<p> Gamma-ray astronomy is the study of very energetic photons, from <i> E</i> = <i>m_ec</i>² &ap;0.5&times;10⁶ eV to > &ge;10²⁰eV. Due to the large span of the energy range, the field focuses on non-thermal processes that include the acceleration and propagation of relativistic particles, which can be found in extreme environments such as pulsars, supernova remnants, molecular clouds, black holes, etc.</p><p> The High Altitude Water Cherenkov (HAWC) observatory is an instrument designed for the study of gamma rays in the energy range of &sim;100 GeV to 100 TeV. Using data from the HAWC observatory, a study for the search of very high energy gamma rays in the northern <i>Fermi</i> Bubble region was made. The <i>Fermi</i> Bubbles are large extended regions in the gamma-ray sky located above and below the galactic plane that present a hard emission between 1 GeV and 100 GeV. No significant excess is found an upper bounds at 95% C.L. are obtained. The implications of this result are that certain processes explaining the Fermi Bubble formation from the center of our galaxy are excluded. I will discuss and compare the scenarios that still present a possible hypothesis of the Fermi Bubble origin.</p>

<p>Hierarchical structure formation in the standard &Lambda;+cold dark matter (CDM) model produces gravitationally bound clumpy halos with abundant substructure. These subhalos are the remnants of dark matter halos that have been accreted by their host halo over cosmic time, and have survived tidal destruction. Understanding halo substructure is extremely important, as subhalos are believed to host satellite galaxies, boost the dark matter annihilation signal, cause tidal heating of fragile structures such as stellar streams and disks, and are potentially responsible for interesting phenomena in gravitational lensing. Most importantly, the demographics of subhalos contain information of the Universe, thus providing a stringent testbed for the cosmological model.</p><p> This thesis provides a comprehensive study of dark matter subhalos, using a combination of cosmological <i>N</i>-body simulations and semi-analytic modeling. We start with developing a new, semi-analytic model describing halo assembly and subhalo evolution. The model combines Monte-Carlo techniques of generating halo merging histories and simple analytical descriptions for the evolution of subhalos, thus offering extremely fast computation, the agility to experiment with different cosmologies, and the control of specific physical processes. The model accurately predicts the distributions of subhalo mass and structural parameters in cosmological simulations, and outperforms simulations in terms of mass resolution and statistical power. Taking advantage of the speed and agility of the model, we present universal fitting formulae for subhalo mass and maximum circular velocity (^&ngr;max) functions that are valid for a broad range in host halo mass, redshift and CDM cosmology. </p><p> The remainder of the dissertation makes use of the model, together with a number of state-of-the-art <i>N</i>-body simulations, to study the statistics of halo substructure. Recent high-resolution CDM simulations reveal ~10 massive Galactic subhalos whose central potential wells are too deep to be consistent with those of the ~10 brightest Milky-Way (MW) satellite galaxies. This inconsistency, dubbed the `too-big-to-fail' problem (TBTF), has become a persistent challenge to the standard ACDM cosmology. However, the number of well resolved Galactic halos in simulations is too small to fully capture the halo-to-halo variance in substructure content, which hinders the interpretation of the inconsistency. Unleashing the power of the semi-analytic model, we generate thousands of MW-size halos with well-resolved subhalo populations, and explicitly demonstrate that a reliable assessment of TBTF requires such large samples. We argue that existing statistics used to address TBTF suffer from the look-elsewhere effect and/or disregard certain aspects of the data on the MW satellite population. We devise a new statistic that is not hampered by these shortcomings, and, using data of the MW satellites with vmax > 15 km s<sup>-1</sup>, demonstrate that 1.4<sup>+3.3</sup>-1.1% of MW-size host halos have a subhalo population in statistical agreement with that of the MW. We also discuss how the severity of TBTF depends on halo mass and cosmology.</p><p> We conclude the thesis with a study of unprecedented statistical power regarding the halo-to-halo variance of substructure. First, we study the mass fraction (<i>f</i><sub>sub</sub>) in subhalos as a function of host halo mass, formation redshift, and halo-centric distance. We note that recent measurements of <i>f</i><sub>sub</sub> from gravitational lensing are much higher than the average but within the 90th percentile of the <i> f</i><sub>sub</sub> distribution. Second, we quantify the deviation of the occupation statistics of subhalos from Poissonian statistics, which is widely assumed in halo occupation distribution (HOD) models. In particular, we clearly reveal the sub-Poissonian statistics at [special characters omitted] &le; 3, aside from the already-known super-Poissonity at [special characters omitted] &raquo; 1, with [special characters omitted] the average number of subhalos. we also quantify the effect of the sub-Poissonity on the galaxy clustering predictions from HOD models. We further show that the extent of nonPoissonity depends on subhalo selection and on halo formation time - selecting subhalos by their mass or vmax at accretion yields weaker super-Poissonity for large [special characters omitted] but stronger sub-Poissonity for small [special characters omitted], compared to selecting by their present-day mass or vmax; earlier-formed halos exhibit less non-Poissonity than later-formed ones. Finally, we use the occupation statistics of the most massive satellites of the MW to put constraint on the mass and formation redshift of the MW halo. In particular, the `^&ngr;_max gap' of MW satellites between ~ 30 km s^-1 and 60 km s^-1 favors a low-mass, late-formed MW halo, with 0.25 &lt; <i>M</i>_vir/10¹² <i> h</i>^-1M[special characters omitted] &lt; 1.4 and 0.1 &lt; <i>z</i>_f &lt; 1.4 at 90% confidence.</p>

