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The Hot Interstellar Medium in Normal Elliptical Galaxies

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THE HOT INTERSTELLAR MEDIUM IN NORMAL ELLIPTICAL GALAXIES A dissertation presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Steven Diehl August 2006 This dissertation entitled THE HOT INTERSTELLAR MEDIUM IN NORMAL ELLIPTICAL GALAXIES by STEVEN DIEHL has been approved for the Department of Physics & Astronomy and the College of Arts and Sciences by Thomas S. Statler Associate Professor of Physics & Astronomy Benjamin M. Ogles Dean, College of Arts and Sciences DIEHL, STEVEN, Ph.D., August 2006, Physics & Astronomy THE HOT INTERSTELLAR MEDIUM IN NORMAL ELLIPTICAL GALAXIES (250 pp.) Director of Dissertation: Thomas S. Statler I present a complete morphological and spectral X-ray analysis of the hot interstel- lar medium in 54 normal elliptical galaxies in the Chandra archive. I isolate their hot gas component from the contaminating point source emission, and adaptively bin the gas maps with a new adaptive binning technique using weighted Voronoi tesselations. A comparison with optical images and photometry shows that the gas morphology has little in common with the starlight. In particular, I observe no correlation between optical and X-ray ellipticity, contrary to expectations for hydrostatic equilibrium. Instead, I find that the gas in general appears to be very disturbed, and I statis- tically quantify the amount of asymmetry. I see no correlations with environment, but a strong dependence of asymmetry on radio and X-ray AGN luminosities, such that galaxies with more active AGN are more disturbed. Surprisingly, this AGN– morphology connection persists all the way down to the weakest AGN, providing strong morphological evidence for AGN feedback in normal elliptical galaxies. I con- clude that the hot gas in elliptical galaxies is generally not in hydrostatic equilibrium; instead, it is continually disturbed by intermittent outbursts of the central AGN. I extract radial temperature profiles, revealing surprisingly complex structures with positive and negative gradients, or even combinations of both. I find that the outer temperature gradient is determined by galaxy environment, while the inner temperature profiles shows a strong correlation with radio luminosity and a weaker with stellar mass. While our data are consistent with compressive heating in cooling flow models or supernova heating, AGN feedback is a more likely explanation. I suggest that the change of sign for the temperature gradient indicates either how localized different AGN heat, or where AGN heating becomes unimportant. Despite the disturbed gas morphology, I also report on the discovery of a tight correlation, the X-ray gas fundamental plane (XGFP), linking temperature, half-light 0.28 0.22 radius, and average surface brightness as TX ∝ RX IX , reducing the large scatter in the closely related luminosity-temperature relation. The XGFP has a small intrinsic width of only 0.07 dex, and represents a new constraint on the hydrodynamic history of the gas. Approved: Thomas S. Statler Associate Professor of Physics & Astronomy Preface I would like to note that each chapter, except for chapter 1 (Introduction) and chapter 7 (Conclusions), is an individual paper by Diehl & Statler. These papers are either already published, submitted or to be submitted very soon. As a result of our collaboration, the introduction to chapter 2 was written by my adviser Thomas Statler, and is not my own work. As each chapter was designed to be primarily published in a paper, we will not reference back to different chapters, but rather quote the appopriate paper. In addition, chapters 4, 5, and 6 are published as a series, which is why we usually refer back to Paper I, II and III of the series. To avoid confusion, here are the appropriate references for each individual chapter, along with their typically used “shortcuts”: - Chapter 2 (Diehl and Statler 2005): S. Diehl and T. S. Statler. A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies. ApJ, 633:L21-L24, November 2005. - Chapter 3 (Diehl and Statler 2006a): S. Diehl and T. S. Statler. Adaptive binning of X-ray data with weighted Voronoi tessellations. MNRAS, 368:497-510, May 2006a. - Chapter 4 (Paper I, Diehl and Statler 2006b): S. Diehl and T. S. Statler. The Hot Interstellar Medium in Normal Ellipti- cal Galaxies I: A Chandra Gas Gallery and Comparison of X-ray and Optical Morphology. ApJ, submitted, 2006b. - Chapter 5 (Paper II, Diehl and Statler 2006c): S. Diehl and T. S. Statler. The Hot Interstellar Medium in Normal Elliptical Galaxies II: Morphological Evidence for AGN feedback. ApJ, to be submitted, 2006c. - Chapter 6 (Paper III, Diehl and Statler 2006d): S. Diehl and T. S. Statler. The Hot Interstellar Medium in Normal Elliptical Galaxies III: The Thermodynamic Structure of the Gas. ApJ, to be submitted, 2006d. To my wife Anke Acknowledgments First of all, I would like to thank my adviser Tom Statler, who has not only been an exceptional mentor, but has also become a good friend during the last five