SKA pathfinders (e.g., ASKAP and MeerKAT) are performing large HI surveys, surpassing previous-generation radio surveys in angular resolution, volume, and depth — going nearly two orders of magnitude deeper. The vast amount of data being produced by these telescopes creates a need for new tools and techniques, such as automated source-extraction. One particular science goal is to trace the large scale structure of galaxies in the local Universe behind the Zone of Avoidance (ZoA). This is difficult to do in the optical and infrared bands because of the thick dust and stellar crowding along the plane of the Milky Way. HI surveys are ideal because the 21 cm spectral line emission of neutral hydrogen (HI) atoms is unaffected by these features. This dissertation presents two HI surveys in the ZoA: the MeerKAT16 Early Science project, conducted before the completion of the full array, with 16-dish subarrays and the ROACH-32k correlator; and later, the Galactic Plane Legacy Survey (GPS), a larger MeerKAT survey project utilising 60-dish subarrays and the SKARAB-4k correlator. The motivation of the MeerKAT16 survey was two-fold: serving as a pilot project for the envisioned large MeerKAT64 Vela Supercluster (VSCL) survey; and a test-bed for optimising galaxy identification processes using automated pipelines. The survey aimed to map the rich galaxy cluster VC04, embedded in one of the prospective VSCL walls. We searched two mosaic cubes for HI sources by means of a deep visual search, and then using the Source Finding Application (SoFiA; version 1.3.2). The purpose of using both methods was to understand and optimise the SoFiA pipeline on real data, which is important for the development of source finding strategies for future large surveys. We catalogued 119 reliable galaxy detections (and an additional 37 candidates with lower certainty) within the early science data. SoFiA found galaxies all the way at the VSCL distance (V(hel) ≈ 18 000 km/s), where we detected hints of two walls, and HI deficiency in the centre of VC04. GPS surveyed a long narrow strip (|b| ≤ 2°) along the Galactic plane, with the aim of penetrating the most obscured part of the ZoA. A segment of the data spanning 302° ≤ l ≤ 332° was reduced, imaged, and analysed as part of this project. This region is interesting because it encompasses the Great Attractor (GA) — a massive overdensity highly influential to the local flow of galaxies. Aided by the newer version of SoFiA (v2.3.1), we found 477 galaxy candidates, most of which are new discoveries, and performed an in-depth comparison against the largest systematic HI survey covering the southern ZoA up to this point — the Parkes HIZOA survey. Additionally, we compared our results to simulations that follow the method used for the SKA HI science case. The GPS redshift distribution reveals a striking overdensity at the GA distance (V(hel) ≈ 4000 km/s), inconsistent with a uniform galaxy distribution, whereas the high-redshift end is more underdense than predicted by the simulation. These deep interferometric HI surveys provided a new glimpse of highly interesting structures crossing the Galactic plane, demonstrating that large scale structure can be mapped even in the deepest part of the ZoA. The success of MeerKAT16 and GPS is a preview of the HI science potential of the upcoming SKA, which will surpass MeerKAT in depth, sensitivity and resolution.

Rotation and structure in the universe are studied in two investigations. The first investigation is a theoretical exploration of structure formation in the young universe, when localized angular momentum is present. The second investigation is an exploration of the "signature" localized large scale rotation or transverse motion might leave in observational "redshift maps" of structure in the universe. In the first investigation, analytic approximations to the virial equations of equilibrium for rotating, self-gravitating gaseous bodies are studied in the multi-equilibrium regime. Such bodies may collapse via a "phase transition" before reaching dynamical instability. The phase transition mass is calculated for a simple cosmological scenario and found to be the size of a small galaxy. The dynamical collapse mass, by contrast, is much larger and grows rapidly as the universe expands. In the second investigation, a redshift space artifact is studied which results from an "infall" field surrounding a rich galaxy cluster. In a simulation, a redshift map of an infall region shows an artificial "ring" of galaxies encircling the "finger of god" artifact produced by the simulated cluster core. When the observer has transverse motion or the infall region has rotation, the ring tilts on the finger. Redshift maps of two nearby clusters, Virgo and Fornax, are compared against the simulation. Redshift space structure surrounding Virgo shows a strong resemblence to the simulation's ring, although structure surrounding Fornax is less similar. Both Virgo's and Fornax's "rings" tilt. Taken together, the tilt directions are not consistent with a uniform translation of the rest frame. The directions are consistent with either a steep tidal shear produced by a "Great Attractor" or with rotation about each cluster.

No description available.

