Physics

We are studying the population of active galaxies residing both in and out of groups along the Coma-Abell 1367 supercluster to look at the occurrence of Seyfert galaxies. We are also measuring the level of activity, as defined by active galactic nuclei (AGN) and star formation rates in the galaxy groups. Our goal is then to relate this information to determine any environmental correlation of these two features. We report on the distribution of Seyfert galaxies as a function of environment across the supercluster and probe the characteristics of the population of groups that currently host at least one Seyfert. 

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has the principal goal of detecting gravitational waves (GWs) in the nanohertz part of the spectrum using pulsar timing. This thesis presents results from radio campaigns at frequencies from 322 MHz to 10 GHz aimed at both multi-messenger constraints on GW sources and improving the timing sensitivity. The primary expected source of GWs at the nanohertz frequencies to which pulsar timing is sensitive are supermassive black hole (SMBH) binaries. We investigate a purported SMBH displaced from the galactic photocenter in NGC 3115. We explore the possibilities that the source is a SMBH binary or a post-merger recoiling SMBH. We place constraints on a possible SMBH companion using observations taken with the NRAO Very Large Array. If a companion SMBH can be confirmed, this system could be a future GW source detectable with pulsar timing. In order to detect such sources, our pulsar timing array must be as sensitive as possible, requiring the mitigation of all other astrophysical delays, including those from the interstellar medium (ISM). Using NANOGrav wideband multifrequency observations obtained with the Green Bank Telescope and Arecibo Observatory, we characterize frequency-dependent dispersion. This effect is quantified by the dispersion measure (DM). We analyze trends in the DM time series, propose sources of these trends, and identify timescales over which the DM varies beyond measurement errors and therefore can no longer be modeled as constant in timing. Analyzing DM variations aids in characterizing properties of the ISM and informs our timing observation strategy. Multi-telescope observations around the globe and at complementary frequencies can be used to more sensitively constrain DMs. We compare DMs measured with dual-frequency observations obtained using the Giant Metrewave Radio Telescope (GMRT) to those calculated in the NANOGrav 11-year data release to assess the possible precision of frequency-dependent noise measurements with the GMRT. We discuss the possibility of incorporating the GMRT into international pulsar timing efforts and anticipated challenges in future data combination. First, I would like to thank Maura McLaughlin for all of her guidance over the past five years, and for telling me time and time again that everything was going to be alright. I cannot imagine a better or more supportive advisor. I would also like to thank my committee members for their investment in my doctoral success: Jim Cordes for his seemingly infinite wisdom and willingness to share it, Sarah Burke-Spolaor for being so cool and sharing her data wizardry with me, and Kathryn Williamson for helping me build my confidence in front of a crowd and in life. I would like to thank my undergraduate advisor Eric Wilcots for giving me my start in astronomy research as a plucky teenager, and for helping me find my way both in undergrad and after. Thank you for always giving me good advice, and for guiding me when I could not see the forest through the trees. Thank you to my numerous colleagues in the NANOGrav collaboration. Among you are my friends, mentors, and role models; it takes a village, and you all are my village. Thank you also to my friends and fellow graduate students at WVU. Grad school would not have been surmountable without our camaraderie and beer therapy. Particular thanks to my best friend Tom, who is always up for long chats, late night proofreading, and who always makes me laugh. A huge thank you to Michael Lam for being my professional and emotional co-pilot, and for keeping me fueled with the best ramen ever while writing this thesis. Above all, I would like to thank my parents, Gail and Richard, for their unceasing, unwavering, and unconditional support. They taught me to see no limitations and that the stars were (literally) the limit. I would not be the person I am today without their care and guidance.

The physics of Space Elevators connecting the Earth with outer space has recently attracted increased attention, in part due to the discovery of ultra-strong materials such as carbon nanotubes and diamond nano-thread structures. In this article we review a new venue in space elevator physics: Rotating Space Elevators (RSE) [Golubovic, L. & Knudsen, S. (2009). Classical and statistical mechanics of celestial scale spinning strings: Rotating space elevators. Europhysics Letters 86(3), 34001.]. The RSE is a double rotating system of strings reaching outer space. Objects sliding along the RSE string (sliding climbers) do not require internal engines or propulsion to be transported far away from the Earth's surface. The RSE thus solves a major problem in the space elevator technology which is how to supply the energy to the climbers moving along the string. RSE strings exhibit interesting nonlinear dynamics and statistical physics phenomena. Satellites and spacecraft carried by slidi...

The physics of manmade celestial scale objects, such as Space Elevators connecting the Earth with outer space, has recently attracted increased attention of diverse researchers. In this article we review basic physics of celestial scale dumbbells such as the Analemma Tower suspended from an asteroid orbiting the Earth (Clouds, 2017). Celestial dumbbells involve two large masses (top and bottom) connected by strings. The two masses move geosynchronously with the Earth, with the bottom mass remaining close to the Earth and the top mass moving above the Earth’s geosynchronous satellite orbit. Appealing examples of celestial scale dumbbells are untied Rotating Space Elevators (RSE) (Knudsen…

We investigate a novel and unique dynamical system, the Rotating Space Elevator (RSE). The RSE is a multiply rotating system of strings reaching beyond the Earth geo-synchronous satellite orbit. Objects sliding along the RSE string ("climbers") do not require internal engines or propulsion to be transported far away from the Earth's surface. The RSE thus solves a major problem in the space elevator technology which is how to supply the energy to the climbers moving along the string. The RSE is a double rotating floppy string. The RSE can be made in various shapes that are stabilized by an approximate equilibrium between the gravitational and inertial forces acting in the double rotating frame. The RSE exhibits a variety of interesting dynamical phenomena studied in this thesis.

