I carried out systematic studies on the assembly history of dark matter halos, using numerical simulations and semi-analytical methods. First, I look into dark halo mass assembly history. I confirmed that the halo mass assembly is divided into a fast accretion phase and a slow accretion phase. These two phases are found to be separated by the epoch when the dark halo potential reaches its maximum. The fast accretion phase is dominated by mergers, especially major mergers; the slow accretion phase is dominated by slow mass accretion. Each halo experiences about 3±2 major mergers since its main progenitor had a mass equal to 1 percent of halo mass. However, the average redshift at which these major mergers occur is strongly mass dependent. Secondly, I investigate the formation times and the assembly bias of dark halos. I use eight different definitions of halo formation times to characterize the different aspects of the halo assembly history. I find that these formation times have different dependence on halo mass. While some formation times characterize well the hierarchical nature of halo formation, the trend is reversed for other definitions of the formation time. In addition, the formation-time dependence of halo bias is quite strong for some definitions of formation time but weak or absent for others. Thirdly, I study sub-halo mass function in the halo assembly history, with the generally known unevolved sub-halo mass functions (USMFs). I find that for subhalos that merge into the main progenitor of a present-day halo, their USMF can be well described by a universal functional form; the same conclusion can also be reached for the USMF of all sub-halos that have merged during the entire halo merging history. In these two cases, the USMFs do not seem to depend on the redshift of the host halo either. However, due to the mass loss caused by dynamical effects, only small part of the accreted halos survived and became sub-structures in the present-day dark halos. In the cluster-sized halos, 30% survived sub-halos are sub-subhalos. The sub-halo mass function at given accretion time (redshift) is also investigated to find the origin of the statistics mentioned above.

abstract: The spectra of brown dwarfs are key to exploring the chemistry and physics thattake place in their atmospheres. Late T dwarf (950 - 500 K) spectra are particularly diagnostic due to their relatively cloud free atmospheres and deep molecular bands. With the use of powerful atmospheric retrieval tools, these properties permit constraints on molecular/atomic abundances and temperature profiles. Building upon previous analyses on T and Y dwarfs (Line et al. 2017; Zalesky et al. 2019), I present a uniform retrieval analysis of 50 T dwarfs via their low-resolution near infrared spectra. This analysis more than doubles the sample of T dwarfs with retrieved properties. I present updates on current compositional trends and thermal profile constraints amongst the T dwarf population. My analysis shows that my collection of objects form trends that are consistent with solar grid model expectations for water, ammonia, methane, and potassium. I also establish a consistency between the thermal structures of my objects with those of grid models. Moreover, I explore the origin of gravity-metallicity discrepancies that are observed in some of my brown dwarf candidates. / Dissertation/Thesis / Masters Thesis Astrophysics 2020

abstract: Galaxy structural properties such as size, morphology, and surface brightness bear the imprint of galaxies' evolutionary histories, and so are related with other properties such as stellar mass, star formation rate, and emergent spectra. In this dissertation, I present three studies exploring such relationships. In the first, I investigated the relationships between 4000 Å break (D4000) strength, colors, stellar masses, and morphology in a sample of 352 galaxies at intermediate redshifts based on photometric and spectroscopic data from the Hubble Space Telescope (HST). I explored several diagrams such as UVJ color space combined with the D4000 strengths and the structural parameters of sample galaxies. The analysis shows that the presence of a bulge component is a necessary but not sufficient requirement for star formation quenching at intermediate redshifts. In the second study, I investigated the central 250 pc UV star formation intensity (SFI, star formation rate per unit area) of a sample of 40 Green Pea (GP) galaxies and 15 local Lyman Break Galaxy Analogs (LBAs) to understand the Lyα escape mechanisms and the associations with the SFI in Lyα-emitters (LAEs). I utilized the Cosmic Origins Spectrograph near-ultraviolet (COS/NUV) images from the HST. I found that the Lyα equivalent width (EW(Lyα)) and the Lyα escape fraction are positively correlated with the ratio of SFI to galaxy stellar mass. These correlations suggest the importance of the central SFI in Lyα photon escape. In the third study, I investigated the UV photometric properties of a sample of 40 GPs and the possible associations with Lyα escape mechanisms. I measured the UV-continuum size and luminosity of the sample galaxies by employing the COS/NUV images. The circularized half-light radius of GPs shows compact sizes and it further shows the statistically significant anti-correlations with EW(Lyα) and the Lyα escape fraction. The size comparison of GPs to those of high-redshift LAEs shows that their sizes are similar, once spatial resolution effects are properly considered. These results show that a compact size is crucial for escape of Lyα photons, and that Lyα emitters show constant characteristic size independent of their redshift. Therefore, the results presented in this dissertation emphasize the importance of galaxy structural properties in star formation quenching and in Lyα escape. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2020