years. Without his guidance, this project would have been impossible. He has inspired me from the beginning with his enthusiasm for teaching and his concise analytic skills. He left me the space to explore in my own ways, but always kept me firm on target. I would also like to thank Christopher Fryer to give me the opportunity to visit the Los Alamos National Laboratory twice during my time as a graduate student, and to offer me the chance to continue the collaboration after my PhD. I am also indebted to the other astronomy professors in our department, Joe Shields, Brian McNamara and Markus B¨ottcher, who always had an open door for questions and greatly helped in many situations. A great deal of gratitude is also due for the members of my dissertation committee, who stayed patient with me near the end, when I was short on time. I also owe a special note of gratitude to our system administrator Don Roth, who kept my computer alive, no matter how hard I tried to break it. In that sense, I would also like to thank all of my colleagues Justin, Swati, Manasvita, Myriam, Nick, Anca, Tom and Mangala, whose computers were crunching my data for extended periods of time, without them complaining. There are also many friends back home in Germany, that I simply want to thank for keeping in contact, despite the long distance: Kersten, Torsten, Sven, Andreas, Jens, Michael, Holger, and Frank. Of course, I am also grateful to my friends at Ohio University, Rocco, Karthik, Manasvita, Justin, Jack, Christina, Swati, Eric, Greg, Nick, Stu, Anca, Chris, Ozan, and Zach, who were always there to help, or just to talk and keep me from working too much. Finally, I’d like to thank my family. From the beginning, my parents Rosi and Reiner, encouraged me to go after the chance to study in the U.S., well knowing that I may not make it back soon. I would also like to thank my grand-mothers Trude and Ilse, who have accepted my decision to leave Germany to study without trying to hold me back. My brother Bj¨orn and my sister Annika were always there, when I needed someone to talk. All of them always supported my decisions and have stood by my side whenever I needed them most, which I will be always thankful for. But most of all, I would like to thank my wife Anke, who took a chance before we got married, and followed me to Ohio after one year of a long-distance relationship. She left her family and a good job behind to be with me, without ever looking back afterwards or regretting her decision. This took a lot of courage and I will be forever in her debt for taking this leap of faith for me. 10 Table of Contents Page Abstract ...................................... 3 Preface ....................................... 5 Dedication ..................................... 7 Acknowledgments ................................. 8 List of Tables ................................... 13 List of Figures ................................... 14 1 Introduction .................................. 21 1.1 Structural and Photometric Properties of Normal Elliptical Galaxies . 21 1.2 X-ray Observations of Elliptical Galaxies ................ 23 1.2.1 Historical Background ...................... 23 1.2.2 The Importance of Understanding the Point Source Component 24 1.3 The Hot Interstellar Medium ....................... 25 1.3.1 Origin ............................... 25 1.3.2 Evolution ............................. 26 1.3.3 Hydrostatic Equilibrium ..................... 28 1.4 The Role of the Central AGN in Clusters ................ 30 1.5 Open Questions for Normal Elliptical Galaxies ............. 32 1.6 Outline of the Project .......................... 33 2 A Fundamental Plane Relation for the X-Ray Gas ............. 36 2.1 Introduction ................................ 36 2.2 Data Reduction and Analysis ...................... 38 2.2.1 Chandra Archive Sample and Pipeline Reduction ....... 38 2.2.2 Isolating the Gas Emission .................... 38 2.2.3 Physical Parameters for the Gas ................. 40 2.3 The X-Ray Gas Fundamental Plane ................... 44 2.4 Discussion ................................. 46 2.4.1 Relation to Known Scaling Laws ................ 46 2.4.2 Independence of the XGFP and the SFP ............ 48 3 Adaptive Binning of X-ray data with Weighted Voronoi Tesselations ... 51 3.1 Introduction ................................ 51 3.2 Existing Adaptive Binning Algorithms ................. 53 11 3.2.1 Quadtree Binning ......................... 53 3.2.2 Voronoi Binning .......................... 54 3.3 Adaptive Binning with Weighted Voronoi Tesselations ......... 57 3.3.1 Introduction to Weighted Voronoi Tesselations (WVT) .... 57 3.3.2 Adaptive Binning Algorithm ................... 58 3.4 Performance ................................ 61 3.4.1 Comparison with Quadtree .................... 61 3.4.2 Comparison with VT ....................... 64 3.5 Applications to X-ray data ........................ 68 3.5.1 Intensity Maps .......................... 68 3.5.2 Hardness Ratio Maps ....................... 70 3.5.3 Maps of Temperature (or other Spectral Parameters) ....
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