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Item A multi-wavelength study of dwarf galaxies with active massive black holes(Montana State University - Bozeman, College of Letters & Science, 2023) Kimbrell, Seth Jordan; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.Dwarf galaxies which host massive black holes with M less than or equivalent to 106M circled dot give us an opportunity to better understand the formation mechanism behind the supermassive black holes that live in the center of galaxies. Studying how common massive black holes in dwarfs are is an important step in constraining the channels that led to those supermassive black holes. An important part of that study is understanding in what types of dwarf galaxies we can expect to find massive black holes. I present a multi-wavelength study of dwarf galaxies which attempts to find any trends in the morphologies of the hosts of active massive black holes. I begin by modeling the structures of a sample of galaxies which have been identified as black hole hosts; I then perform an identical modeling on a sample of galaxies which show no signs of hosting a massive black hole. I finish by describing an X-ray search for massive black holes among irregular/disturbed galaxies, including the discovery of a very bright X-ray source which is extremely likely to be a massive black hole in a dwarf-dwarf merger. This is one of the first active massive black holes discovered in such a late-stage merger, and it is also notable for radiating at nearly its upper limit.Item A deeper look into X-ray-selected AGN candidates in dwarf galaxies with Chandra(Montana State University - Bozeman, College of Letters & Science, 2023) Sanchez, Adonis Arismendy; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.The ability to accurately discern active massive black holes (BHs) in local dwarf galaxies is paramount to understanding the origins and processes of "seed" BHs in the early universe. We present Chandra X-ray Observatory observations of a sample of three dwarf galaxies (M* < or = 3 x 109 M circled dot, z < or = 0.15) pre-selected by Latimer et al. (2021a) as candidates for hosting active galactic nuclei (AGN). The galaxies were selected from the NASA-Sloan Atlas (NSA) with spatially coincident X-ray detections in the eROSITA Final Equatorial Depth Survey (eFEDS). Our new Chandra data reveal X-ray point sources in two of the three galaxies with luminosities of log(L 2-10keV/[erg s -1]) = 39.1 and 40.4. In the target galaxy with the nondetection, we calculate an upper limit on the luminosity for a potential source. We observed notably higher fluxes and luminosities from the two detected X-ray sources compared to their original eFEDS observations, pointing to possible X-ray variability on the scale of a few years. We plot and fit the spectra of the X-ray sources with a power-law model, finding the likely presence of intrinsic absorption. The X-ray luminosities are above that expected from XRBs, but we cannot definitively rule out stellar-mass compact objects with the data on hand. Assuming the X-ray sources are accreting massive BHs with masses that scale with the stellar mass of the host galaxies, the Eddington ratios are on the order of a few x 10 -3.Item Exploring the low-frequency gravitational-wave universe with pulsar timing arrays(Montana State University - Bozeman, College of Letters & Science, 2022) Becsy, Bence; Chairperson, Graduate Committee: Neil J. Cornish; This is a manuscript style paper that includes co-authored chapters.Pulsar timing arrays monitor millisecond pulsars to detect gravitational waves with nanohertz frequencies. They provide valuable information about various astophysical processes inaccessible to electromagnetic observations. In particular, they could shed light on unsolved problems related to the formation and evolution of supermassive black holes. We present several new methods which will help us fully realize the detection potential of pulsar timing arrays. We explore how the large collection of supermassive black hole binaries in the Universe can appear as a stochastic gravitational wave background, and how it might also result in a few individually detectable binaries. We describe a new method to efficiently search for such individual binaries, and also how we can detect multiple binaries in the presence of the confusion noise from the stochastic background. Finally, we introduce a new approach to search for generic gravitational-wave bursts, which enables us to hunt for unexpected new types of sources on the nanohertz gravitational-wave sky.Item Black hole scaling relations and feedback in dwarf galaxies(Montana State University - Bozeman, College of Letters & Science, 2022) Schutte, Zachary Willis; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.Over the past decade the discovery of a population of dwarf galaxies which host massive black holes (BH) has prompted the study of how these systems interact, grow and evolve. To address these questions I first present a new relationship between central black hole mass and host galaxy stellar bulge mass extending to the lowest BH masses known in dwarf galaxies (MBH < or ~ 10 5 M circle dot, M* ~ 10 9 M circle dot). I have obtained Hubble Space Telescope imaging of seven dwarf galaxies with optically selected broad-line active galactic nuclei (AGNs) and BH mass estimates from single-epoch spectroscopy. I perform modeling to decompose the structure of these galaxies and find that the majority have an inner bulge/pseudo-bulge component with an exponential disk that dominates the total stellar mass. Using the modeling results, I determine the stellar mass of each photometric component in each galaxy. I determine the M BH - M bulge relation using a total of 12 dwarf galaxies hosting broad-line AGNs, along with a comparison sample of 125 higher-mass galaxies. I find a strong correlation between BH mass and bulge mass which is in good agreement with correlations found