No description available.

abstract: Reionization is the phase transition of intergalactic atoms from being neutral to becoming fully ionized. This process began ∼400 Myr after the Big Bang, when the first stars and black holes began emitting ionizing radiation from stellar photospheres and accretion disks. Reionization completed when all of the neutral matter between galaxies became ionized ∼1 Gyr after the Big Bang, and the Universe became transparent as it is today. Characteristics of the galaxies that drove reionization are mostly unknown. The physical mechanisms that create ionizing radiation inside these galaxies, and the paths for this light to escape are even more unclear. To date, only a small fraction of the numerous searches for this escaping light have been able to detect a faint signal from distant galaxies, and no consensus on how Reionization was completed has been established. In this dissertation, I discuss the evolution of the atomic matter between galaxies from its initially ionized state, to its current re-ionized state, potential sources of re-ionizing energy, and the theoretical and observational status of the characteristics of these sources. I also present new constraints on what fraction of the ionizing radiation escapes from galaxies using Hubble Space Telescope UV imaging, theoretical models of the stellar and accretion disk radiation, and models of the absorption of ionizing radiation by the intergalactic medium. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2019

Type Ia supernovae (SNe Ia) play a vital role in the discrimination of different cosmological models. These events have been shown to be standardizable based on properties of their light curves during the early-time photospheric phase. However, the distribution of types of progenitor system, the explosion trigger, and the physics of the explosion are still an active topic of discussion. The details of the progenitors and explosion may provide insight into the variation seen in Type Ia supernova light curves and spectra, and therefore, allow for additional methods of standardization among the group. Late-time near-infrared spectral observations for SNe Ia show numerous strong emission features of forbidden line transitions of cobalt and iron, tracing the central distribution of iron-group burning products. As the spectrum ages, the cobalt features fade as expected from the decay of 56Co to 56Fe. This work will show that the strong and isolated [Fe II] emission line at 1.644 μm provides a unique tool to analyze near-infrared spectra of SNe Ia. Several new methods of analysis will be demonstrated to determine some of the initial conditions of the system. The initial central density, ρc, and the extent of mixing in the central regions of the explosion have signatures in the line profiles of late-time spectra. An embedded magnetic field, B, of the white dwarf can be determined using the evolution of the lines profiles. Currently magnetic field effects are not included in the hydrodynamics and radiation transport of simulations of SNe Ia. Normalization of spectra to the 1.644 μm line allows separation of features produced by stable versus unstable isotopes of iron group elements. Implications for potential progenitor systems, explosion mechanisms, and the origins and morphology of magnetic fields in SNe Ia, in addition to limitations of the method, are discussed. Observations of the late-time near-infrared emission spectrum at multiple epochs allow for the first ever analysis of the evolution of the 1.644 μm line profile for a SNe Ia. These late-time data are really pushing the observational limits of current ground-based telescopes in terms of a dim target and low signal-to-noise. The new analysis method presented in this work is used on observations of SN 2005df to constrain the initial conditions of those systems. Finally, the details and limitations of the method are presented for use with SN 2014J and future time-series observations, which will dramatically increase in number and signal-to-noise with the next-generation of telescopes and missions. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester, 2015. / April 13, 2014. / near-infrared, spectroscopy, supernovae / Includes bibliographical references. / Peter Hoeflich, Professor Directing Dissertation; Eric Chicken, University Representative; David Collins, Committee Member; Harrison Prosper, Committee Member; Mark Riley, Committee Member.

