Studies of the Interstellar Medium Towards Dark TeV Gamma-ray Sources James Cheuk-Heng Lau School of Physical Sciences University of Adelaide A thesis submitted for the degree of Doctor of Philosophy October 2017 Abstract Recent advances involving ground based observations of the very-high-energy sky have led to the discovery of a growing number of Galactic tera-electron volt (TeV) γ-ray sources. Many of these sources have been associated with other high energy phenomena, such as supernova remnants (SNRs) and pulsar wind nebulae. However, a number of TeV γ-ray sources are `dark', with no strong counterparts seen at other wavelengths. Astrophysical TeV γ-rays are produced via two channels; leptonic and hadronic. The leptonic process involves the upscattering of background photons by high energy elec- trons through the inverse-Compton effect. The hadronic process involves the decay of neutral pions produced by interactions between highly accelerated cosmic-rays (CRs) and the ambient interstellar medium (ISM). Dark TeV γ-ray sources may result from CR and ISM interactions, and studying these sources will shed light on the nature of the parent particle accelerators. Some of the sources selected for study in this thesis are considered candidate PeVatrons, an emerging class of extreme accelerators which produce CRs with energies in the peta-electron volt (PeV) range. A detailed understanding of the ISM towards dark TeV γ-ray sources is vital in order to investigate possible origin scenarios. In addition to providing contextual clues, morphological similarities between TeV γ-rays and the ISM can provide strong evidence of hadronic CR interactions. This may lead to positive associations between TeV sources and other sources observed at different wavelengths. The work in this thesis primarily involves the use of radio telescopes to investigate the distribution of the ISM towards dark TeV γ-ray sources. The data has been used to constrain the mechanisms behind these objects, and underpins the investigations into their mysterious origins. Five TeV γ-ray sources were studied as part of the work in this thesis. HESS J1640 465 and the PeVatron candidate HESS J1641 463 are a pair of γ-ray − − sources each coincident with a SNR bridged by bright Hii regions. The study of the ISM suggests that the TeV emission from both sources may be hadronic in origin, and provides evidence to support the PeVatron nature of HESS J1641 463. − HESS J1614 518 has no obvious counterparts seen in other wavelengths. The inves- − tigation of the ISM reveals an overlapping ring of gas potentially associated with an undiscovered SNR or the stellar cluster Pismis 22, which may power the TeV emis- sion. The nearby TeV source HESS J1616 508 has several potential counterparts, − including two SNRs and three pulsars, though there was no evidence in the study of the ISM to support any association. Spatially matching gas, however, is found coinci- dent with the TeV source, suggesting a possible link with an undiscovered accelerator. HESS J1702 420 is both a dark TeV γ-ray source and a PeVatron candidate. Though − a SNR and pulsar are seen near the outskirts of this source, no evidence in the study of the ISM is found to support any association. Morphologically favourable gas overlap- ping the TeV source, however, suggests possible scenarios involving nearby acclerators, such as stellar winds from massive stars or an undetected SNR. ii Statement of Originality I, James Cheuk-Heng Lau, certify that this work contains no material which has been accepted for the award of any other degree or diploma in my name, in any university or other tertiary institution and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due refer- ence has been made in the text. In addition, I certify that no part of this work will, in the future, be used in a submission in my name, for any other degree or diploma in any university or other tertiary institution without the prior approval of the Uni- versity of Adelaide and where applicable, any partner institution responsible for the joint-award of this degree. I give consent to this copy of my thesis when deposited in the University Library, being made available for loan and photocopying, subject to the provisions of the Copyright Act 1968. I acknowledge that copyright of published works contained within this thesis resides with the copyright holder(s) of those works. I also give permission for the digital version of my thesis to be made available on the web, via the Universitys digital research repository, the Library Search