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Probing Magnetic Fields in Clusters of Galaxies

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Probing Magnetic Fields in Clusters of Galaxies Probing Magnetic Fields in Clusters of Galaxies 'Iiacy E. Clarke A Thesis submitted in codormity with the requirements for the degree of Doctor of Philosophy Graduate Department of Astronomy University of Toronto @ Copyright by Ztaq Clarke 1999 National Library Bibiiiothège naîiode 14 ofcanada du Cana uisitiis and Acquisitions et "fZS6 bgnphic &N~S senrices bibliographiques The author has granted a non- L'auteur a accordé une Licence non exclusive licence allowing the exclusive permettant à la National L&my of Canada to Bibliothèque nationale du Canada de teproduce, loan, disûi'bute or seii reproduire, prêter, distn'buer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/nlm, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conSave la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fkom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son autorisation. O81 2-50043-8 Probing Magnetic Fields in Clusters of Galaxies nacy Clarke Doctor of Phihophy, 1999 Department of Astronomy, University of Toronto ABSTRACT Gdaxy clusters represent the largest gravitationdy bound objects in the universe. De- tailed morphological studies of galaxy clusters indicate that they are relatively young ob- jects, current ly in the formation process. Observations toward these objects reveal that the major components of the clusters are the galaxies, the diffuse, thermal cluster gas, and the dark matter. There is, however, evidence that the intracluster medium aisa contains magnetic fields distributed through the intracluster gas. The presence of magnetic fields within the intracluster medium can lead to significant effects on the dynamics, and energy transport throughout the cluster. Embedded magnetic fields can modify the pressure of the intraciuster medium, suppress thermal conduction, which may lead to the omet of cooling instabilities, and magnetic fields may even bias the initial mass function of star formation. Therefore, in order to understand the overdl picture of the evolution of galaxy clusters, it is necessary to understand the role magnetic fields have played in this evolution. The most direct method of pmbing intrac1uster magnetic fields relies on the Faraday rotation effect. As luiearly polarized radiation passes through a magneto-ionic region, the rotation of the plane of polarization dlsplays a characteristic wavdength dependence. This thesis combines radio and X-ray observations, in a statistical study of Faraday rotation toward a sample of nearby galaxy dusters. A cornparison of the Faraday rotation - - measures of sources viewed through the intracluster medium, to the rotation measmes of sources falling beyond the edge of the X-ray emitting gas, reveals the presence of excess Faraday rotation in the former sampie. This excess rotation is interpreted as originating in the diffuse intracluster medium. A detailed investigation of the Faraday rotating medium indicates that the Faraday screen is well represented by a mode1 of Faraday cells distributecl through the intracluster medium. An analysis of the clus ter impact parame ter distribution over the rotation measure sample indicates that the Faraday ce11 scales vary boom < 1 kpc in the central regions of the clusters, to over 100 kpc at large impact parameter. Further, the rotation measure distribut ion indicates the widespread existence of intracluster magnet ic fields out to the edges of the X-ray emitting gas. The overd distribution of magnetic field strengths interred nom the data suggests that the fields may play a dynamically important role in the evolutioa of galaxy clusters. Acknowledgements It seems only appropriate that thanks go £ktto my supenrisor, Prof. Phiiipp Kronberg, who provided me with an enthusiasm for, and understanding 9 the relemce of magnetic fields in astmphysics. This project began as Phii's vision and, thmugh his encouragement for independence in tesearch, has evolved into the current work as Phil provided me wi th the keedom to pursue my own vision of this project. I'd &O iike to thank PMfor providing me with numerous opportunities to attend coderences and workshops, and develop connections in the research cornrnunity. I would like to thank the members of my PhD examining cornmittee for their interesting insights and clarifying suggestions on the thesis. Thanks Prof. Simon Lilly, Prof. Peter Martin, and Prof. Ernie Seaquist. I'd &O We to partidarly thank my extemal examiner, Dr. Gregory Taylor, for numerous suggestions and comments which have helped smooth out some of the bump on the thesis. Iri addition, I am particularly grateful to the staff at the Array Operations Center of the VLA in Socorro, New Mexico. During each visit to the VLA 1 was assured to be geeted with lots of fnendly smiles aad enthusiastic scientific discussion. In particular 1would Like to thenk Michael Rupen for his patience in teaching me AlPS and his unwavering enthusiasm for ail aspects of astronomy. I'd also like to thank Chris Carilli, Jean Eilek, and Ftaaer Owen for many interesthg discussions which always left me thinking of new approaches to my work. A special thanks also to Dan Mertely for helping me to dodge RF1 wherever possible. Finally, I'd like to thank NRAO for the genemus allocation of VLA tirne to undertake this project . This research ha9 aho greatly benefited kom an extended stay at the Max-PIanck- Institut flu extraterrestrische Physik in Garching, Germany. 