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Observational Signatures and Effects of Magnetic Fields in Astrophysical Systems Observational signatures and effects of magnetic fields in astrophysical systems Alvina Yee Lian On Thesis submitted for the degree of Doctor of Philosophy of University College London Mullard Space Science Laboratory Department of Space and Climate Physics University College London May 2020 2 Declaration of originality I, Alvina Yee Lian On, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. In Chapters3 and4, I collaborated with Jennifer Chan on the polarised radiative transfer calculations, where I focused on the galaxy cluster scales, and Jennifer on the cosmological aspect. David Barnes kindly provided the galaxy cluster data obtained from his GCMHD+ simulations for my analysis in Chapter 4. In Chapter5, I collaborated with Ellis Owen on the ionisation and heating effects of cosmic rays in molecular clouds, where I focused on the effects caused by the pres- ence of magnetic fields, and Ellis on the microscopic interactions between cosmic ray particles and the medium. Some parts of this thesis were published/submitted in the following articles: (i) Alvina Y. L. On, Jennifer Y. H. Chan, Kinwah Wu, Curtis J. Saxton and Lidia van Driel-Gesztelyi, \Polarised radiative transfer, rotation measure fluctuations and large-scale magnetic fields”, December 2019, MNRAS 490 (2): 1697{1713 (ii) Jennifer Y. H. Chan, Kinwah Wu, Alvina Y. L. On, David J. Barnes, Jason D. McEwen and Thomas D. Kitching, \Covariant polarized radiative transfer on cos- mological scales for investigating large-scale magnetic field structures", April 2019, MNRAS 484 (2): 1427{1455 (iii) David J. Barnes, Alvina Y. L. On, Kinwah Wu and Daisuke Kawata, \SPMHD simulations of structure formation", May 2018, MNRAS 476 (3): 2890{ 2904 (iv) Ellis R. Owen, Alvina Y. L. On, Shih-Ping Lai and Kinwah Wu, \Interac- tions of energetic cosmic rays in molecular clouds: astrophysical implications and Declaration of originality 3 observational signatures", submitted March 2020 to MNRAS Signed: Date: 4 Abstract Magnetic fields are ubiquitous, permeating across all scales from interstellar space to voids. Their origins and evolution still remain as open questions. On galactic scales and beyond, Faraday rotation measure (RM) at radio wavelengths is commonly used to diagnose magnetic fields, and its spatial correlation gives the characteristic length scales of the field variation. This work shows how the RM is derived from the polarised radiative transfer equations under restrictive conditions and assesses the merit of RM fluctuations (RMF) for large-scale magnetic field diagnostics. The interpretation of RMF analyses is ambiguous for an ill-defined characteristic den- sity, such as lognormal-distributed and fractal-like structures. The RMF approach also falls short under radiative absorption, emission, Faraday mixing and the con- tribution of non-thermal electrons. Notably, correlations along the line-of-sight and across the sky plane are generally dissimilar, therefore the context of RMF must be clarified when inferring from observations. Magnetic fields can also imprint observa- tional signatures in the radio synchrotron emission, whose total intensity reveals the field strength and polarisation traces the field orientation. A point-by-point com- parison between the X-ray and radio emissions of a simulated galaxy cluster follows a linear best-fit slope of almost unity, indicating that the magnetic field scales with density locally. On smaller scales, magnetic fields may have an important role dur- ing the formation and evolution of molecular clouds. The effects of magnetic fields on the ionisation and heating rates of cosmic rays in IC 5146 are quantified, assum- ing that the fields are traceable via optical and near-infrared starlight polarisations. While the ionisation rate is fairly constant across the cloud, cosmic-ray heating is capable of raising temperatures by order of 1 K in a Galactic environment, or even higher in actively star-forming regions. This may lead to an increase in the Jeans mass and consequently affect the onset of star formation. 5 Impact statement The origins and evolution of cosmic magnetism is still an astrophysical puzzle. This research has shifted our current understanding of cosmic magnetic fields. Notably, this work has shown that the conventional diagnostic of large-scale magnetic fields is not universally applicable. As such, it has raised awareness within the scientific community of the validity of assumptions which have been previously overlooked in literature. This consequently motivates the work to develop a more robust theoret- ical method of radiation transport as a cosmic magnetic field probe. These studies are crucial as a means to probe the magnetic fields in galaxy clusters and beyond, which are particularly timely with the advent of the world's largest radio telescope, the Square Kilometre Array. As part of this research, the conventions of polarisation and magnetic field have been clarified. These definitions were previously