Theses and Dissertations from UMD
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Item SUPPRESSION OF ELECTRON THERMAL CONDUCTION IN THE HIGH β INTRACLUSTER MEDIUM OF GALAXY CLUSTERS(2019) Roberg-Clark, Gareth; Drake, James F; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the thermodynamic state of the hot intracluster medium (ICM) in a galaxy cluster requires a knowledge of the plasma transport processes, especially thermal conduction. The basic physics of thermal conduction in plasmas with ICM-like conditions has yet to be elucidated, however. We use particle-in-cell simulations and analytic models to explore the dynamics of an ICM-like plasma (with small gyroradius, large mean-free-path, and strongly sub-dominant magnetic pressure) induced by the diffusive heat flux associated with thermal conduction. Linear theory reveals that whistler waves are driven unstable by electron heat flux, even when the heat flux is weak. The resonant interaction of electrons with these waves then plays a critical role in scattering electrons and suppressing the heat flux. In a 1D model where only whistler modes that propagate parallel to the magnetic field are captured, the only resonant electrons are moving in the opposite direction to the heat flux and the electron heat flux suppression is small. In 2D or more, oblique whistler modes also resonate with electrons moving in the direction of theheat flux. The overlap of resonances leads to effective symmetrization of the electron distribution function and a strong suppression of heat flux. The results suggest that thermal conduction in the ICM might be strongly suppressed. In a numerical model with continually supplied heat flux in the system, two thermal reservoirs at different temperatures drive an electron heat flux that destabilizes off-angle whistler-type modes. The whistlers grow to large amplitude, 𝛿B=B0, and resonantly scatter the electrons. A surprise is that the resulting steady state heat flux is largely independent of the thermal gradient. The rate of thermal conduction is instead controlled by the finite propagation speed of the whistlers, which act as mobile scattering centers that convect the thermal energy of the hot reservoir. The results are relevant to thermal transport in high β astrophysical plasmas such as hot accretion flows and the intracluster medium of galaxy clusters. When the plasma β is reduced in the numerical model, we find that a transition takes place between whistler-dominated (high-β) and double-layer-dominated (low-β) heat flux suppression. Whistlers saturate at small amplitude in the low β limit and are unable to effectively suppress the heat flux. Electrostatic double layers suppress the heat flux to a mostly constant factor of the free streaming value once this transition happens. The double layer physics is an example of ion-electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of β explored. The range of plasma-βs studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including not just the intracluster medium but hot accretion flows, stellar and accretion disk coronae, and the solar wind.Item The Origins and Ionization Mechanisms of Halpha Filaments in the Cool Cores of Galaxy Groups and Clusters(2011) McDonald, Michael Adam; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a survey of 10 galaxy groups and 23 galaxy clusters aimed at explaining the presence of warm, ionized filaments in the cool cores of galaxy clusters. By combining deep, high spatial resolution H-alpha, far-UV, and X-ray data from the Maryland Magellan Tunable Filter, Hubble Space Telescope, and Chandra X-ray Observatory, respectively, we have assembled the most complete picture of these mysterious filaments to date. This extensive database has allowed us to shed new light on two critical questions: i) Where does the cool gas in these filaments come from? ii) What process or processes are responsible for ionizing the cool filaments. As a pilot project, we obtained high-resolution H-alpha and far-UV data for Abell1795 and find that the previously-discovered H-alpha filament is, in fact, two very thin, intertwined filaments extending 50 kpc and with a width < 700 pc. The clumpy UV morphology and UV/H-alpha flux ratios of these filaments suggest that they may consist of chains of star-forming regions. Based on these data we conclude that the H-alpha emission is a result of photoionization by young stars and that the cool gas filaments are a byproduct of the intracluster medium cooling onto a fast-moving central galaxy. When we consider the full sample of 23 galaxy clusters, we find several strong correlations between the X-ray and H-alpha data. In general, complex, extended H-alpha filaments are found in clusters with cool, low-entropy cores. Furthermore, the morphology of the warm gas is correlated with the soft X-ray morphology and the filaments are found to occupy regions where, locally, the intracluster medium is cooling fastest. Finally, we find a strong correlation between the mass of of gas cooling below X-ray temperatures and the mass of gas in the warm filaments. These results provide strong evidence that the ionized filaments are a result of highly-asymmetric runaway cooling in the intracluster medium. By extending this sample to include 10 galaxy groups we are able to probe more than 2 orders of magnitude in mass and cooling rate. We find that there is only a weak correlation between the presence of ionized filaments and the total mass of the system. Instead, the presence of ionized filaments appears to depend almost entirely on the core properties, specifically whether there is a cool, low-entropy core. We find that groups are, in general, cooling more efficiently that clusters, due to their lower starting temperature. This can only be the case if cool-core groups are experiencing exclusively weak AGN