previously when only considering higher-mass systems. Second, I investigated the role of AGN feedback in dwarf galaxies by studying Henize 2-10, a dwarf starburst galaxy previously reported to have a central massive black hole. At a distance of ~9 Mpc, it presents an opportunity to resolve the central region and determine if there is evidence for a black hole outflow impacting star formation. I found a ~150 pc long ionized filament connecting the region of the black hole with a site of recent star formation. Spectroscopy reveals a sinusoid-like position-velocity structure that is well described by a simple precessing bipolar outflow. I conclude that this black hole outflow triggered the star formation. The results from the work presented in this thesis show that the coevolution of massive BHs and their host galaxies extends into the lowest mass regime and that AGN feedback plays an important and complex role in dwarf galaxies.Item Searching for signatures of accreting massive black holes in dwarf galaxies(Montana State University - Bozeman, College of Letters & Science, 2022) Latimer, Lilikoi Jean; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.Identifying and analyzing massive black holes (BHs) in dwarf galaxies can advance our understanding of the formation and development of supermassive BHs. Searching for these objects can prove troublesome, however. Massive BHs, with masses of M BH < or ~ 10 6 M circled dot, are smaller than their supermassive cousins (M BH ? 10 6-10 9 M circled dot), and the dwarf galaxies they reside in can yield very different environments than what one might find in a more standard massive galaxy, resulting in a different set of challenges than searching out active galactic nuclei (AGN) powered by supermassive BHs. Here we describe several methods for both identifying AGN candidates powered by massive BHs and searching for evidence to confirm the presence or absence of AGN candidates in dwarf galaxies. For the latter, we use different combinations of radio, X-ray, and mid-IR observations to search for AGN signatures, elucidating the various benefits and difficulties that lie with each one, and reporting the results when applied to our varying samples. For the former, we analyze the efficacy of a mid-IR color-color AGN selection method when applied to dwarf galaxies instead of massive galaxies, and use X-ray observations from a newly-released catalog to search for new AGN candidates in dwarf galaxies.Item X-ray analysis and modeling of NGC 3227(Montana State University - Bozeman, College of Letters & Science, 2022) Newman, Jared James; Chairperson, Graduate Committee: Yves U. Idzerda and Sachiko Tsuruta (co-chair)The 1.5 Seyfert galaxy NGC 3227 has been observed by several X-ray missions. We carried out combined analysis of the data from a total of twenty one observations. Six of the observations were preformed by Suzaku, eight by XMM-Newton, and seven by NuStar. A unified model was constructed which is consistent with all twenty one of the observations by the three satellites with large intensity and spectral changes. The model consists of a hard power law which is interpreted as the Comptonized emission from the corona above an accretion disk. In the high flux states an additional soft excess component dominates, which is consistent with a model with either a steeper power law or a warm Comptonization component. These emissions from the central engine are absorbed by multiple distinct absorbers at various ionization levels. A reflection component and several emission lines are also present. This is the most robust model of this source to date.Item Numerical methods for rotating compact objects in modified gravity theories(Montana State University - Bozeman, College of Letters & Science, 2020) Sullivan, Andrew Patrick Kyung; Chairperson, Graduate Committee: Neil J. Cornish and Nicolas Yunes (co-chair); Nicolas Yunes was a co-author of the article, 'Slowly-rotating neutron stars in massive bigravity' in the journal 'Classical and quantum gravity' which is contained within this dissertation.; Nicolas Yunes, and Thomas Sotiriou were co-authors of the article, 'Numerical black hole solutions in modified gravity theories: spherical symmetry case' in the journal 'Physical review D' which is contained within this dissertation.; Nicolas Yunes, and Thomas Sotiriou were co-authors of the article, 'Numerical black hole solutions in modified gravity theories: axial symmetry case' submitted to the journal 'Physical review D' which is contained within this dissertation.Detailed observations of phenomena involving compact objects will provide us with a new avenue to test general relativity in the strong field regime. So as to not bias our analysis of these new experiments, we require knowledge of the spacetimes around these objects both within and beyond general relativity. Here I will describe work that applies two specific methods to solve the modified Einstein's equations that describe the exotic spacetimes beyond general relativity for neutron stars and black holes. The first method is a fourth-order Runge-Kutta-Fehlberg ordinary differential equation numerical integrator method. The second method is a relaxed Newton- Raphson method applied to a system of nonlinear partial differential equations. Using these methods, we solve for the spacetimes of slowly rotating neutron stars in massive bigravity and rotating black holes in scalar Gauss-Bonnet gravity in a theory independent methodology. We validate our numerical methods by applying them to compact objects in general relativity and using them to recover known perturbative solutions. We can then compare the fully nonlinear solutions to these perturbative solutions and comment on their differences. We then use these numerical solutions to calculate the physical observables of these systems and finally construct analytic fitted models that can be used in rapid computation methods that future experiments may use to constrain the