Two leading theories exist to explain the progenitor models of Type Ia supernovae. In the single-degenerate scenario (SDS), a carbon-oxygen white dwarf slowly accretes matter from a non-degenerate binary companion that is exceeding its roche lobe until the mass of the white dwarf reaches the Chandrasekhar limit (M ∼ 1.4 solar masses). At this point a deflagration wave begins in the core, eventually turning into a detonation wave that reaches the surface and annihilates the white dwarf, causing the supernova event. In the double-degenerate scenario (DDS), two white dwarfs lose angular momentum due to the emission of gravitational waves and merge together, exceeding the Chandrasekhar limit and causing a supernova. In this study, we explore the observational evidence indicative of only the single-degenerate scenario by looking at the long-term effects caused by the interaction between the supernova debris and the non-degenerate companion. We model the interaction in two dimensions using the PROTEUS code that utilizes adaptive mesh refinement. Our simulations involve one supernova type interacting with one of seven different companion types -- four main-sequence-like stars (MS), one subgiant (SG), and two red giants (SY). During the interaction, a region mostly devoid of material is formed behind the companion. We find that the structure of this 'hole' formed behind the companion is similar across each of these models, with an angular size extending 30°-45°. The structure of the supernova remnant is affected out to 90°-100° as a result of the interaction with the companion. Each companion type has a characteristic percentage of mass stripped from it by the end of the simulation with MS stars losing about ∼20% of their mass, the SG star losing about ∼10%, and the SY stars losing about ∼40%, where in the SY case only the denerate core and a small portion of the stellar envelope is left over. We find that the interaction contaminates the companion with trace amounts of Nickel-56. 10⁻¹⁶ to 10⁻⁸ solar masses of Nickel-56 are found in the MS stars, 10⁻⁷ solar masses are found in the SG star, and 10⁻¹⁹ to 10⁻²¹ solar masses are found in the SY stars' leftover envelope, though this contamination may be purely numerical. In the initial stages of the interaction, we find that the superheated material trapped between the companion and the expanding ejecta is capable of prompt X-ray emission through the evacuated hole once it travels around the companion and along the border of the hole. The temperature of this material is expected to decrease once it reaches the hole, but at each model's most energetic time, all three sample systems considered are capable of producing soft X-rays (for the MS38 system, the bulk of potential X-ray emission lies in the range 2.73 to 4.32 KeV, for the SG system, 1.08 to 2.43 KeV, and for the SY319 system, 0.09 to 0.27 KeV). / A Thesis submitted to the Department of Scientific Computing in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester, 2014. / October 30, 2014. / Close Binaries, General Supernovae, Hydrodynamics, Numerical Methods / Includes bibliographical references. / Tomasz Plewa, Professor Directing Thesis; Gordon Erlebacher, Committee Member; Anke Meyer-Baese, Committee Member.

A galaxy group catalogue for the recently-completed 2MASS Redshift Survey (2MRS, Macri et al., 2019) is presented which consists of 44572 redshifts, including 1041 new measurements for galaxies mostly located within the Zone of Avoidance. The galaxy group catalogue is generated using a novel, graph-theory based, modified version of the Friends-of-Friends algorithm. Several graph-theory examples are presented throughout this paper, and include a new method to identify substructures within groups. The results and graph-theory methods have been thoroughly interrogated against previous 2MRS group catalogues and a Theoretical Astrophysical Observatory (TAO) mock by making use of cutting-edge visualization techniques including immersive facilities, a digital planetarium, and virtual reality. This has resulted in a stable and robust catalogue with on-sky positions and line-of-sight distances within 0.5 Mpc and 2 Mpc of the group respectively. It has recovered all major groups and clusters. The final catalogue consists of 3022 groups, resulting in the most complete “whole-sky” galaxy group catalogue to date. The 3D-positions of the groups are presented, as well as their luminosity and comoving distances, observed and corrected number of members, richness metric, velocity dispersion, and estimates of R200 and M200. Three additional catalogues are provided: 2MRS galaxies found in groups, a catalogue of subgroups, and the catalogue of 687 new group candidates which had no counterparts in previous 2MRS-based analyses.

