Astronomy Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2746
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Item THE FORMATION OF METAL-FREE POPULATION III STARS IN X-RAY AND LYMAN-WERNER RADIATION BACKGROUNDS(2024) Park, Jongwon; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal-free Population III (or Pop III) stars are instrumental in shaping the early universe, influencing the formation of the first galaxies. The formation of Pop III stars depends on the fraction of molecular hydrogen (H2), which is regulated by X-ray and Lyman-Werner (LW) radiation backgrounds. Therefore, gaining insight into the impact of these radiation backgrounds is essential for unraveling the mysteries surrounding Pop III star formation and their impacts on the first galaxies. In this dissertation, I investigate the interaction between X-ray/LW backgrounds and the formation of Pop III stars. To conduct this investigation, I employed the radiative hydrodynamics code RAMSES - RT. I implemented various physical processes governing Pop III star formation, such as primordial chemistry, radiation background, secondary ionization/heating, and self-shielding. Performing a grid of simulations covering a large parameter space of X-ray/LW intensity, I systematically explored the effects of radiation backgrounds on Pop III stars. I found that a moderate X-ray background boosts the H2 fraction in dark matter halos, facilitating Pop III star formation in low-mass halos. In contrast, a LW background dissociates H2 and prevents star formation in low-mass halos. This result suggests that the number of Pop III supernovae detected by the JWST is enhanced by an X-ray background. Furthermore, I discovered that an X-ray background reduces the characteristic mass and multiplicity of Pop III stars. This leads to a top-heavier initial mass function and may have a potential impact on galaxy formation. Moreover, I made further improvements to the simulations by incorporating radiative feedback from Pop III protostars. This study confirmed previous works that radiation from protostars suppresses their growth, thereby playing a significant role in determining the mass of Pop III stars theoretically. I also found that hierarchical binaries (binaries of binaries), eccentric orbits, and outward migration are common occurrences in Pop III star formation. Eccentric orbits induce variability of Pop III protostars and this is observable by the JWST when light is magnified through gravitational lensing. In a follow-up study, I investigated the origin of outward migration and found that the gas disks around the protostars accrete gas with high angular momentum and transfer the angular momentum to the binary stars through torques. This finding paves the wayfor studies of migration behaviors across different stellar populations. Finally, I explored the X-ray effects on the number of Pop III stars using cosmological simulations. Developing methods to calculate the intensity of the radiation background on the fly and realistically accounting for the X-ray feedback loop, I found that a weak X-ray background develops and this background ionizes the intergalactic medium, thereby moderately increasing the number density of Pop III stars (by a factor of ∼ 2). This rise in the number of Pop III stars due to X-ray radiation lowers the star formation rate of metal-enriched Pop II stars, highlighting the significance of the X-ray background in galaxy formation.This thesis covers various aspects of Pop III star formation and the effect of X-ray radiation backgrounds which has been overlooked by previous studies. It lays a foundation for future research aimed at connecting the theoretical understanding of Pop III star formation and observations targeting Pop III stars and the first galaxies.Item Probing the Central Regions of Active Galactic Nuclei(2014) Lohfink, Anne; Reynolds, Christopher S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Active Galactic Nuclei (AGN) are one of the key players in the Universe. Their energy output can strongly affect the growth of their host galaxy and can promote or suppress star formation on galactic scales. Most of the processes that determine the power of an AGN as well as the form in which that power is released take place in the immediate surroundings of its supermassive black hole, a region that is still not entirely understood. A comprehension of these inner regions is, however, crucial to any ultimate understanding of the AGN's vast influence. This dissertation explores these close-in environments of the black hole using two approaches: X-ray spectroscopy and variability studies. We begin by summarizing our current understanding of why AGN play such a significant role in galaxy formation. This is followed by a discussion of why X-ray spectroscopy is one of the best means to investigate them. We point out that, in particular, the X-ray reflection spectrum is interesting