1 would iike to thank Ham Bohringer for his patience and support as 1 learned the ropes of X-ray astronomy. 1 muid ahiike to sincerely thank Thomas Reiprich for taking valuable time nom his resed scheduie to provide extensive help and patience in teaching a radio astronomer how impor- tant eacb photon really is! The Department of Aetronomy at U of T has provîded a wealth of knowledge, insight, anci particuiariy fiends which 1 nin not hrget. Although I'd like to thank ail members of the department for making my stay here enjoyable, there are a few feilow students who provided the majorîty of the laughter and companiomhip 1 relied ou that I must single out. Whenever 1lost sight of rny enthusiasm for research, Mike GLadders always provided a bright reminder of the fascination that Astronomy holds. Felipe Barrientos was an endesa source of humour and enthusiasm throughout my time at U of T, qualities which allowed me to overlook his dislike of chocolate. The pitfslls during rny PhD aluays seemed las menachg after the words of support and encouragement fiom Marcin Sawicki. Marcin provided me with a role mode1 both as a person and as a researcher. Mike Allen was essential to my sanity during the long days (and night) over the past year. His unique sense of humour and iight heart made 1404 a fun place to work. Gabriela Md&-Ornelas helped me to discover the beauty of Early Music through her love of music and extensive knowledge of the Toronto music scene. Gabriela also proved to be the ideal person to share the experiences of job searches and thesis completion with. With these people 1 have the strangest, and most enjoyable conversations possible over vast quanti ties of Chinese food and tea. I'd also like to say a special thanks to Barbara Haynes and Elizabeth Rescher for pro- viding much needed reali ty checks t hat t here is a life outside the university. 1 also have to thank Kathy Dumli for sharing in an infinite number of strange conversations and boxes of Kraft Dinner, and Boys in the Bright White Sporkrcur for always knowing the proper distraction when 1needed it. Thanks &O to Sandra Scott for her advice on cornputers and lif'e, and for her generous tirne spent reading this manuscript. 1 would particularly iike to thank my family for their support and encouragement during my many years in university. Although it was a long road fiom Moncton, New Brunswick to my PhD, tiiey never doubted that 1 would be successfulLIn fact, 1 would iike to dedicate my thesis to the memory of my grandmot her, Olga Mae Richard, who recently passed away. Nan was a grandmothm in every sense of the terni, she was a beautifid, intelligent, and resourceful woman who always told me tbat 1could accompIish anything 1 wanted to do, and who supported me with her love and strength throughout my life. Although I'm sad that I cannot share the joy of complethg rny PhD with her, 1 Lnon how proud she would be. 1 have made use of the ROSAT Data Archive of the Max-Planck-Institut fiir extrater- restrische Physik (MPE) at Garching, Germany. This research has also made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propuision Lab- oratory, Caüfornia Institute of Technology, under contract with the National Aeronautics and Space Adminihration. This work was generously supported by scholarships fiom the National Sciences and Engineering Research Council of Canada, the province of New Brunswick, the I.O.D.E., the Sumner Foundation, the O'Brien Foundation, and the University of Toronto. Contents 1 Introduction 1 1.1 Why Study Intracluster Magnetic Fields? ................... 2 1.2 Overview of Previous Statistical Faraday Studies ............... 4 1.3 This Thesis .................................... 6 3 Th- 8 2.1 Emission Mechmisms .............................. 8 2.1.1 Synchrotron Radiation .......................... 8 2.1.2 Thermal Bremsstrahlung ........................ 11 2.1.3 InverseCompton ............................. 12 2.2 Radio Sources ................................... 13 2.3 Clusters of Galaxies ............................... 14 2.3.1 Propert ies of the Intracluster Medium ................. 14 2.4 Detecting Intracluster Magnetic Fields ..................... 17 2.4.1 Synchrotron Halos: Equipartition ...................
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