Here, we utilize Quantum Chemistry (QC) approaches to predict the structures, vibrational frequencies, infrared intensities and Raman activities of unusual molecular species using the General Atomic and Molecular Structure System (GAMESS(US)) package. A Python-based software, AutoGAMESS, was developed to automate the workflow and take advantage of High Throughput Computing (HTC) techniques enabling the automated generation of spectroscopic data from hundreds of calculations. This approach was utilized to determine these properties for a series of carbon oxides (C2On; n = 3 to 4), anticipated to be produced during the radiation of pure carbon dioxide ices, under conditions relevant to the interstellar medium. Beyond generating predicted spectroscopic results, we additionally performed a benchmark study of 70 different basis sets across multiple levels of theory (including Density Functional Theory, Moller–Plesset, and Coupled Cluster calculations), in QC to identify the method with the best balance between obtaining the lowest error in predictions while being mindful of the computation resources required.

Since its arrival to Saturn in 2004, the Cassini spacecraft has utilized its suite of sophisticated instruments to further our understanding of the Saturnian ring system. We analyze occultation data from Cassini's Ultraviolet Imaging Spectrograph (UVIS) in order to measure the particle size distribution and place limits on the minimum particle sizes in Saturn's rings. Throughout the ring system, particle accretion is countered by collisional and tidal disruption and Keplerian shear. Therefore, the particle size distribution of the rings is continually evolving. The presence of sub-centimeter particles, which have short lifetimes due to these processes, is indicative of ongoing dynamics in the rings. Sub-centimeter-sized particles efficiently diffract light at ultraviolet wavelengths, and thus produce signatures of diffraction in the occultation data. The shape and intensity of the diffraction signatures are indicative of the sizes of the particles that produce them. The UVIS wavelength bandpass, 51.2 - 180 nm, contains the shortest wavelengths of the Cassini instruments, making it most sensitive to the smallest particles in the rings. We have developed a computational model that reconstructs the geometry of a UVIS observation and produces a synthetic diffraction signal for a given truncated power-law particle size distribution, which we compare with the observed signal. We implement this model for two sets of observations: (1) diffraction spikes at sharp ring edges during stellar occultations and (2) the light curve due to attenuated and diffracted sunlight by particles in Saturn's F ring during solar occultations. Near sharp ring edges, ring particles can diffract light such that there is a measurable increase in the signal of an unocculted star exterior to the ring. In Saturn's A ring, diffracted light can augment the stellar signal by up to 6% and can be detected tens of kilometers radially beyond the edge. The radial profile of the diffraction signal is dependent on the size distribution of the particle population near the ring edge. These diffraction signals are observed at sharp edges throughout Saturn's rings, although in this work we focus on diffraction at the outer edge of Saturn's A ring and at the edges of the Encke Gap. We find an overall steepening of the power-law size distribution and a decrease in the minimum particle size at the outer edge of the A ring when compared with the Encke Gap edges. This suggests that interparticle collisions caused by satellite perturbations in the region result in more shedding of regolith or fragmentation of particles in the outermost parts of the A ring. We rule out any significant population of sub-millimeter-sized particles in Saturn's A ring, placing a lower limitation of 1-mm on the minimum particle size in the ring. We also model the light curves produced as Saturn's F ring occults the Sun. We consider both the attenuated signal and the light diffracted by the particles in the ring during the occultation. Five of the eleven solar occultations analyzed show a clear signature of diffracted light that surpasses the unocculted solar signal. This includes a misaligned solar occultation that placed most of the solar disk outside of the instrument's field of view, reducing the solar signal by 97.5% and resulting in the serendipitous detection of diffracted light. We measure a large variation in the the size distribution of the particles that fill the broad, ~500 km region surrounding the F ring core. We find that smaller particles ( < 50 micrometers) are present during solar occultations for which diffraction was detected, and place a lower limit on the minimum particle size of 100 micrometers for occultations during which diffraction was not detected. A comparison with images of the F ring observed by the Cassini Imaging Science Subsystem near the times of the occultations reveals that the detections of small particles in the UVIS data correspond with locations of collisional events in the F ring. This implies that collisions within the F ring core replenish the sub-millimeter-sized dust in the 500-km region that encompasses the F ring core.

