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Probing the Intergalactic Medium with High-Redshift

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Probing the Intergalactic Medium with High-Redshift UNIVERSITY OF CAMBRIDGE INSTITUTE OF ASTRONOMY ATHESISSUBMITTEDFORTHEDEGREE OF DOCTOR OF PHILOSOPHY [X\ PROBING THE INTERGALACTIC MEDIUM WITH HIGH-REDSHIFT QUASARS [X\ ALEXANDER PETER CALVERLEY DARWIN COLLEGE Submitted to the Board of Graduate Studies 19th October 2011 UNDER THE SUPERVISION OF DR. GEORGE D. BECKER “Science is what we understand well enough to explain to a computer. Everything else we do is art.” - DONALD KNUTH DECLARATION I hereby declare that my thesis entitled Probing the Intergalactic Medium With High-Redshift Quasars is not substantially the same as any that I have submitted for a degree or diploma or other qualifi- cation at any other University. I further state that no part of my thesis has already been or is being concurrently submitted for any such degree, diploma or other qualification. This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration except where specifically indicated in the text. I note that Chapters 1 and 2 are intended as reviews, and as such contain little, if any, original work. They contain a number of images and plots extracted from other published works, all of which are clearly cited in the appropriate caption. Those parts of this thesis which have been published or accepted for publication are as follows: • Much of the work contained in Chapters 4 and 5 formed the basis of the work published as Calverley, A. C., Becker, G. D., Haehnelt, M. G., and Bolton, J. S. (2011) ‘Measurements of the ultraviolet background at 4.6 < z < 6.4 using the quasar proximity effect’, Monthly Notices of the Royal Astronomical Society 412, 2543-2562 and was completed in collaboration with the named authors. This thesis contains fewer than 60,000 words. Alex Calverley Cambridge, October 19, 2011 v ACKNOWLEDGEMENTS This thesis would not exist without the continuing help and support of those around me, and there are many people who deserve a mention. The greatest thanks are reserved for my supervisor, George Becker, who chose to take me on as his student, and who has been a great source of ideas and in- spiration during these past three years. I am also relatively rare amongst grad students in the IoA, in that I have had regular contact with my secondary supervisor, Martin Haehnelt. He first became in- volved during the writing of my paper in my second year, and has kept a careful eye on my work ever since. In many ways, the fact my thesis has been completed on time is largely down to his constant encouragement (and relentless deadlines!). Before I could choose a supervisor though, I needed a place in the IoA first. I will forever be grateful to Bob Carswell for sending me that email offering me a place, and to the examiners who looked at my work during each year of my PhD (Max Pettini, Ian McCarthy, Richard McMahon, Anthony Challinor and Andrew Fox) and decided to let me stay! Similarly, I must thank Paul Hewett for showing himself to be the quasar expert, and for always being willing to answer my questions on both basic astrophysics, and topics tangential to my research. Of course, the IoA is more than just a building full of researchers. Working behind the scenes are a dedicated team of support staff whose excellent work keeps the department working with incredible efficiency. Particular mentions must go to Siân Owen, who is quite simply the source of all knowledge, and Margaret Harding, whom I have to thank for all her sage advice when planning my observing and conference trips. I must also mention the Helpdesk staff (Hardip, Andy B, Andy D and Sue) who have always come to my rescue when my inability to use the command-line has caused my computer to throw a tantrum. I also give thanks to STFC, who have funded my PhD studentship and given me the opportunity to spend three years studying astronomy. They, together with Darwin College, have also covered the costs of my adventures abroad, for conferences and observing. Via them I have visited both India and Hawai’i during my PhD, and these were experiences I will never forget. My time at the IoA has been greatly enhanced by my friendships with other grad students. The most notable of these are my year group (Adrian, Amy, Becky, Dom, James M, James O, Jon, Ryan, Stephanie, Warrick and Yin-Zhe) with whom I have some great shared memories, ranging from games of ‘foilball’ to hiding under a particular professor’s desk. A special mention must go to the subset known as the ‘West’s Crew’ (Becky, James M and Jon), who ensured that I could spend my lunchtimes discussing