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Cosmic Rays in Star-Forming Galaxies

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Cosmic Rays in Star-Forming Galaxies Cosmic Rays in Star-Forming Galaxies Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Brian Cameron Lacki Graduate Program in Astronomy The Ohio State University 2011 Dissertation Committee: Professor Todd Thompson, Advisor Professor John Beacom Professor Christopher Kochanek Copyright by Brian Cameron Lacki 2011 ABSTRACT Cosmic rays (CRs) are high energy particles that are found wherever in the Universe star formation is occurring. I investigate several problems in the propagation of CRs in star-forming galaxies. By applying analytic models and numerically solving the “leaky box” differential equation, I calculate the population of primary and secondary CR protons, electrons and positrons in model star-forming galaxies and their nonthermal emission. Observations show that the synchrotron radio emission of star-forming galaxies grows linearly with the infrared emission from dust-obscured young stars; this is the FIR-radio correlation (FRC). To explain the correlation, I constructed one-zone models of galaxies over the dynamic range of the FRC. I found that the FRC is caused by conspiracies of several factors, including CR escape from galaxies, ultraviolet (UV) dust opacity, non-synchrotron cooling, and secondary electrons and positrons generated by CR protons. The conspiracies have great implications for the evolution of the FRC at high redshift, preserving it and allowing variations in the FIR-radio ratio for submillimeter galaxies. ii Recent gamma-ray observations of M82 and NGC 253 indicate that CR protons lose much of their energy to collisions in these galaxies’ dense gas, where they generate unstable pions that decay into gamma rays and secondary particles. The ratio of gamma-ray to radio luminosity indicates that secondary electrons mostly do not cool by synchrotron emission, supporting a conspiracy origin of the FRC. I also compare the intensities of the diffuse cosmic gamma-ray background to the X-ray and radio backgrounds. From this comparison, I find that Inverse Compton is a minority of the X-ray background, and that the radio background is probably not from starbursts. Finally, I modeled the nonthermal X-ray emission from starburst galaxies, both synchrotron from TeV electrons and Inverse Compton from GeV electrons. The synchrotron emission is enhanced by γγ pair production in the intense infrared radiation of starbursts. Synchrotron and Inverse Compton emission make up 1 - 10% of the observed diffuse hard X-ray emission observed in starburst galaxies. iii For science! And my parents. iv ACKNOWLEDGMENTS I’ll start off by thanking my advisor and mentor, Todd Thompson. He has been both energetic and approachable. Most of what I’ve learned about being a theoretical astrophysicist, I have learned from him. Todd has shared with me many insights into astronomy and physics and presentation. He has also encouraged me when I set out on my own rogue research projects. John Beacom has served as my other mentor through my years here. Much of the rest I have learned about being a theoretical astrophysicist I learned from him, both on physics and presentation. We’ve had many interesting discussions and ideas over the years. Our collaboration has led to one published paper, on dark matter haloes around primordial black holes, and one work in preparation. Now is my opportunity to thank my other co-authors of the papers compiled in this dissertation: Eliot Quataert, Eli Waxman, and Avi Loeb. They have provided helpful comments on presentation, encouragement, and useful insight into the propagation of cosmic rays, especially in using analytic calculations. While working on the papers that formed the core of this dissertation, a number of additional people provided critical comments that contributed to our works: v Rainer Beck, Boaz Katz, Matthew Kistler, Michael Micha lowski, and Diego Torres. I would also like to acknowledge the interesting and informative discussions and exchanges I’ve had on cosmic rays with Marco Ajello, Roland Crocker, Shane Davis, Chuck Dermer, Liu Fan, Gary Ferland, Shunsaku Horiuchi, Christopher McKee, Norm Murray, Padelis Papadopoulos, Vasiliki Pavlidou, Troy Porter, Jack Singal, Aristotle Socrates, Andy Strong, Meng Su, Heinrich V¨olk, Peter Williams, and Dong Zhang. More generally, I would like to thank the events and institutions that have hosted me as a speaker: CCAPP, Berkeley, the IAS, CITA, the “Infrared Emission, Interstellar Medium and Star Formation” conference in Heidelberg, and the “Cosmic Ray Interactions: Bridging High and Low Energy Astrophysics” workshop in Leiden. Not all of my work has been on galactic cosmic rays. I acknowledge Bernd Br¨ugmann, who offered me my first research project at Penn State. Jane Charlton was my mentor during my undergraduate years, when I worked on quasar absorption lines. She gave me both experience handling real data and a taste of theory, for which I thank her. Christopher Kochanek guided me through my first graduate project here, and has served on my candidacy and dissertation