Contemporary galaxy simulations are currently capable of resolving the dense molecular phase of the interstellar medium. The behaviour of this molecular gas is complicated by the ability for molecular hydrogen to self-shield, protecting the deeper layers of clouds from dissociating radiation that would otherwise break the molecules into constituent parts and heat the gas. An accurate model of molecular hydrogen needs to couple to the local radiation field of the galaxy while also accounting for the effects of self-shielding. I present a self-consistent chemical network to model the formation and destruction of molecular hydrogen and related primordial gas species (H, He, and their ions). The model is designed to couple to a realistic UV radiation field modeled using discrete bands. It is intended for use in the GASOLINE N-body hydrodynamics code for galaxy simulations alongside the TREVR/TREVR2 ray-tracing radiative transfer routines. When combined with these routines, my model offers a correct treatment for shielding that allows for radiation from multiple sources to be shielded independently. I include several tests to ensure the fidelity of this model, including simulated HII regions, photodissociation regions, and the evolution of primordial gas prior to galaxy formation. This model is applicable in the simulation of a realistic interstellar medium in isolated disk galaxies and the evolution of dwarf galaxies. A proper model for molecular hydrogen with radiation in a galaxy enables simulations to produce observable quantities that can be used to evaluate the quality of our simulated galaxies. This model will provide opportunities to explore the connection between molecular hydrogen and models of star formation. / Thesis / Master of Science (MSc)

Planetary nebulae (PNs) that evolve from relatively high mass progenitor stars can "masquerade" as low mass objects. We simulate the evolution of PNs and their central stars based on simple models, using various wind speeds and mass loss rates. Even when our nebulae become ionized beyond the characteristic dense inner shell, a faint halo can comprise most of ionized matter while contributing little luminosity. For such PNs, standard techniques severely underestimate ionized mass. Curiously, ionized masses that would be observationally derived for our model nebulae ("Shklovsky masses") are insensitive to variations in the model's input parameters. For evolved PNs, the Shklovsky mass remains a few tenths of a solar mass, despite the total ionized mass varying over two orders of magnitude in our simulations. We show that this is consistent with the range of masses determined for PNs with independent distance estimates. This small mass variance should produce only $\sim$30% distance errors using Shklovsky's constant mass method and may explain why this method is successful despite the incorrect assumption of low ionized mass. We describe a new distance method for PNs based on a theoretical/empirical relationship between their radii and radio surface brightnesses. This method requires only readily available radio flux and angular size measurements. We use Galactic bulge PNs along with PNs with independent distances to establish, calibrate, and test this method. Our distance method appears to yield errors of only $\sim$20% using the best available data. We also find that the Shklovsky method predicts the distances of large, low surface brightness PNs well, but overestimates distances of smaller PNs. We have also made deep radio observations of two PNs, NGC 6804 and NGC 6826, to examine their halo masses. Despite large dynamic ranges, we detect inner halos of both nebulae. Derived halo-to-shell mass ratios demonstrate that the halos contain $>$60% of the total ionized mass while contributing $<$25% of the emission. We further test our distance method by comparison with kinematic distances derived using measured radial velocities of a sample of PNs. Our method agrees with Galactic kinematics within limits of measurement uncertainties and velocity dispersion.

Circumstellar disks with masses comparable to the primeval solar nebula have been discovered around numerous pre-main sequence stars; it is believed the disks are a natural byproduct of star formation. If most stars originally have massive circumstellar disks, it is very likely planetary systems are common. Orbiting planets are not directly observable owing to their relatively cool temperatures and meager surface area. However, in the early stages of planetary formation, the surface area of debris in the disk may exceed the surface area of the star by many orders of magnitude. Material in the terrestrial zone emits primarily at near-infrared wavelengths; sufficient disk debris may produce detectable excess emission at these wavelengths. As clearing mechanisms, including possible planetary formation, remove the small particles in the disk, the strong infrared emission diminishes. By observing the excess infrared emission as a function of stellar age and spectral type, timescales for inner disk processes which create or remove small particles can be established. This dissertation presents sensitive, simultaneous, near-infrared broadband continuum observations of old pre-main sequence and young main-sequence cluster stars. The stellar ages range from 1-600 Myr, spanning the predicted epoch of planetary formation for solar-type stars. A wide range of spectral types were observed. We detect no excess emission after an age of about $3\times 10\sp6$ yr. Using a model to predict the infrared emission from an optically thin dust disk, we find our measurements are sensitive to 10$\sp{20}$-10$\sp{21}$ g of micron-radius dust grains ($\rho$ = 2 g cm$\sp{-3}$) distributed within the terrestrial zone. Adapting this result to a more realistic particle size distribution, we believe we can detect debris in extra-solar systems until the terrestrial planets are 90-95% complete. Older models of the formation of the Earth which assume orderly growth predict the Earth is 90% complete after about 80 Myr. Newer models allow runaway growth, which shortens the timescale to ${\sim}10\sp5$ yr. If the observed clearing in the inner disk reflects the formation of terrestrial planets, our results strongly support models of planetary formation which incorporate runaway growth. Implications are discussed.