current affairs away from the IoA, instead of eating alone at my desk. On a day to day basis though, it has been my officemates in H27, past and present (Caitlin, Simeon, Julie, Maciek, Adriano, Kim, Stephen and Warrick), who have kept me smiling, even when my code refuses to work. A special thank you goes out to Caitlin for teaching me some of her IDL skills (plus giving me her entire IDL library), and to Warrick for putting up with my endless requests for command-line help, questions on grammar, and showing me how to use Mathematica. During my final year, I became much more involved in the outreach activities that take place at the IoA. These are all organised by Carolin Crawford, so I am very grateful to her for letting me take part. vii viii Acknowledgements Indeed, if it wasn’t for Carolin, chances are I would never have heard about the job that I ultimately took, so I will always be thankful to her for forwarding on that email. My world has not just revolved around the IoA during my time in Cambridge. I have met some great people in my college, particularly in my first year, who have been instrumental in helping me adjust to life in Cambridge. Similarly, student societies have helped me to balance my work with play. One of my greatest regrets in undergrad was never picking up rowing, so I’m very glad that Darwin College Boat Club let me make amends for that! Also, I am particularly grateful to the excellent characters I have met on the Cambridge music scene, and I will always feel honoured to have made music with such a talented band of musicians. There have been several great science teachers during my life who have inspired me to pursue my dream of studying astronomy, although none more so than my MSci thesis supervisor, Alastair Edge. My project with him helped me realise that I enjoyed doing research, and with his encouragement I sent in my application to the IoA. I hope that through this thesis, I have lived up to the potential that he saw in me. No acknowledgements would be complete without mentioning the tireless love and support of my parents. They have always been there for me, and I cannot thank them enough for everything they have done. They also formed one half of my diligent team of thesis proof-readers. The other half is someone very special to me, who has been at my side through it all. Sarah, thank you so much for all that you have brought to my life. This thesis has been typeset in LATEX using TEXnic Center and JabRef. Thanks to all the former IoA members who have contributed to the thesis template on which this is based, and particularly to Mark Booth from whom I received this version. Alex Calverley Cambridge, October 19, 2011 SUMMARY Probing the Intergalactic Medium with High-Redshift Quasars Alex Calverley Clues about the timing of reionization and the nature of the ionizing sources responsible are im- printed in the ionization and thermal state of the IGM. In this thesis, I use high-resolution quasar spectra in conjunction with state-of-the-art hydrodynamical simulations to probe the IGM at high redshift, focusing on the ionization and thermal state of the gas. After reionization, the ionization state of the IGM is set by the intensity of the ultraviolet back- ground (UVB), quantified by the hydrogen photoionization rate, Γbkg. At high redshifts this has been estimated by measuring the mean flux in the Lyα forest, and scaling Γbkg in simulations such that the simulated mean flux matches the observed value. In Chapter 3 I investigate whether the precision of these estimates can be improved by using the entire flux probability distribution function (PDF) instead of only the mean flux. Although I find it cannot improve the precision directly, the flux PDF can potentially be used to constrain other sources of error in observational estimates of Γbkg, and so may increase the precision indirectly. The ionizing output of a quasar will locally dominate over the UVB, and this leads to enhanced transmission bluewards of the quasar Lyα line, known as the proximity effect. In Chapter 4 I present the first measurements of Γbkg at z > 5 from the proximity effect. The UVB intensity declines smoothly with redshift over 4.6 < z < 6.4, implying a smooth evolution in the mean free path of ionizing pho- tons. This suggests that reionization ends at z > 6.4. There is a drop in Γbkg by roughly a factor of five, which corresponds to a drop in the ionizing emissivity by about a factor of two. Such a redshift evolution in the emissivity cannot continue to much higher redshift without reionization failing to complete, which suggests that reionization cannot have ended much higher than z = 6.4.
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