feedback, which we show is the case. Finally, using new far-UV data from the Hubble Space Telescope, we find that 12/15 systems with H-alpha emission are consistent with being photoionized by young stars. The three remaining systems are under-luminous in the far-UV for their H-alpha luminosity, suggesting an alternative ionization source such as fast shocks or significant internal reddening. When we supplement this sample with UV data from GALEX and IR data from Spitzer we find a correlation between the star formation rate and the ICM cooling rate for 32 systems. The inferred efficiency of stars forming out of the cooling ICM is 14(+18/-8)%, which is consistent with the Universal fraction of baryons in stars. These results suggest that, for the majority of cases, young star formation provides sufficient UV flux to ionize the cool filaments.Item Hydrodynamic Models of AGN Feedback in Cooling Core Clusters(2008-05-20) Vernaleo, John C.; Reynolds, Christopher S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)X-ray observations show that the Intra Cluster Medium (ICM) in many galaxy clusters is cooling at a rapid rate, often to the point that it should have radiated away all of its energy in less than the age of the cluster. There is however a very clear lack of enough cool end products of this gas in the centers of the clusters. Energetic arguments indicate that Active Galactic Nuclei (AGN) should be capable of heating the inner regions of clusters enough to offset the radiative cooling; truncating massive galaxy formation and solving the cooling flow problem. We present three sets of high resolution, ideal hydrodynamic simulations with the ZEUS code to test this AGN heating paradigm. For the first set of simulations, we study the dependence of the interaction between the AGN jets and the ICM on the parameters of the jets themselves. We present a parameter survey of two-dimensional (axisymmetric) models of back-to-back jets injected into a cluster atmosphere. We follow the passive evolution of the resulting structures. These simulations fall into roughly two classes, cocoon-bounded and non-cocoon bounded. We find that the cocoon-bounded sources inject significantly more entropy into the core regions of the ICM atmosphere, even though the efficiency with which the energy is thermalized is independent of the morphological class. In all cases, a large fraction of the energy injected by the jet ends up as gravitational potential energy due to the expansion of the atmosphere. For the second set, we present three-dimensional simulations of jetted AGN that act in response to cooling-mediated accretion of an ICM atmosphere. We find that our models are incapable of producing a long term balance of heating and cooling; catastrophic cooling can be delayed by the jet action but inevitably takes hold. At the heart of the failure of these models is the formation of a low density channel through which the jet can freely flow, carrying its energy out of the cooling core. Finally, we present a set of simulations with both feedback and precessing jets. The addition of jet precession is not sufficient to couple the jets to the ICM energetically although it can deposit a large amount of energy in sound waves. These sound waves are lost to the system in ideal hydrodynamics, but ultimately may provide a powerful heating mechanism for clusters cores by AGN when additional physical effects are taken into account.Item Elemental Abundances via X-ray Observations of Galaxy Clusters and the InFOCuS Hard X-ray Telescope(2004-04-30) Baumgartner, Wayne; Mushotzky, Richard F; Leventhal, Marvin; AstronomyThe first part of this dissertation deals with the oxygen abundance of the Milky Way interstellar medium. Previous measurements had shown that oxygen in the ISM was depleted compared to its abundance in the sun. This dissertation presents new measurements of the ISM oxygen abundance taken in the X-ray band by observing the oxygen 0.6 keV photoionization K-edge in absorption towards 10 galaxy clusters. These measurements show that the ISM oxygen abundance is 0.9 solar, much greater than earlier depleted values. The oxygen abundance is found to be uniform across our 10 lines of sight, showing that it is not dependent on the depth of the hydrogen column. This implies that the galactic oxygen abundance does not depend on density, and that it is the same in dense clouds and in the more diffuse ISM. The next part of the dissertation measures elemental abundances in the galaxy clusters themselves. The abundances of the elements iron, silicon, sulfur, calcium, argon, and nickel are measured using the strong resonance K-shell emission lines in the X-ray band. Over 300 clusters from the ASCA archives are analyzed with a joint fitting procedure to improve the S/N ratio and provide the first average abundance results for clusters as a function of mass. The alpha elements silicon, sulfur, argon and calcium are not found to have similar abundances as expected from their supposed common origin. Also, no combination of SN Ia and SN II yields can account for the cluster abundance ratios, perhaps necessitating a contribution from a cosmologically early generation of massive population III stars. The last part of this dissertation details the development of the Cadmium Zinc Telluride (CZT) detectors on the InFOCuS hard X-ray telescope. InFOCuS is a balloon-borne imaging spectrometer that incorporates multi-layer coated grazing-incidence optics and CZT detectors. These detectors are well suited for hard X-ray astronomy because their large bandgap and high atomic number allow for efficient room temperature detection of photons in the 20-150 keV band. The InFOCuS CZT detectors achieve an energy resolution of 4.0 keV. A 2000 flight to measure the inflight background is discussed, as well as the results of a 2001 flight to observe Cyg X-1.