<p>Giant molecular clouds (GMCs) and cores, which are the sites of star formation in the Galaxy today, are supported against their own gravity and an external pressure by a combination of thermal and magnetic pressures, and by an internal turbulence which is characterized by gas velocity dispersions that increase outwards within a given cloud. Until now, however, models of isothermal spheres (in which velocity dispersion is independent of radius) have been used as the starting point for theories of star formation. Thus, this thesis presents a generalization of the classic Bonnor-Ebert stability analysis of pressure-bounded, self-gravitating spheres of isothermal gas, to include clouds with arbitrary equations of state (EOS). The results are applied to GMCs and cores in order to model their internal structure. It is found that the simplest EOS which is consistent with all the salient features of these clouds is a "pure logotrope," in which P/Pⅽ=1+A ln(ρ/ρc). Detailed comparisons with data are made to estimate the value of A, and an excellent fit to the observed dependence of velocity dispersion on radius in cores is obtained with A≃0.2. This phenomenological model is then used to construct a new theory for gravitational collapse and star formation in turbulent molecular gas. Points of similarity and contrast between this and the collapse of an isothermal sphere are discussed. In particular, it is found that the rate of mass accretion onto a protostar in a logotrope increases with time (as M∝ t³), rather than remaining constant as in an isothermal sphere. Low-mass stars therefore take longer to form than previously suspected, while high-mass stars form more rapidly. This result has implications for the interpretation of observations of young stellar objects (the so-called "Class 0" protostars are discussed explicitly), for the origin and form of the stellar initial mass function, and for the process of star formation in clusters.</p> <p>GMCs and cores are further characterized by a mass spectrum, N(m)=dN/dm, which scales roughly as m⁻¹⋅⁵ and is reminiscent of the distribution of globular cluster masses in the globular cluster systems (GCSs) observed around most galaxies. This suggests that star formation in protogalaxies may have followed the same basic pattern that it does today, i.e., that globular clusters formed in clumps of gas embedded in much larger (∼10⁸-10⁹M⨀) protogalactic fragments. Within this framework, the first theory of the GCS N(m) (which is directly related to the more frequently discussed globular cluster luminosity function, dN/dMᵥ) is developed here. A kinetic equation is formulated to statistically follow the collisional build-up and subsequent disruption (by the side-effects of internal massive-star formation) of protocluster clumps within a parent fragment, and solved for the mass spectrum which gives a detailed balance between the rates of these processes. This N(m) agrees well with observations of the Milky Way, M31, and M87 GCSs, for cluster masses in excess of ∼10⁵M⨀. In addition, the theory naturally accounts for observations which suggest that, at least at these high masses, N(m) is largely independent of galactocentric radius within any one GCS. The model makes specific predictions, regarding the lifetimes of cluster-forming cores, that should be applicable to present-day Galactic GMCs and cores as well.</p> / Doctor of Philosophy (PhD)

QC 351 A7 no. 27 / This edition has been prepared in order to include more recent information than was available at the time of the first printing. New material is not incorporated into the text directly but is placed at the end of each table. It will be found on pages B-133, C-12, D-12, and E-8.

abstract: One of the most fundamental questions in astronomy is how the Universe evolved to become the highly structured system of stars and galaxies that we see today. The answer to this question can be largely uncovered in a relatively unexplored period in the history of the Universe known as the Epoch of Reionization (EoR), where radiation from the first generation of stars and galaxies ionized the neutral hydrogen gas in the intergalactic medium. The reionization process created "bubbles" of ionized regions around radiating sources that perturbed the matter density distribution and influenced the subsequent formation of stars and galaxies. Exactly how and when reionization occurred are currently up for debate. However, by studying this transformative period we hope to unravel the underlying astrophysics that governs the formation and evolution of the first stars and galaxies. The most promising method to study reionization is 21 cm tomography, which aims to map the 3D distribution of the neutral hydrogen gas using the 21 cm emission lines from the spin-flip transition of neutral hydrogen atoms. Several radio interferometers operating at frequencies below 200 MHz are conducting these experiments, but direct images of the observed fields are limited due to contamination from astrophysical foreground sources and other systematics, forcing current and upcoming analyses to be statistical. In this dissertation, I studied one-point statistics of the 21 cm brightness temperature intensity fluctuations, focusing on how measurements from observations would be biased by different contaminations and instrumental systematics and how to mitigate them. I develop simulation tools to generate realistic mock 21 cm observations of the Hydrogen Epoch of Reionization Array (HERA), a new interferometer being constructed in the Karoo desert in South Africa, and perform sensitivity analysis of the telescope to one-point statistics using the mock observations. I show that HERA will be able to measure 21 cm one-point statistics with sufficient sensitivity if foreground contaminations can be sufficiently mitigated. In the presence of foreground, I develop a rolling foreground avoidance filter technique and demonstrate that it can be used to obtain noise-limited measurements with HERA. To assess these techniques on real data, I obtain measurements from the legacy data from the first season observation of the Murchison Widefield Array (MWA) and perform additional high-precision radio interferometric simulations for comparison. Through these works, I have developed new statistical tools that are complementary to the power spectrum method that is currently the central focus of the majority of analyses. In addition to confirming power spectrum detections, one-point statistics offer additional information on the distribution of the 21 cm fluctuations, which is directly linked to the astrophysics of structure formation. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2019

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (p. 127-140).