free parameters in these theories.Item Study of MRK 110 using x-ray spectroscopy(Montana State University - Bozeman, College of Letters & Science, 2019) Liang, Xiao (Jennifer); Chairperson, Graduate Committee: Anne LohfinkJets are one of the most mysterious, and fascinating subjects in the study of relativistic outflow of the high energy particles produced by the Active Galactic Nuclei (AGN). AGNs are the center of active galaxies, where a very massive black hole is actively accreting matter into it. The relationship between the black hole mass and the luminosity is produced through accretion is known as the fundamental plane of black hole activities. The goal is to probe the characteristics of the black hole and its close vicinity by studying the X-ray produced by this inner region. We report the finding of high energy X-ray photons of a low radio luminous galaxy Mrk 110 from NuSTAR observations. We've found there are almost no variations in the X-ray flux during time scale observation, and there is a linear correlation between 3-5 keV and 5-10 keV energy band. We also find that the energy spectrum is best modeled by a cutoff power law and a red-shifted Gaussian modeling the fluorescence emission. Fluorescence emission is one of the signatures when X-rays are reflected by the disk. However the other significant reflection component, --X-rays signals reflected by the disk, is missing. We suspect the cause of the unexpected amount of high energy X-ray flux is due to jet emissions.Item The structure of energy-extracting black hole magnetospheres(Montana State University - Bozeman, College of Letters & Science, 2019) Thoelecke, Kevin; Chairperson, Graduate Committee: Yves U. IdzerdaSpinning black holes can store enormous amounts of rotational energy. Efficiently extracting that rotational energy can lead to significant energy outflows capable of powering very high energy astrophysical phenomena, such as gamma-ray bursts and active galactic nuclei. Black holes are unique in that they do not exist as physical objects in the same way a rock, planet, or star exists; instead, black holes exist only as spacetime curvature. As such processes for extracting a black hole's rotational energy are largely unique to black holes. This work explores one such process, the extraction of a black hole's rotational energy via an appropriately configured magnetosphere. Both analytic perturbation techniques and numerical codes are developed in order to solve for thousands of energy-extracting black hole magnetospheres. Those magnetospheres broadly sample the relevant solution space, allowing correlations to be drawn between different rates of black hole rotational energy and angular momentum extraction and global magnetosphere structure. The most fundamental behavior discovered is that magnetospheres that extract the most energy per unit angular momentum direct that energy away from the black hole's rotational axis, while magnetospheres that extract the least amount of energy per unit angular momentum direct that energy into jet-like structures aligned with the black hole's rotational axis. Exploration of the solutions obtained also suggests that magnetospheres most compatible with nearby accreting matter can very naturally launch jets, implying that black hole energy extraction and jet launching are likely to be concurrent and common features of astrophysical black hole magnetospheres.Item Testing alternative theories of gravity using low frequency gravitational waves(Montana State University - Bozeman, College of Letters & Science, 2019) O'Beirne, Logan Tyler; Chairperson, Graduate Committee: Neil J. Cornish; Bennett Link (co-chair); Logan O'Beirne, Stephen R. Taylor and Nicolas Yunes were co-authors of the article, 'Constraining alternative theories of gravity using pulsar timing arrays' in the journal 'Physical review letters' which is contained within this thesis.; Neil J. Cornish were co-authors of the article, 'Constraining the polarization content of gravitational waves with astrometry' in the journal 'Physical review D' which is contained within this thesis.; Neil J. Cornish, Sarah J. Vigeland and Stephen R. Taylor were co-authors of the article, 'Constraining alternative polarizations of continuous gravitational waves using pulsar timing arrays' submitted to the journal 'Physical review D' which is contained within this thesis.General Relativity aptly describes current gravitational observations. However, there is great theoretical interest in its validity in untested regimes. Alternative theories of gravity attempt to relax some of the assumptions made, leaving distinct signatures that are absent in Einstein's theory, namely the presence of alternative polarizations of gravitational waves that manifest from the emission of gravitational scalar and vector dipole radiation in black hole binaries. To study this lower order multipole of radiation, it is desirable to work in a regime where the quadrupolar tensor radiation of general relativity is as quiet as possible. This motivates working with supermassive black hole binaries in their slowly evolving inspiral phase, when they are well separated from merger, emitting low frequency gravitational waves. Using a frequentist framework, we study the detectability of a stochastic background of each polarization using pulsar timing arrays, which is currently the most technically developed and viable method for studying low frequency gravitational waves, correlating the observed time delays of pulsars. We also find that astrometry, which measures transverse displacements of the apparent position of stars, turns out to have a very similar correlation structure as the time delays measured by pulsar timing arrays. We lastly study how effective using a pulsar timing array is at studying a loud, foreground binary with these alternative polarizations, using a Bayesian framework. Low frequency gravitational wave astronomy proves advantageous for studying these exotic signatures.