comparison of the GBT data with complementary VLA data shows significant HI excess in the GBT spectra. We discuss the origin of the excess gas and explore the possible physical conditions in such systems. Finally, in the forth chapter we explore the connection of Lyman alpha absorbers to galaxies, groups of galaxies and the large-scale structures. We use data the data and galaxy and galaxy-group catalogs. We find the strength of the Lyman alpha absorbers to be inversely proportional to the impact parameter of the nearest galaxy (ρ) or galaxy-group when corrected for the galaxy luminosity (L) or group mass (Mhalo), respectively. We also used the cross-correlation function to explore the connection of Lyman alpha absorbers to galaxies.