Metallicity Variations and Conduction in the Intracluster Medium THIS DISSERTATION IS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY March 2003 Roger Glenn Morris INSTITUTE OF ASTRONOMY & ROBINSON COLLEGE UNIVERSITY OF CAMBRIDGE For J. A. M. and J. K. M. DECLARATION I hereby declare that my thesis entitled Metallicity Variations and Conduction in the Intracluster Medium is not substantially the same as any that I have submitted for a degree or diploma or other qualification 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. Those parts of this thesis which have been published or accepted for publication are as follows. Sections of Chapters 2 and 4 were published as: • Morris R. G., Fabian A. C., 2003, MNRAS, 338, 824. Some of the issues discussed in Chapter 4 were presented in preliminary form in: • Morris R. G., Fabian A. C., 2002, in Matteucci and Fusco-Femiano (2002), pp. 85–90. Work on thermal conduction (Chapter 5) has benefited from discussions with Lisa Voigt, published • as: Fabian A. C., Voigt L. M., Morris R. G., 2002b, MNRAS, 335, L71. Various figures throughout the text are reproduced from the work of other authors, for illustration or discussion. Such figures are always credited in the associated caption. This thesis contains fewer than 60,000 words. R. Glenn Morris Cambridge, 10th March 2003 v SUMMARY This thesis focuses mainly on an investigation of possible explanations for the lack of low-temperature emission lines seen in recent X-ray spectra of cooling flow clusters. I consider two main hypotheses. Firstly, I examine the idea that the metals in the intracluster medium might be distributed inhomoge- neously on small, as yet unresolved scales. I make a theoretical analysis of the effects that such a metal distribution would have on the X-ray spectra of cooling plasmas such as are found in the centres of galaxy clusters. Using the standard isobaric cooling flow model, I quantify the suppression in low-temperature line emission that can be achieved in this way. Different species are found to behave in different fashion, as governed by the temperature dependence of their equivalent widths. I go on to describe my implementation of a one-dimensional Lagrangian numerical scheme that can be used to model the intracluster medium in either a single-phase or multi-phase mode. Synthetic X-ray observations can be produced and analysed using standard tools. This scheme is used to investigate the spatial and temporal evolution of simulated clusters with small-scale metallicity variations. I stress the effects on the observed spectra, and the deductions that would be made from them. I find that such metal distributions may give rise to interesting effects in observed abundance profiles, in that apparent abundance gradients with central abundance drops and off-centre peaks, similar to those seen in several real clusters, can be produced. The importance of observational testing of this hypothesis is stressed. Secondly, I study some of the possible effects of efficient thermal conduction in the intracluster plasma. The numerical code is modified to include the effects of thermal conduction. The level of con- ductivity is controlled by the structure of the cluster magnetic field, and I include a simple parametrization to represent this. The influence of conduction on the evolution of the temperature profile for high- and low-temperature clusters is demonstrated, as well as its ability to delay the onset of cooling in cluster cores over a wide range of cluster parameters. The simulations highlight the difficulty in constructing stable solutions to the cooling flow problem based on thermal conduction, but do show that the properties of clusters are such that conduction is potentially an important process for their evolution. vii ACKNOWLEDGEMENTS∗ Or, “This is all your fault, you know” Well, here we are. To be honest, there were times when I wondered if I’d ever get to this point. That I have done is largely due, I think, to the people listed below. This is poor recompense for putting up with me, but I hope to make it up to you all in the future. It’s been a long three-and-some years, and what I’ve learnt about astronomy is far outweighed by the other things you have taught me. I may go on for too long here, but what the hell, nobody’s forcing you to read this. If I were to write fully what I owe to all of you, it would be a second thesis, but one is quite enough. It’s a cliche´ to say that this thesis wouldn’t exist without my supervisor, Andy Fabian, but that doesn’t make it any less true. Thanks to him for suggesting many interesting lines of inquiry, and for tolerating my appalling slowness in following them up. Thanks to Andy Amos for putting up with me during my A-levels, for appreciating the value of sarcasm, and for not complaining too much when I turned my back on biology. I’d like to thank/blame Alastair Edge for getting me started in the field of X-ray astronomy in the first place, all those years ago. It was him who made me think this seemed like it could be a fun thing to do. Alastair is also responsible for my firm belief that unfeasibly large tea mugs and astronomy should go hand in hand. I’m also grateful for the observing trip to IRAM Granada, to see how the other half lives. Thanks to Thomas Hove Pedersen for helping me get through my undergraduate years, though I fear our personalities were too similar to be good for either of us. Still, another pair of hands to steady the mortar is always useful. It looks like I haven’t learnt your “get out of Cambridge” lesson yet Tom, so you’ll have to keep explaining it to me. Thanks to Felipe Menanteau for being one of those to welcome me to the Hoyle 19 family on my arrival to the IoA, and for being a part of the project to educate me with the palabra del d´ıa. I’m sorry I turned out to be such a poor pupil. To Meghan Gray for a lot of sensible advice, many emails, some fun conversations, and generally for taking an interest. As another one of the original senior figures (sorry!) of H19 she contributed enormously to the great (and much missed) atmosphere. Thank you also Meghan for some much-needed support during some difficult times. To Mateusz “the Disco King” Ruszkowski for his shared appreciation of Desperados beer, and for managing to teach me one word of Polish. Sadly your dancing shoes remain unfilled in the Group. Thanks to Richard Wilman, who despite having the singular misfortune in life to have been born a Yorkshireman, remains a thoroughly decent bloke. To Raquel Morales for an awful lot of things, including some very civilised lunches, a lot of fruit tea, some fine walks and many good conversations. Heartfelt thanks for being a pillar of the small but select group that tried to keep me sane, and apologies for making it such a difficult job at times. Thanks to Andrea Merloni and Alessandra Corchia for tolerating so well my decidedly non-Italian temperament, some good parties, and for making it possible for me to see Rome. To Enrico Ramirez Ruiz and Kristin Truse I owe more than I can say. Thank you for many fine dinners, for your tolerance and understanding, for Mexico, for putting up with your unsociable flatmate, ∗The OED recommends ªacknowledgmentsº. What is the world coming to? ix x Acknowledgements for Borges, and above all for your friendship. El deseo de expresar gratitud aventaja a mi capacidad de enunciarlo, desgraciadamente en cualquier idioma. To Jon Gair, Fraser Clarke and Dave Gittins must go collective thanks for many hours of slacking, as the co-inventors of the truly beautiful concepts of Pre-tea Tea, Pre-pre-tea Tea, Post-tea Tea. I’m sorry we never got round to the twenty-four hour tea-a-thon, but we surely put the training in. Thanks guys for making life at the IoA enjoyable, and for some good nights at the St. Radegund. Individually, thanks to Jon for trying to keep me fit with the occasional bout of squash, to Dave for being a games programmer extraordinaire, and to Fraser for inviting me over to watch his TV (several times), and buying a round (once). Dave gets another mention as a fellow webcomic-devotee, and the most prolific (OK, only) suggester of features for “the script”. Thanks Dave, and also Colin Frayn and Fraser, for sharing my obsession (to some degree), and having at least a vague idea of what I was talking about in my references to Bobbins et al. My thanks to Poshak “The Shak” Gandhi for the gifts of his friendship, and his bicycle. The former carries me farther than the latter ever will. I dread to think what impression of British culture we’ve left you with Poshak, and how badly we must have corrupted your vocabulary. To Richard Barker, for a lot of things over a lot of years, including many fine walks in the Lakes that provide me with fond memories. I’m looking forward to many more in the future. Thanks also for being a man of letters Richard, and reminding me of the virtues of pen and paper.