and also through web search engines, unless permission has been granted by the University to restrict access for a period of time. I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship. Acknowledgements This PhD would not have been possible without the support of many people, to whom I am deeply indebted. Here, I'd like to take the time to acknowledge all those who have been a part of my PhD journey; I would not have been able to do it without you. Firstly, I'd like to thank my supervisor, Gavin Rowell, for his continued guidance over the years. His advice and support allowed for my PhD journey to be a smooth and rewarding experience. I very much appreciate Gavin's feedback he provided on my innumerous drafts, as well as the many unique opportunities that he provided for me to travel both nationally and internationally. I'm very grateful to the academics of the High Energy Astrophysics Group, who have been excellent mentors throughout my entire time as both an undergraduate and postgraduate student. Special mention goes to Bruce Dawson and Roger Clay who have been there for me since day one. Thanks to Gary Hill for allowing himself to be roped in as my co-supervisor. Many thanks goes to the academics and students outside Adelaide who helped me through my PhD, especially Michael Burton and the members of TeamMopra. I am thankful to my peers in the Radeladio research group. You have been a amazing group of people, and I am glad that we were able to share an office during my PhD. Many thanks goes to my seniors: Fabien Voisin, Phoebe de Wilt and Rebecca Blackwell. I'd be remiss if I did not say that I learnt an incredible amount from all of you. Thank you for all the time we spent discussing our research, as well as for the (very enjoyable) random tangents we'd inevitably find ourselves on. I'd also like to thank all my juniors: Stephanie Pointon, Luke Bowman, Cameron Snoswell, Andrew Curzons, Kirsty Feijen and Brayden Pullen, as well as all the other students in the High Energy Astrophysics Group. It was a pleasure working (and procrastinating) with you all, and I wish you well on your future endeavours. Of course, I need to make special mention of the other astrophysics students in my year: Patrick van Bodegom, Phong Nguyen and Simon Blaess. The four of us started together as first year Space Science and Astrophysics students, and have been together through our undergrad, honours and PhDs. Special mention to Alex Chambers, who despite being a theorist, we considered the honanary `5th member' of our group. From working on assignments together (perhaps too closely), to the late nights playing video games, to even just me popping by your office for a chat, many thanks to you guys for keeping my journey an entertaining one. Thank you to all my friends from my school days for the continued support. It is heart-warming to know that, regardless of the paths we take, we can always find a way to spend time together to laugh and enjoy life. Many thanks to all my friends from Church who have been with me throughout my PhD, and indeed, most of my life. Your wide-eyed wonder (and jokes) at my research certainly encouraged me to the finish line. While most of my extended family does not live in Australia, I must express my gratitude for the care they have for me. Thank you for your messages of support throughout my studies. I'd like to make a special thanks to my grandparents. Though they passed away before I completed my PhD, the joy they had as they spoke with me about my studies is something I will never forget. To my parents, Kitty and Quentin, and my brothers Ian and Samuel. You've provided me with the most wonderful of homes to live in. The years of support and love that you have given me have had the most profound impact on my life, and I am deeply grateful for it. Thank you all so much. Finally, I'd like to thank God for his endless provision in my life, including blessing me with wonderful mentors, peers, friends and family. vi Contents Abstract i Statement of Originality iii Acknowledgements v 1 Cosmic-rays, Gamma-rays and High Energy Astrophysics 1 1.1 Introduction and Motivation . .1 1.2 Astrophysical Sources of VHE Gamma-rays . .2 1.2.1 Supernova Remnants . .3 1.2.2 Pulsars and Pulsar Wind Nebulae . .4 1.2.3 Binary Systems . .4 1.2.4 Massive Stellar Clusters . .5 1.2.5 Diffuse VHE Gamma-ray Emission at the centre of the Galaxy5 1.2.6 PeVatrons . .5 1.2.7 Dark and Unidentified Sources . .6 1.3 Cosmic Rays and Electrons . .7 1.3.1 The Cosmic-Ray Spectrum . .8 1.3.2 Cosmic-Ray and Electron Propagation .