committees. I thank him for his work, his patience, and his acting as a soundboard for my Cherenkov telescope paper. I want to acknowledge Kris Stanek for his work on the difference imaging paper as well. Scott Gaudi also provided much advice, ideas, and encouragement for the Cherenkov telescope paper, for which I am grateful. I acknowledge David Weinberg who served on my candidacy committee as well and attended my defense. vi Finally, I would like to thank Kohta Murase, for his work on the circumstellar shell transient paper that I was a co-author on. I have relied on the technical assistance of many in getting my code working and verifying that it worked properly. I would like to thank the GALPROP team for making their code freely available, and codifying the pionic secondary production cross sections. Igor Moskalenko in particular shared the total Galactic gamma-ray luminosity, which was an important calibration for our efforts. I also thank the Kamae et al. (2006) team for making public their own pionic secondary production cross sections, which include neutrinos. I gratefully acknowledge recent technical discussions on the spectrum of γγ pair production e± with Felix Aharonian, Dmitry Khangulyan, and Markus B¨ottcher. Closer to home, I relied on the assistance of the OSU Astronomy Department’s Computing Support staff. Thanks especially to David Will who maintained Condor. Rick Pogge provided valuable assistance with getting my tables working with this LaTex template. Finally, I would like to acknowledge those tools that I relied on so much: the Astrophysical Data System and arXiv. They are both free to use and indispensable. On the administrative side, I would like to thank the staff of the Astronomy Department, including Kristy Krehnovi, who assisted me with travel and other administrative matters during my years. I acknowledge Evan Sugarbaker, who served as the Graduate Faculty Representative during my defense. vii In addition to my GTA and GRA positions, I acknowledge my Elizabeth Clay Howald Presidential Fellowship, which supported my last year of work here. I would like to thank the NRAO and IAS who will be hosting me at my next position as a Jansky Fellow. I also am grateful to CITA and the Hubble Fellowship who offered me positions. To my fellow grad students: thanks for giving me a chance to be Sir Isaac. And on a different note, I hope you put those rainbow sprinkles to good use. Finally, I’d like to thank my parents, who have put up with this dream of mine for a good 20 years now. viii VITA July 3, 1984 . Born – Dover, New Jersey, USA 2006 . B.S., Astronomy & Astrophysics and Physics The Pennsylvania State University 2006 – 2008 .................... Graduate Teaching Associate, The Ohio State University 2008 – 2011 . Graduate Research Associate, The Ohio State University 2010 – 2011 . Elizabeth Clay Howald Presidential Fellow, The Ohio State University PUBLICATIONS Research Publications 1. Brian C. Lacki, Christopher S. Kochanek, Krzysztof Z. Stanek, Naohisa Inada, and Masamune Oguri, “Difference Imaging of Lensed Quasar Candidates in the Sloan Digital Sky Survey Supernova Survey Region”, ApJ, 698, 428, (2009). 2. Brian C. Lacki and Jane C. Charlton, “The z = 0.0777 CIII absorber towards PHL 1811 as a case study of a low-redshift weak metal line absorber”, MNRAS, 403, 1556, (2010). 3. Brian C. Lacki, “The End of the Rainbow: What Can We Say About the Extragalactic Sub-Megahertz Radio Sky?”, MNRAS, 406, 863, (2010). 4. Brian C. Lacki, Todd A. Thompson, and Eliot Quataert, “The Physics of the FIR-Radio Correlation: I. Calorimetry, Conspiracy, and Implications”, ApJ, 717, 1, (2010). ix 5. Brian C. Lacki and Todd A. Thompson, “The Physics of the FIR-Radio Correlation: II. Synchrotron Emission as a Star-Formation Tracer in High-Redshift Galaxies”, ApJ, 717, 196, (2010). 6. Brian C. Lacki and John F. Beacom, “Primordial Black Holes as Dark Matter: All or Nothing”, ApJL, 720, L67, (2010). 7. Brian C. Lacki, “The Gamma-Ray Background Constrains the Origins of the Radio and X-Ray Backgrounds”, ApJL, 729, L1, (2011). 8. Brian C. Lacki, Todd A. Thompson, Eliot Quataert, Abraham Loeb, and Eli Waxman, “On The GeV & TeV Detections of the Starburst Galaxies M82 & NGC 253”, ApJ, 734, 107, (2011). FIELDS OF STUDY Major Field: Astronomy x Table of Contents Abstract..................................... ii Dedication.................................... iv Acknowledgments................................ v Vita ....................................... ix ListofTables .................................. xvii ListofFigures.................................. xx Chapter 1 Introduction ............................ 1 1.1 Cosmicrays................................ 1 1.1.1 TheOriginofCRs ........................ 4 1.1.2 Understanding CR Propagation and Emission . 8 1.1.3 TheEffectsofCRs ........................ 13 1.2 What Did We Know on CRs in Star-Forming Galaxies? . 15 1.2.1 Previous observations of CRs in other galaxies . 15 1.2.2 PrevioustheoriesoftheFRC . 21 1.2.3 Previous models of CR propagation in other galaxies . 23 1.2.4 The Cosmic Backgrounds from Star-Formation Cosmic Rays .
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