ambiguous across multi-disciplinary literature, including plasma physics and astrophysics. Furthermore, this research has contributed to our knowledge and understanding of magnetic fields in the intr- acluster medium and molecular clouds. The work presented in this thesis has been presented at local, national, and international conferences. The research works have also been published and submitted to leading, international, peer-reviewed academic journals. The cumulative academic reach1 of the three publications, from February 2018, are 9 citations, 486 reads, and 232 downloads, as of May 2020. Additionally, this research has established a network of local and international collaborations, across the UK, Taiwan (ROC), USA, Canada, Israel, France, and Hungary. Beyond academia, this research has stimulated broader public interest and en- gagement through posters, talks, and social media. From the extensive public en- gagement activities undertaken during the duration of this research, it is clear that 1Using the metrics from the NASA's Astrophysics Data System Impact statement 6 astronomy is a subject that fascinates people of all ages. Parts of this research have been widely communicated through Facebook, VoxCharta, and the UCL-MSSL As- trophysics Blog, where posts on topics including \The invisible world of magnetic fields” and scientific experiences in various conferences, have been well-received. Moreover, the opportunity to tutor GCSE and A-Level students, as well as discuss with visitors to the laboratory about research student life, has allowed this work to be disseminated in informal and non-specialist settings. This has ensured that the results of this research are accessible to wider audiences, including school children and prospective BSc/MSc/PhD students. In some cases, this has inspired younger students to pursue higher education in physics and astronomy. Finally, there has been fruitful knowledge and research transfer between astrophysics and industry, by consulting start-ups in renewable energy and technology on big data analytics. 7 Acknowledgements First of all, I would like to express my deepest gratitude towards Prof Kinwah Wu and Prof Lidia van Driel-Gesztelyi for their unwavering support and encourage- ment throughout my PhD journey. I am incredibly grateful to be working with two supervisors who inspired me to become a better scientist through their passion, knowledge, insight and wisdom. Special thanks to Dr Curtis Saxton for the help- ful science discussions, and for patiently teaching me IDL and various statistical techniques. I am appreciative of Dr Wim van Driel for being a wonderful mentor in radio astronomy. I would also like to thank Prof Daisuke Kawata, Prof Mark Cropper and Dr Ignacio Ferreras for their technical help, scientific discussions and bits of life anecdotes. I thank Prof Lucie Green and Prof Chung-Ming Ko for exam- ining this thesis and for their helpful suggestions. Very special thanks to Mr Kwan Keng Kwek for introducing me to the wonders of physics at Pengiran Jaya Negara Pengiran Haji Abu Bakar Secondary School. I am lucky to have met many amazing people who have assisted me along the way and contributed immensely to my research experience at MSSL. Many thanks to Dr David Barnes, Dr Idunn Jacobsen, Dr Jennifer Chan, Dr Ellis Owen, Dr Kirthika Mohan, Dr Luke Pratley and Brian Yu for their help, discussion and companionship in achieving our PhD degrees together. Special thanks to Dr Jennifer Chan for being my long-time collaborator and good friend; and to Dr Ellis Owen for the fruitful collaboration and his generosity in sharing ideas, techniques and methods in science. I am happy to have had the opportunity to work with and alongside other past and present members of the MSSL Astrophysics group, especially Dr Ziri Younsi, Dr Awat Rahimi, Dr Stefano Pasetto, Dr Robert Grand and Dr Jason Hunt. I would also like to particularly thank Dr Curtis Saxton, Dr Ziri Younsi, Dr Idunn Jacobsen, Dr Ellis Owen, Dr Martin Spinrath, Dr Jennifer Chan, Jane Yap and SiNan Long for proofreading my chapters. Acknowledgements 8 I would like to thank the hospitality of the Institute of Astronomy, National Tsing Hua University (NTHU), ROC (Taiwan), where I spent substantial time as a visitor and wrote parts of this thesis. Special thanks to Prof Albert Kong for kindly hosting and supporting my visits under the Ministry of Science and Technology of the ROC (Taiwan) grant 105-2119-M-007-028-MY3. I have also been supported by the Center for Informatics and Computation in Astronomy (CICA) at NTHU through a grant from the Ministry of Education of the ROC (Taiwan). I am grateful for the opportunity to learn from Prof Albert Kong, Prof Tomotsugu Goto, Dr Martin Spinrath, Prof Shih-Ping Lai, Dr Kuo-Chuan Pan and Dr Hsiang-Yi Yang during my visits. I enjoy the astro-ph sessions and Tomo-san's group meetings, especially the science discussions and banter with Dr Tetsuya Hashimoto, Dr Seong- Jin Kim, Ting-Wen Wang, Daryl Santos, Chien-Chang Ho, Ting-Yi Lu, Yu-Yang Hsiao and Chih-Teng Ling.
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