The seeds of planetesimals that formed in the gaseous protoplanetary disk (PPD) have many barriers to overcome in their growth from millimeter to meter-sized and larger bodies. Centimeter-sized aggregates are weakly bound and self-gravity is almost non-existent so surface forces play a critical role in holding small loosely-bound rubble-piles together. Their orbital motions and effects form disk processes impart relative velocities leading to collisions so understanding the macroscopic disk environment is also necessary. To this end we analyze the dynamics of particles in Saturn's F ring as an analogue to understanding the orbital evolution of proto-planetesimals embedded in a PPD. We also study how the mechanical, material, and collisional properties affect the dynamical accretion of cm-sized bodies. The collisional outcomes can be determined by a set of definable collision parameters, and experimental constraints on these parameters will improve formation models for planetesimals. We have carried out a series of microgravity laboratory collision experiments of small aggregates to determine under what conditions collisional growth can occur for protoplanetary aggregates. We measure coefficients of restitution, sticking and fragmentation thresholds, compressive strengths, and sticking probabilities for collision velocities of 1 - 200 cm/s, then compare the results of our experiments with results from a collisional N-body code that includes adhesion between particles. We find that cm-sized aggregates are very weakly bound and require high internal cohesion to avoid fragmentation in agreement with simulations. The threshold for sticking is found to be under 10 cm/s and the fragmentation threshold near 1 m/s. Quiescent regions in the mid-plane of the disk may cultivate abnormally low relative velocities permitting sticking to occur (~1 cm/s), however, without a well-defined path to formation it is difficult to determine whether collisional accretion as a mechanism can overcome low thresholds for sticking and fragmentation. We discuss this research's implications to both the meter-barrier and planetesimal formation.

Adhesive forces play a significant role on airless bodies due to their weak gravities. Investigating adhesion at the surface of asteroids and their constituent components is vital to understanding their formation and evolution. Previous research has been done to understand the interaction of micron-sized spheres to planar surfaces and sphere-to-sphere interactions, which have been used to develop models of asteroid surfaces. Our investigation experimentally investigates adhesion through atomic force microscopy (AFM) measurements between JSC-1 simulant particles and several AFM tips, including a typical pyramidal gold tip and microspheres of sizes 2 μm and 15 μm. The samples of JSC-1 consist of three size ranges: < 45 μm, 75-125 μm, and 125-250 μm. For each sample we looked at the magnitude and distribution of the measured adhesive forces. Results show that the pyramidal tip produced larger forces than the spherical tips generally, and the sample that produced larger forces and a larger distribution of those force was the smaller, more powder-like sample with sizes < 45 μm.

<p> The heliosphere is a &ldquo;comet-like&rdquo; bubble of plasma reaching from &sim;10² to over 10³ astronomical units in size. It is created by the outflow of solar wind (SW) plasma and its interaction with the partially-ionized local interstellar medium (LISM). Due to its large size, it is unfeasible to take <i>in situ</i> measurements at the edges of this interaction. Therefore it is necessary to develop sensing techniques to remotely probe the heliosphere and its boundaries. </p><p> The NASA-funded <i>Interstellar Boundary EXplorer</i> (<i>IBEX</i>) mission is aimed at improving our understanding of the heliospheric interface. Launched in 2008 October, <i>IBEX</i> measures fluxes of energetic neutral atoms (ENAs) that are created through the SW&ndash;LISM interaction, as well as interstellar neutral atoms that permeate the heliospheric boundary. Out of the neutral atom species that <i> IBEX</i> can detect, hydrogen (H) atoms are the most abundant in interstellar space and the heliosphere. Hydrogen ENAs, in particular, are created when relatively energetic protons from the heliospheric plasma charge-exchange with interstellar H atoms. Due to their high energies, and thus large mean free paths, H ENAs can propagate large distances before ionizing (i.e., on the order of the size of the heliosphere), and can be detected by <i> IBEX.</i> </p><p> The purpose of this study is to simulate H ENA flux measurements at 1 AU and relate these to the <i>IBEX</i> mission. Three goals of this study that are of particular interest to <i>IBEX</i> are: (1) to simulate H ENA fluxes measured in the solar (inertial) and <i>IBEX</i> spacecraft frames of reference in order to better understand <i>IBEX </i> measurements made in different frames of reference; (2) to study the effects of pickup ions, i.e., non-thermalized ions, on H ENA fluxes, and determine how <i>IBEX</i> observations can reveal the properties of PUIs in the distant heliosphere; (3) to analyze the effects of a time-dependent solar cycle on <i>IBEX</i> H ENA measurements, particularly the &ldquo;ribbon&rdquo; of enhanced flux encircling the sky. The simulations are performed by post-processing a pre-simulated, &ldquo;background&rdquo; heliosphere containing plasma and neutral H properties (e.g., density, temperature, velocity) produced from a three-dimensional magnetohydrodynamic/kinetic simulation of the SW&ndash;LISM interaction.</p>