as it can directly probe parameters such as the black hole spin or the inclination of the accretion disk. Since the reflection spectrum is a broad band component, that usually only contributes a fraction of the total observed X-ray flux, the entire X-ray spectrum requires careful modeling. To perform such modeling and gain access to the parameters of the reflection spectrum, we first select a target in which the spectral decomposition is simplified by the absence of absorption - the Seyfert 1 galaxy Fairall 9. We apply a multi-epoch fitting method that uses more than one spectrum at a time to get the best possible results on the parameters of the reflection spectrum that are invariant on human timescales. This technique enables us to tightly constrain the reflection parameters and leads us to conclude that Fairall 9 most likely possesses a composite soft X-ray excess, consisting of blurred reflection and a separate component such as Comptonization. The reflection spectrum also provides a way to enhance our knowledge of jet formation. We present a multi-wavelength study of the broad line radio galaxy 3C120 centered around a study of the reflection spectrum from two Suzaku and one XMM observation. Our results confirm that jet formation is linked to changes in (and possibly the destruction of) the inner accretion disk, and the high measured spin suggests that the rotational energy could very well be the energy source required to launch the jet. Finally, we present results from variability studies, which present another window into the processes taking place close to the black hole. A 10 year RXTE monitoring of Fairall 9 allows us to discover very rapid flux dips in the X-ray band which only last 5-15 days. While we are unable to determine the exact nature of the dips, we discuss a range of possible models, including the idea that the accretion disk in this radio-quiet AGN may be undergoing sporadic disruptions (via some yet-to-be-determined global instability) in much the same manner as is inferred to occur in 3C120 and other broad-line galaxies. Lastly we turn to the UV variability of Fairall~9 and its connection to the X-ray variability. From 2.5 months of Swift monitoring, we find that Fairall~9 shows significant variability on 4 day timescales, and the analysis of XMM-OM data shows that variability is present even on the time scales of hours. Folding in the X-ray variability, we determined that this fast UV variability can be explained as reprocessing of X-rays. We conclude by explaining how these studies fit into the field of AGN science as a whole and how they can be followed up with future observations.Item A Spectral Survey of Black Hole Spin in Active Galactic Nuclei(2007-09-20) Brenneman, Laura; Reynolds, Christopher S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation explores the question of whether broad iron lines from the accretion disk can be used as viable diagnostic tools for constraining black hole spin. We begin by giving an overview of the importance of black hole angular momentum as a signature of General Relativity and as a means of testing this theory in the strong-field limit. We discuss the anatomy of the typical black hole/accretion disk system, focusing on the complex environments of active galactic nuclei, and in particular Seyfert-1 systems which we pursue in this work. After developing a robust technique for fitting the continuum and absorption parameters through a rigorous analysis of the XMM-Newton spectrum of the Sy-1 galaxy NGC 4593, we then discuss a new model we have developed that fits broad emission lines from the inner accretion disk. This model, kerrdisk, is fully relativistic and allows the black hole spin to be a free parameter in the fit. Using this model, we carefully analyze the 350 ks XMM-Newton spectrum of the Sy-1 source MCG--6-30-15, which has the broadest and best-studied iron line observed to date. Fitting for the black hole spin in this source, we conclude that a > 0.987 to 90% confidence. We then extend our source list to analyze the XMM-Newton spectra of nine other radio-quiet Sy-1 AGN that have previously been observed to harbor broad iron lines. We find that, given enough photons and a broad line indicative of an origin in the inner disk where relativistic effects are important, our new model enables us to place robust constraints on black hole spin. Four of our sampled AGN meet the criteria necessary to constrain spin. Those constraints are given, along with the full spectral fit to each source. Interestingly, the spins of these sources range from moderate (a ~ 0.5−0.7) to very high (a > 0.95), and we do not find any AGN consistent with non-rotating black holes. For those objects that had marginal spin constraints or none at all, we discuss the spectral fits and the probable reasons for the lack of robustness of our results. This is the first ever survey of black hole spin in type-1 AGN.