The origins of Type Ia supernovae (SNe Ia) remain an unsolved problem of contemporary astrophysics. Decades of research indicate that these supernovae arise from thermonuclear runaway in the degenerate material of white dwarf stars; however, the mechanism of these explosions is unknown. Also, it is unclear what are the progenitors of these objects. These missing elements are vital components of the initial conditions of supernova explosions, and are essential to understanding these events. A requirement of any successful SN Ia model is that a sufficient portion of the white dwarf plasma must be brought under conditions conducive to explosive burning. Our aim is to identify the conditions required to trigger detonations in turbulent, carbon-rich degenerate plasma at low densities. We study this problem by modeling the hydrodynamic evolution of a turbulent region filled with a carbon/oxygen mixture at a density, temperature, and Mach number characteristic of conditions found in the 0.8+1.2 solar mass (CO0812) model discussed by Fenn et al. (2016). We probe the ignition conditions for different degrees of compressibility in turbulent driving. We assess the probability of successful detonations based on characteristics of the identified ignition kernels, using Eulerian and Lagrangian statistics of turbulent flow. We found that material with very short ignition times is abundant in the case that turbulence is driven compressively. This material forms contiguous structures that persist over many ignition time scales, and that we identify as prospective detonation kernels. Detailed analysis of the kernels revealed that their central regions are densely filled with material characterized by short ignition times and contain the minimum mass required for self-sustained detonations to form. It is conceivable that ignition kernels will be formed for lower compressibility in the turbulent driving. However, we found no detonation kernels in models driven 87.5 percent compressively. We indirectly confirmed the existence of the lower limit of the degree of compressibility of the turbulent drive for the formation of detonation kernels by analyzing simulation results of the He0609 model of Fenn et al. (2016), which produces a detonation in a helium-rich boundary layer. We found that the amount of energy in the compressible component of the kinetic energy in this model corresponds to about 96 percent compressibility in the turbulent drive. The fact that no detonation was found in the original CO0812 model for nominally the same problem conditions suggests that models with carbon-rich boundary layers may require higher resolution in order to adequately represent the mass distributions in terms of ignition times. / A Dissertation submitted to the Department of Scientific Computing in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2016. / November 22, 2016. / Includes bibliographical references. / Tomasz Plewa, Professor Directing Dissertation; Mark Sussman, University Representative; Gordon Erlebacher, Committee Member; Jorge Piekarewicz, Committee Member; Sachin Shanbhag, Committee Member.

The presence of extraplanar diffuse ionized gas (eDIG) and galactic winds and outflows in the late Universe has been observed to be ubiquitous in nearby star-forming spiral galaxies. Optical integral field spectroscopic data from the Wide Field Spectrograph (WiFeS) on the 2.3m Australian National University Telescope is used to determine if the nearby edge-on galaxy J1447-17 from the SINGG survey hosts eDIG, winds or outflows. Tests used to detect their existence include the presence of multiple components in the Hα signal, an increased velocity dispersion in the off-plane region relative to the disk, residuals in the velocity asymmetry map and an enhanced line-ratio ([N II]λ6583/Hα) corresponding to a decrease in the Hα intensity in the case of eDIG. The absence of profiles meeting the line-ratio condition and absence of multiple Hα components compounded with strong residuals in the velocity asymmetry map are not indicative of the galaxy possessing diffuse ionized gas. Ionization cone signatures in the velocity dispersion map together with the velocity asymmetry map and greater dispersion in the off-plane regions than in the disk indicate that the galaxy hosts a galactic wind.