For a plasma in the collisionless regime, test-particle modelling can lend some insight into the macroscopic behavior of the plasma, e.g conductivity and heating. A common example for which this technique is used is a system with electric and magnetic fields given by B = {dollar}\delta y{dollar}cx x + xcx y + {dollar}\gamma{dollar}cx z and E = {dollar}\epsilon{dollar}cx z, where {dollar}\delta{dollar}, {dollar}\gamma{dollar}, and {dollar}\epsilon{dollar} are constant parameters. This model can be used to model plasma behavior near neutral lines, ({dollar}\gamma{dollar} = 0), as well as current sheets ({dollar}\gamma{dollar} = 0, {dollar}\delta{dollar} = 0). The integrability properties of the particle motion in such fields might affect the plasma's macroscopic behavior, and we have asked the question "For what values of {dollar}\delta{dollar}, {dollar}\gamma{dollar}, and {dollar}\epsilon{dollar} is the system integrable?" to answer this question, we have employed Painleve singularity analysis, which is an examination of the singularity properties of a test particle's equations of motion in the complex time plane. This analysis has identified two field geometries for which the system's particle dynamics are integrable in terms of the second Painleve transcendent: the circular O-line case and the case of the neutral sheet configuration. These geometries yield particle dynamics that are integrable in the Liouville sense (i.e. there exist the proper number of integrals in involution) in an extended phase space which includes the time as a canonical coordinate, and this property is also true for nonzero {dollar}\gamma{dollar}. The singularity property tests also identified a large, dense set of X-line and O-line field geometries that yield dynamics that may possess the weak Painleve property. In the case of the X-line geometries, this result shows little relevance to the physical nature of the system, but the existence of a dense set of elliptical O-line geometries with this property may be related to the fact that for {dollar}\epsilon{dollar} positive, one can construct asymptotic solutions in the limit {dollar}t \to \infty{dollar}.

Previous density functional theory (DFT) calculations of hyperfine coupling constants (HFCC) on single nucleic acid base radicals agree well with the EPR/ENDOR experiments’ values on radiation induced nucleic acid constituents radicals, except for four problem cases,1 namely the N1-deprotonated cytosine cation radical, the native guanine cation radical, the N3-deprotonated 5’-dCMP cation radical and the N7-H, O6-H protonated 5’-GMP anion. The main effort of the present work is to address these four discrepancies by using the highly parameterized density functional M05/6-2X and by including the crystalline environment’s H-bonding effects in the calculations. The geometries of the four model radicals are optimized within their single crystal environment using ONIOM technique. Then the spin density distributions and HFCCs of the radicals are examined within various scales of cluster models. The results obtained by including H-bonding environment are in strong agreement with the experimental values. The calculations show advantages of using the M05/62X functional rather than the B3LYP functional in obtaining more satisfactory HFCC results. However, the delocalization errors are encountered with both M05/6-2X and B3LYP functionals. Further development in eliminating delocalization errors in practical DFT approximations is suggested.

A group of prospective drugs with the aromatic di-N-oxide (ANO) functionality as the common feature are currently undergoing testing for the ability to selectively target tumors surrounded by normal tissues. It has been long recognized that the mechanism of biological activity of these drugs involves DNA damage by free radical species generated through one-electron reduction, although the exact nature of the reactive intermediate responsible for DNA damage remains uncertain. It is believed, however, that one of the key factors defining, in particular, hypoxic selectivity of these drugs is the rate of N-O bond scission in the one-electron reduced intermediate. In this study we have made an attempt to verify whether the predictions made in the literature regarding the N-O bond dissociation rate in a related class of derivatives are applicable to the same process in ANO. For that purpose both theoretical (electronic structure calculations) and experimental (Electron Spin Resonance spectroscopy) have been employed. While our results are not conclusive, they have laid the groundwork for future studies.