The discovery of ultra-strong materials such as carbon nanotubes and diamond nano-thread structures has recently motivated an enhanced interest for the physics of Space Elevators connecting the Earth with outer space. A new concept has recently emerged in space elevator physics: Rotating Space Elevators (RSE) [Golubović, L. & Knudsen, S. (2009). Classical and statistical mechanics of celestial scale spinning strings: Rotating space elevators. Europhysics Letters 86(3), 34001.]. Objects sliding along rotating RSE string (sliding climbers) do not require internal engines or propulsion to be transported from the Earth's surface into outer space. Here we address the physics of a special RSE family, Uniform Stress Rotating Space Elevators (USRSE), characterized by constant tensile stress along the string. From the point of view of materials science, this condition provides the best control of string’s global integrity. We introduce an advanced analytic approach to obtain the dynamic ...

Properly regularized second-order degenerate perturbation theory is applied to compute the contribution of higher Landau levels to the low-energy spectrum of interacting electrons in a disk-shaped quantum dot. At “filling factor” near 1/2, this contribution proves to be larger than energy differences between states with different spin polarizations. After checking convergence of the method in small systems, we show results for a 12-electron quantum dot, a system which is hardly tractable by means of exact diagonalization techniques.

The problem of computational structure prediction of materials is approached using the Firefly algorithm. Starting from the chemical composition and optionally using prior knowledge of similar structures, the Firefly method is able to predict not only known stable structures but also a variety of novel competitive metastable structures. This paper focuses on the strengths and limitations of the algorithm as a multimodal global searcher. The algorithm has been implemented in the software package Py-Chemia 1 , an open source python library for materials analysis. We present applications of the method to van der Waals clusters and crystal structures. The Firefly method is shown to be competitive when compared to other population-based global searchers.

The problem of computational structure prediction of materials is approached using the Firefly algorithm. Starting from the chemical composition and optionally using prior knowledge of similar structures, the Firefly method is able to predict not only known stable structures but also a variety of novel competitive metastable structures. This paper focuses on the strengths and limitations of the algorithm as a multimodal global searcher. The algorithm has been implemented in the software package Py-Chemia 1 , an open source python library for materials analysis. We present applications of the method to van der Waals clusters and crystal structures. The Firefly method is shown to be competitive when compared to other population-based global searchers.

Mixed metal oxides of earth-abundant 3d transition metals are an interesting class of materials that show interesting magnetic properties and a significant synergistic effect as catalysts for electrochemical oxygen evolution compared to simple unary oxides. However, the exact atomic-scale nature of such mixed oxide phases and the link to their interesting physico-chemical properties are poorly understood. Here, a combination of scanning tunneling microscopy and x-ray photoemission spectroscopy reveals that Fe species embed in a facile way into CoO bilayers on Au(111) resulting in an Fe doped oxide. Density functional theory and the spectroscopic fingerprint from x-ray photoemission spectroscopy reveal that the Fe dopants in the cobalt oxide matrix assume a higher oxidation state than in the structurally corresponding unary bilayer oxide. Furthermore, the substituted Fe is structurally displaced further away from the Au than the metal in either of the corresponding pure unary oxides. Both O and to a smaller extent Co in the nearest coordination shell are also structurally and electronically perturbed. The interesting effects observed in the bilayer binary oxides may enable a better fundamental understanding of the nature of doping of metal oxides, in general, and promotion effects in catalytic applications.

The photophysics of interstellar ices and condensed molecules adsorbed on grains is of primary importance for studies on the origin of the specific handedness of complex organic molecules delivered to the early Earth and of the homochirality of the building blocks of life. Here, we present quantum mechanical calculations based on time-dependent density functional theory for the absorption and circular dichroism (CD) of isovaline and its chiral precursor 5-ethyl-5-methylhydantoin, both observed in meteoritic findings. The systems are considered in their geometrical conformation as extracted from a full solid (icy) matrix, as a shortcut to understand the behaviour of molecules with fixed orientation, and/or taking into account the full solid matrix. In the context of a possible 'condensation-warming plus hydrolysis-recondensation' process, we obtain that: (i) for low-energy excitations, the 'condensed' precursor has a stronger CD with respect to the amino acid, suggesting that the handedness of the latter could be biased by asymmetric photolysis of the precursor in cold environments; (ii) enantiomeric excess could in principle be induced more efficiently in both systems for excitation at higher energies (VUV). X-ray absorption near-edge spectroscopy and related CD results could serve as support for future experiments on ionization channels.

Properly regularized second-order degenerate perturbation theory is applied to compute the contribution of higher Landau levels to the low-energy spectrum of interacting electrons in a disk-shaped quantum dot. At "filling factor" near 1/2, this contribution proves to be larger than energy differences between states with different spin polarizations. After checking convergence of the method in small systems, we show results for a 12-electron quantum dot, a system which is hardly tractable by means of exact diagonalization techniques.